Overcurrent Protection And The 2002 National Electrical Code NE02 Questions & Answers To Help You Comply ® On-Line Training available on www.bussmann.com See inside cover for details In the pdf format this electronic version can be navigated by: 1. Scrolling using the side bar. 2. From the Contents, pages 3 & 4, click either the Section Number or Heading Topic of interest. 3. Using Catch Phrases—found on pages A1 & A2 at the end of the document or using the button on this page. 4. Go to the edit menu, find and type phrase. Click “find again” to find repeated occurrences.
Transcript
Overcurrent Protection And The 2002 National Electrical Code
NE02Questions & Answers To Help You Comply
®
On-Line Trainingavailable on www.bussmann.com
See inside cover for details
In the pdf format this electronic version can be navigated by:1. Scrolling using the side bar.2. From the Contents, pages 3 & 4, click either the Section Number or Heading Topic of
interest.3. Using Catch Phrases—found on pages A1 & A2 at the end of the document or using the
button on this page.4. Go to the edit menu, find and type phrase. Click “find again” to find repeated
occurrences.
2
A presentation format of questions and answers has been
used in this bulletin to focus on the factors which are pertinent to
a basic understanding and application of overcurrent protective
devices. Relevant sections of the National Electrical Code® are
referenced and analyzed in detail. Sections are translated into
simple, easily understood language, complemented by one-line
diagrams giving sound, practical means of applying overcurrent
protection, as well as affording compliance with the National
Electrical Code®. This Buss® bulletin is helpful to engineers,
contractors, electricians, plant maintenance personnel, and
electrical inspectors. It also should prove to be a valuable
training aid for formal and informal instruction.
Free Training at www.bussmann.com
Bussmann® offers training modules that complement this NE02
bulletin. Use these learning aids with the NE02 Bulletin.
(1) On-Line Narrated Presentations
Narrated presentations for some key NEC® sections can
be played on-line over the Internet. The audio narration
plays on your computer speakers while the graphic
presentation displays on your computer monitor.
(2) Downloadable PowerPoint® Presentations
PowerPoint® presentations with scripts that can be
downloaded to your computer and shown using
PowerPoint®. You can use these presentations for your
own individual learning or presenters/trainers can use
them for group sessions.
National Electrical Code® and NEC® are registered trademarks of the National Fire ProtectionAssociation (NFPA), Inc., Quincy, MA 02269. This bulletin does not reflect the official position ofthe NFPA.
Great care has been taken to assure the recommendations herein are in accordance with the NEC®
and sound engineering principles. Bussmann® cannot take responsibility for errors or omissions thatmay exist. The responsibility for compliance with the regulatory standards lies with the user.
Copyright March 2002 by Cooper Bussmann, Inc.
Printed U.S.A.
3
ContentsPage
90.2 Scope of the NEC® ………………………………………………………………………………………………5110.3(A)(5), (6) and (8) Requirements for Equipment Selection …………………………………………………………………………5110.3(B) Requirements for Proper Installation of Listed and Labeled Equipment ……………………………………5110.9 Requirements for Proper Interrupting Rating of Overcurrent Protective Devices …………………………6110.10 Proper Protection of System Components from Short-Circuits ……………………………………………10110.16 Flash Protection Field Marking …………………………………………………………………………………14110.22 Field Marking of Series Combination Ratings…………………………………………………………………16210.20(A) and 215.3 Ratings of Overcurrent Devices on Branch Circuits and Feeders Serving Continuous and
Non-Continuous Loads …………………………………………………………………………………………17215.10 Requirements for Ground-Fault Protection of Equipment on Feeders ……………………………………17230.82 Equipment Allowed to be Connected on the Line Side of the Service Disconnect ………………………18230.95 Ground Fault Protection for Services …………………………………………………………………………18240.1 Scope of Article 240 on Overcurrent Protection………………………………………………………………19240.2 Definitions: Coordination, Current-limiting Overcurrent Protective Device, and Tap Conductors ………21240.4 Protection of Conductors Other Than Flexible Cords and Fixture Wires …………………………………22240.5 Protection Flexible Cords, Fixture Cables and Fixture Wires ………………………………………………22240.6 Standard Ampere Ratings ………………………………………………………………………………………22240.8 Protective Devices Used in Parallel and 404.17 Fused Switches …………………………………………23240.9 Thermal Devices …………………………………………………………………………………………………23240.10 Requirements for Supplementary Overcurrent Protection …………………………………………………23240.12 System Coordination or Selectivity ……………………………………………………………………………24240.13 Ground Fault Protection of Equipment on Buildings or Remote Structures ………………………………25240.21 Location Requirements for Overcurrent Devices and Tap Conductors ……………………………………25240.40 Disconnecting Means for Fuses ………………………………………………………………………………29240.50 Plug Fuses, Fuseholders, and Adapters ………………………………………………………………………30240.51 Edison-Base Fuses ………………………………………………………………………………………………30240.53 and254 Type S Fuses, Adapters and Fuseholders ……………………………………………………………………30240.60 Cartridge Fuses and Fuseholders ……………………………………………………………………………31240.61 Classification of Fuses and Fuseholders ……………………………………………………………………31240.83 Circuit Breaker – Markings………………………………………………………………………………………31240.85 Clarifies Requirements for the Use of Slash-Rated Circuit Breakers and
Application of Individual Pole Interrupting Capabilities for Various Grounding Schemes ………………32240.86 Series Ratings ……………………………………………………………………………………………………37240.90 and 240.2 Supervised Industrial Installations ……………………………………………………………………………39240.92(B) Transformer Secondary Conductors of Separately Derived Systems
(Supervised Industrial Installations only)………………………………………………………………………39240.92(B)(1) Short-Circuit and Ground-Fault Protection (Supervised Industrial Installations only) ……………………39240.92(B)(2) Overload Protection (Supervised Industrial Installations only) ……………………………………………39240.92(C) Outside Feeder Taps (Supervised Industrial Installations only) ……………………………………………40240.100 Feeder and Branch Circuit Protection Over 600 Volts Nominal ……………………………………………40240.100(B) Protective Devices ………………………………………………………………………………………………40240.100(C) Conductor Protection ……………………………………………………………………………………………40250 Grounding …………………………………………………………………………………………………………41250.2 Definitions (Grounding) …………………………………………………………………………………………41250.4(A)(4) & (5) General Requirements for Grounded Systems ………………………………………………………………41250.4(B)(4) General Requirements for Ungrounded Systems ……………………………………………………………42250.90 Bonding Requirements and Short-Circuit Current Rating……………………………………………………42250.96(A) Bonding Other Enclosures and Short-Circuit Current Requirements ………………………………………42250.122 Sizing of Equipment Grounding Conductors …………………………………………………………………43310.10 Temperature Limitation of Conductors …………………………………………………………………………44368.11 and 368.12 Busway Reduction and Feeders or Branch Circuits …………………………………………………………44408.16 Panelboard Overcurrent Protection ……………………………………………………………………………44430.1 Scope of Motor Article …………………………………………………………………………………………44430.6 Ampacity of Conductors for Motor Branch Circuits and Feeders …………………………………………44430.8 Marking on Controllers …………………………………………………………………………………………45
Contents (continued)Page
430.32 Motor Overload Protection ……………………………………………………………………………………45430.36 Fuses Used to Provide Overload and Single-Phasing Protection …………………………………………45430.52 Sizing of Various Overcurrent Devices for Motor Branch Circuit Protection ………………………………46430.53 Connecting Several Motors or Loads on One Branch Circuit ………………………………………………47430.62 and430.63 Sizing Fuses for Feeders with Motor Loads …………………………………………………………………47430.71 Motor Control-Circuit Protection ………………………………………………………………………………48430.72(A) Motor Control-Circuit Overcurrent Protection …………………………………………………………………48430.72(B) Motor Control-Circuit Conductor Protection …………………………………………………………………48430.72(C) Motor Control-Circuit Transformer Protection …………………………………………………………………49430.83(E) Requirements for Controllers with Slash Voltage Ratings …………………………………………………50430.94 Motor Control Center Protection ………………………………………………………………………………50430.102 Requirements For Disconnecting Means Within Sight Of Motors …………………………………………51430.109(A)(6) Manual Motor Controller as a Motor Disconnect ……………………………………………………………52440.5 Marking Requirements on HVAC Controllers …………………………………………………………………52440.22 Application and Selection of the Branch Circuit Protection for HVAC Equipment ………………………52450.3 Protection Requirements for Transformers ……………………………………………………………………52450.3(A) Protection Requirements for Transformers Over 600 Volts …………………………………………………53450.3(B) Protection Requirements for Transformers 600 Volts or Less ………………………………………………54450.6(A)(3) Tie Circuit Protection ……………………………………………………………………………………………54455.7 Overcurrent Protection Requirements for Phase Converters ………………………………………………54460.8(B) Overcurrent Protection of Capacitors …………………………………………………………………………54501.6(B) Fuses for Class I, Division 2 Locations ………………………………………………………………………55517.17 Requirements for Ground Fault Protection and Coordination in Health Care Facilities …………………55520.53(F)(2) Protection of Portable Switchboards on Stage ………………………………………………………………55550.6(B) Overcurrent Protection Requirements for Mobile Homes and Parks ………………………………………56610.14(C) Conductor Sizes and Protection for Cranes and Hoists ……………………………………………………56Article 620 Elevators, Dumbwaiters, Escalators, Moving Walks, Wheelchair Lifts, and Stairway Chair Lifts ………56620.51 Disconnecting Means (Elevators) ……………………………………………………………………………57620.61 Overcurrent Protection (Elevators) ……………………………………………………………………………57620.62 Selective Coordination (Elevators) ……………………………………………………………………………58620.91 Emergency and Standby Power Systems (C)Disconnecting Means (Elevators) …………………………58670.3 Industrial Machinery ……………………………………………………………………………………………59700.5 Emergency Systems – Their Capacity and Rating …………………………………………………………60700.16 Emergency Illumination …………………………………………………………………………………………60700.25 Emergency System Overcurrent Protection Requirements (FPN) …………………………………………60701.6 Legally Required Standby Systems – Capacity and Rating ………………………………………………61702.5 Optional Standby Systems – Capacity and Rating …………………………………………………………61705.16 Interconnected Electric Power Production Sources – Interrupting and Short-Circuit Current Rating …61725.23 Overcurrent Protection for Class 1 Circuits …………………………………………………………………61760.23 Requirements for Nonpower-Limited Fire Alarm Signaling Circuits ………………………………………61
Form Series Rating Inspection Form …………………………………………………………………………62 & 63
4
5
90.2 Scope of the NEC®
What does this Section mean?90.2(B) covers installations that are not covered by requirementsof the NEC®. However, the fine print note states that it is the intent
of this section that utility installed utilization equipment located onprivate property is subject to the National Electrical Code®.
110.3(A)(5), (6) and (8) Requirements for Equipment SelectionWhat does 110.3(A)(5), (6) and (8) require?When equipment is selected, its arc-flash protection capability andfinger-safe rating must be evaluated. When equipment isenergized, and the door is open, the possibility exists that an
employee could accidentally create an arcing fault or come intocontact with a live part. Equipment must be evaluated for bothpossibilities, and be chosen for minimum employee exposure toeither danger. See discussion on 110.16.
110.3(B) Requirements for Proper Installation of Listed and Labeled EquipmentWhat is the importance of Section 110.3(B)?Equipment that is listed is subject to specific conditions ofinstallation or operation. The conditions must be followed for safeand proper operation.
What is the protection requirement of an air conditioner when its nameplate specifies Maximum Fuse Size AMPS?
Fuse protection in the branch circuit is mandatory to meet therequirements of the U.L. Listings and the National ElectricalCode®.
Note that the U.L. Orange Book “Electrical Appliance andUtilization Equipment Directory,” April 2000, requires the followingfor heating and cooling equipment: “Such multimotor andcombination load equipment is to be connected only to a circuitprotected by fuses or a circuit breaker with a rating which does notexceed the value marked on the data plate. This marked protectivedevice rating is the maximum for which the equipment has beeninvestigated and found acceptable. Where the marking specifiesfuses, or “HACR Type” circuit breakers, the circuit is intended tobe protected only by the type of protective device specified.” U.L.Standard 1995 also covers this subject.
What about a motor starter heater table (such as that shown below)which specifies Maximum Fuse?
Heater Full-Load Current Max.Code of Motor (Amperes) FuseMarking (40°C Ambient)XX03 .25- .27 1XX04 .28- .31 3XX05 .32- .34 3XX06 .35- .38 3
Like an air conditioner, use of fuse protection is mandatory. Also,the fuse must provide branch circuit protection and be no largerthan the specified size [430.53(C)]. The chart shown, for example,is typical for starter manufacturers and may be found on the insideof the door of the starter enclosure. (See starter manufacturer forspecific recommendations.)
8RY461M3-A
230230
3760
207
-—-—6060
140
Typical Nameplate of a Central Air Conditioning Unit.
LISTED SECTION OF CENTRAL COOLING AIR CONDITIONER
ADME
812H
COMPRESSOR
FAN MOTOR
MINIMUM CIRCUIT AMPACITY
MAXIMUM FUSE SIZE AMPS
MINIMUM OPERATING VOLTAGE
FACTORY CHARGED WITH REFRIGERATORSEE CONTROL PANEL COVER FOR AOF SYSTEM REFRIGERANT
*COMPRESSOR RATED IN RLA
ELECTRICAL RATINGSVAC PH CYC LRA
FOR OUTDOOR USE
UL TYPE NO.®
CIRCUITBREAKERBRANCHCIRCUIT NON-FUSED
DISCONNECT
AIR CONDITIONERMARKED WITH"MAX" FUSE
Violates NEC® & Listing Requirements
AIR CONDITIONERMARKED WITH"MAX" FUSE
BRANCHCIRCUITFUSED DISCONNECT
FUSED FEEDERCIRCUIT
Conforms to NEC® & Listing Requirements
AIR CONDITIONERMARKED WITH"MAX" FUSE
NON-FUSEDDISCONNECT
FUSED BRANCHCIRCUIT
AIR CONDITIONERMARKED WITH"MAX" FUSE
BRANCHCIRCUITFUSED DISCONNECT
CIRCUITBREAKER
Conforms to NEC® & Listing Requirements
Conforms to NEC® & Listing Requirements
6
110.3(B) Requirements for Proper Installation of Listed and Labeled EquipmentWhat violation exists when a “series-rated” panelboard with a “42/10”system rating has the potential to see a fault current less than 4 ft. fromthe loadside circuit breaker?
U.L. 489 Series Rating tests allow a maximum of 4 ft. of rated wireto be connected to the branch circuit breaker. Whenever thepotential for a fault exists closer than 4 ft. from the circuit breaker,i.e., where the 12 AWG wire leaves the enclosure, or amaintenance man is working on the equipment “hot”, a violation of110.3(B) exists, as does a potentially hazardous condition. In thissituation, the interrupting ability of the finely tuned and testedcombination is compromised and a safety hazard may result.
110.9 Requirements for Proper Interrupting Rating of Overcurrent ProtectiveDevicesWhat is the importance of Section 110.9?Equipment designed to break fault or operating currents musthave a rating sufficient to withstand such currents. This articleemphasizes the difference between clearing fault level currentsand clearing operating currents. Protective devices such as fusesand circuit breakers are designed to clear fault currents and,therefore, must have short-circuit interrupting ratings sufficient forfault levels. Equipment such as contactors and switches haveinterrupting ratings for currents at other than fault levels. Thus, theinterrupting rating of electrical equipment is divided into two parts.
Note: Breaking current at other than fault levels.The rating of contactors, motor starters, switches, circuit breakersand other devices for closing in and/or disconnecting loads atoperating current levels must be sufficient for the current to beinterrupted, including inrush currents of transformers, tungstenlamps, capacitors, etc. In addition to handling the full-load currentof a motor, a switch and motor starter must also be capable ofhandling its locked rotor current. If the switch or motor starter hasa horsepower rating at least as great as that of the motor, they willadequately disconnect even the locked rotor current of the motor.
Most people are familiar with the normal current carrying ampere ratingof a fuse or circuit breaker; however, what is a short-circuit interruptingrating?It is the maximum short-circuit current that an overcurrentprotective device can safely interrupt under specified testconditions.
What is a device’s interrupting capacity?The following definition of Interrupting Capacity is from the
IEEE Standard Dictionary of Electrical and Electronic Terms:Interrupting Capacity: The highest current at rated voltage that
the device can interrupt. Because of the way fuses are short-circuit tested (without
additional cable impedance), their interrupting capacity is greaterthan or equal to their interrupting rating. Because of the way circuitbreakers are short circuit tested (with additional cableimpedance), their interrupting capacity can be less than, equal to,or greater than their interrupting rating.
What happens if a fault current exceeds the interrupting rating of a fuseor the interrupting capacity of a circuit breaker?It can be damaged or destroyed. Severe equipment damage andpersonnel injury can result.
In this circuit, what interrupting rating must the fuse have?
At least 50,000 amperes. (Class R, J, T, L and CC fuses have anInterrupting Rating of at least 200,000 amperes. The interruptingrating of a fuse and switch combination may also be 200,000amperes. . .well above the available short-circuit current of 50,000amperes. The interrupting rating of Class G fuses is 100,000amperes; K1 and K5 fuses can be 50,000, 100,000, or 200,000amperes.)
In this circuit, what interrupting rating must the circuit breaker have?
Some value greater than or equal to 50,000 amperes. Seediscussion “Molded Case Circuit Breakers–UL 489 and CSA TestProcedures” later in this discussion of 110.9 for further evaluation.(Faults within four feet of the breaker could cause completedestruction of the breaker if it is applied where the available faultcurrent approaches the tested interrupting capacity of thebreaker.)
Section 110.9 also requires the overcurrent device to have asufficient interrupting rating for both phase-to-phase voltage andphase-to-ground voltage.
What is the significance of this requirement?Molded case circuit breakers typically have lower single-poleinterrupting capabilities than their multi-pole interrupting rating.See discussion for section 240.85 on slash ratings and single poleinterrupting capabilities.
How does one know in practical applications if an overcurrent protectivedevice’s interrupting rating is sufficient? It is necessary to use tables or calculate the maximum short circuitcurrent that is available at the line side of each overcurrentprotective device. Then select an overcurrent protective devicethat has an interrupting rating equal to or greater than themaximum short circuit current. Modern current-limiting branchcircuit fuses have interrupting ratings typically of 200,000 or300,000 amperes, which is sufficient for most applications.
#12 CuWIRE
10KA.I.R.20A CB's
200A Panelboard
Branch Circuit
Fault <4' from BranchCircuit Breaker
200A42KA.I.R
.
40,000 Amperes Available
Available fault current–50,000 amperes
Available fault current–50,000 amperes
7
110.9 Requirements for Proper Interrupting Rating of Overcurrent ProtectiveDevicesHow does Section 110.9 pertain to services?Service equipment must be able to withstand available short-circuit currents. More specifically, the service switchboard,panelboard, etc., and the protective devices which theyincorporate must have a short-circuit rating equal to or greaterthan the short-circuit current available at the line side of theequipment.
In this circuit, what must be the short-circuit rating of the switchboard?
Answer: at least 100,000 amperes.
What must be the interrupting rating of the fuses?100,000 amperes or greater. (Most current-limiting fuses have aninterrupting rating of 200,000 or 300,000 amperes.)
In this circuit, what must be the interrupting capacity of the main circuitbreaker, and the short-circuit rating of the switchboard?
Answer: at least 100,000 amperes.
As shown in the circuit, can fuses be used to protect circuit breakers witha low interrupting rating?
Yes. Properly selected fuses can protect circuit breakers as wellas branch circuit conductors by limiting short-circuit currents to alow level even though available short-circuit current is as high as100,000 amperes. (Buss® LOW-PEAK® YELLOW™ or T-TRON®
fuses give optimum protection.) See the discussion on seriesratings in 240.86 of this booklet.
Application Note:Residential—100 ampere and 200 ampere fused main-branchcircuit breaker panels are commercially available. These loadcenters incorporate the small-sized T-TRON® JJN fuses whichmake it possible to obtain a 100,000 amperes short-circuit currentrating. Mobile home meter pedestals are also availableincorporating the T-TRON® JJN fuses in a Fuse Pullout Unit.
Apartment Complexes—Have high densities of current and,therefore, high short-circuit currents for the typical meters.
Grouped meter stacks are commercially available using the T-TRON® JJN fuses (up to 1200 amperes) to give the proper short-circuit protection. Meter stacks are also available with Class T fusepullouts on the load side of each meter.
METERS
METERS
JJN FUSE(up to 1200A)
JJN FUSE(up to 1200A)
CLASS T FUSES
100,000Aavailablefault current
Fuses must have100,000 amperesinterrupting ratingor greater
100,000Aavailablefault current MAIN
BREAKER
SWITCHBOARD
FEEDER CIRCUIT BREAKERS
100,000Aavailablefault current
200 ampere service entrance panelmust have a short circuit ratingequal to or greater than 100,000 amperes
10,000A.I.C.breakers
For other important related discussions see sections 240.85slash ratings, 240.85 single pole interrupting and 240.86 seriesratings.
8
110.9 Requirements for Proper Interrupting Rating of Overcurrent ProtectiveDevicesDoes an overcurrent protective device with a high interrupting ratingassure circuit component protection?No. Choosing overcurrent protective devices strictly on the basisof voltage, current, and interrupting rating alone will not assurecomponent protection from short-circuit currents. High interruptingcapacity electro-mechanical overcurrent protective devices,(circuit breakers) especially those that are not current-limiting, maynot be capable of protecting wire, cable, starters, or othercomponents within the higher short-circuit ranges. See discussionof Sections 110.10 and 240.1 for the requirements that overcurrentprotective devices must meet to protect components such asmotor starters, contactors, relays, switches, conductors, and busstructures.
Calculating Short-Circuit Current
It is necessary to calculate available short-circuit currents at variouspoints in a system to determine whether the equipment meets therequirements of Sections 110.9 and 110.10. How does one calculate thevalues of short-circuit currents at various points throughout a distributionsystem?There are a number of methods. Some give approximate values;some require extensive computations and are quite exacting. Asimple, usually adequate method is the Buss® Point-To-Pointprocedure presented in Buss® bulletin SPD, Selecting ProtectiveDevices. A program using the Buss® Point-to-Point procedure canbe found on www.bussmann.com and on the Bussmann® ReadyReference CD. The point-to-point method is based on computationof the two main circuit impedance parameters: transformers andcables. Of these two components, the transformer is generally themajor short-circuit current factor for faults near the serviceentrance. The percent impedance of the transformer can varyconsiderably. Thus, the transformer specification should always bechecked. As shown in the illustration of a typical transformernameplate, “%” impedance is specifically designated.
Given the full-load transformer secondary amperage and percentimpedance of a transformer, how can you compute the level of short-circuit amperes that can be delivered at the secondary terminals(Assuming an infinite, unlimited, short-circuit current at the primary)?
ISCA = (F.L.A.) x 100 %Z x .9††
Given: 1.3% impedance from nameplate of 500 KVA transformerwith a 480V secondary
601 Full-Load Amperes (from Table)
ISCA = 601 x 100 = 51,368 Amperes1.3 x .9
What are typical values of transformer short-circuit currents?
Short-Circuit Currents Available from Various Size TransformersVoltage+ KVA Full- % † Short-and Load Impedance †† CircuitPhase Amperes (Name plate) Amperes
† Three-phase short-circuit currents based on "infinite" primary.* Single-phase values are L-N values at transformer terminals. These figures are based
on change in turns ratio between primary and secondary, 100,000 KVA primary, zerofeet from terminals of transformer, 1.2 (%X) and 1.5 (%R) multipliers for L-N vs. L-Lreactance and resistance values, and transformer X/R ratio = 3.
†† U.L. listed transformers 25KVA or greater have a ±10% impedance tolerance. “Short-Circuit Amperes” reflect a worst case scenario.
+ Fluctuations in system voltage will affect the available short-circuit current. Forexample, a 10% increase in system voltage will result in a 10% increase in theavailable short-circuit currents shown in the table.
H2
H1 H3X1 X3
X2H0X0
0° ANGULAR DISP.
X3X2X1H0X0H1H2H3
%Zor
PercentageImpedance
COOPERPower Systems Division
THREE PHASE
VOLTAGE
RATING
CATNO%IMP
TRANSFORMER 60 HERTZ65°CRISE
BIL-KVFULL-WAVE
WDG.MAT LV HV
GALOIL
CLASSOA
LV ENCLOSURE LBS.
KVA 50012470GRD. Y/7200 480Y/277
PCWN 416124-500-L1
LBS. TOTAL
LBS. OIL
LBS.
LBS.
TANK & FITTINGS
UNTANKING
1.3HV
SER.
9
Molded Case Circuit Breakers—U.L. 489 and CSA 5 Test ProceduresU.L. 489 requires a unique test set-up for testing circuit breakerinterrupting ratings. Figure F illustrates a typical calibrated testcircuit waveform for a 20 ampere, 240 volt, 2-pole molded casecircuit breaker, with a marked interrupting rating of 22,000amperes, RMS symmetrical.
Figure F
Figure G illustrates the test circuit as allowed by U.L. 489.
Figure G
Standard interrupting rating tests will allow for a maximum 4 ft.rated wire on the line side, and 10 in. rated wire on the load side ofthe circuit breaker. Performing a short-circuit analysis of this testcircuit results in the following short-circuit parameters, as seen bythe circuit breaker.
• Actual short-circuit RMS current = 9900 amperes RMS symmetrical
• Actual short-circuit power factor = 88%• Actual short-circuit peak current = 14,001 amperes
Conclusion:
This 14,000 ampere (with short-circuit power factor of 20%)interrupting rated circuit breaker has an interrupting capacity of9,900 amperes at a short-circuit power factor of 88%. Unless thereis a guarantee that no fault will ever occur at less than 4'10" fromthe load terminals of the circuit breaker, this circuit breaker mustonly be applied where there are 9,900 amperes or less availableon its line side.
S.C. P.F. = 20%S.C. Avail. = 22,000A
RLINE XLINERCB XCB
20A
RLOAD XLOADRS
XS
SOURCE: 4' Rated Wire (12 AWG Cu)
Note: For calculations, RCB and XCB are assumed negligible.
10" Rated Wire (12 AWG Cu)
P.F. = 20%IRMS = 22,000 Amps
IRMS = 22,000A
Ip = 48,026A
Time
Am
ps
Following is an example of a partial table showing the actual IP andIRMS values to which circuit breakers are tested.
240V–2-Pole MCCB INTERRUPTING CAPACITIES (KA)CB 10KA 14KA 18KA 22KARATING Ip Irms Ip Irms Ip Irms Ip Irms
These values are known as the circuit breaker’s interruptingcapacities.
What about the “bus shot” tests? Won’t those prove that circuit breakerscan safely and properly interrupt their marked interrupting rating?No! Beginning 10/31/2000, UL 489 requires circuit breakers rated100A and less to additionally be tested under “bus bar conditions.”In this test, line and load terminals are connected to 10" of ratedconductor. For single pole circuit breakers, these 10" leads arethen connected to 4' of 1 AWG for connection to the test station.For multipole circuit breakers, the 10" line side leads are connectedto the test station through 4' of 1 AWG. The load side is shorted by10" leads of rated conductor per pole. These “bus shots” still donot fully address the situation where a fault can occur less than4'10" from the circuit breaker.
For example, 7.1.11.6.3.1 of UL 489 states “The inability to relatch,reclose, or otherwise reestablish continuity … shall be consideredacceptable for circuit breakers which are tested under “bus barconditions.” This says the circuit breaker doesn’t have to work aftera close-in fault occurs, and is in violation of the 2002 NEC®
requirement for a circuit breaker which is found in the definition.
The NEC® defines a circuit breaker as:
A device designed to open and close a circuit by nonautomaticmeans and to open the circuit automatically on a predeterminedovercurrent without damage to itself when properly applied withinits rating.
110.9 Requirements for Proper Interrupting Rating of Overcurrent ProtectiveDevices
10
110.10 Proper Protection of System Components from Short-CircuitsWhat is the importance of Section 110.10?The design of a system must be such that short-circuit currentscannot exceed the short-circuit current ratings of the componentsselected as part of the system. Given specific system componentsand level of “available” short-circuit currents which could occur,overcurrent protective devices (mainly fuses and/or circuitbreakers) must be used which will limit the energy let-through offault currents to levels within the short-circuit current ratings of thesystem components. (Current-limitation is treated under 240.2 ofthis bulletin). The last sentence of Section 110.10 emphasizes therequirement to thoroughly review the product standards and toapply components within the short-circuit current ratings in thosestandards.What is component short-circuit current rating?It is a current rating given to conductors, switches, circuit breakersand other electrical components, which, if exceeded by faultcurrents, may result in “extensive” damage to the component. Therating is expressed in terms of time intervals and/or current values.Short-circuit damage can be heat generated or the result ofelectro-mechanical force of high-intensity, magnetic fields.
Conductor Protection
How is the component withstand rating of conductors expressed?As shown in the table below, component withstand of conductorsis expressed in terms of maximum short-circuit current vs. cycles(or time). See discussion in this booklet on 240.1 which providesmore information on conductor short-circuit current withstand.
Table—Copper, 75° Thermoplastic Insulated Cable Damage Table*(Based on 60 HZ).Copper Maximum Short-Circuit Withstand CurrentWire Size (AWG) in Amperes75° For For For ForThermoplastic 1/2 Cycle** 1 Cycle 2 Cycles 3 Cycles**18 900** 700** 500** 400**16 1500** 1100** 700** 600**14 2,400 1,700** 1,200** 1,00012 3,800 2,700** 1,900** 1,55010 6,020 4,300 3,000 2,4508 9,600 6,800 4,800 3,9006 15,200 10,800 7,600 6,2004 24,200 17,100 12,100 9,900Footnotes—*Reprinted from ICEA. **From ICEA formula
In this circuit, what is the maximum permissible available short-circuitcurrent?
2700 amperes. Since the protective device is not current-limiting,the short-circuit current must not exceed the one cycle withstandof the 12 AWG conductor, or 2700 amperes.
In this 20 ampere circuit with a non-current-limiting protective device,what would be the smallest size conductor that would have to be used?
4 AWG wire. Since the protective device is not current-limiting, thewire selected must withstand 12,000 amperes for one cycle.
In this circuit, what type of protective device must be used?
It must be current-limiting. When the available short-circuit currentexceeds the short-circuit current rating of the wire, a protectivedevice such as a current-limiting fuse, properly selected, will limitfault current to a level lower than the wire short-circuit currentrating (3,800 amperes for 1/ 2 cycle). (See Section 240.1 FPN.) Forinstance, a LOW-PEAK® YELLOW™ LPN-RK20SP fuse will limit the12,000 amperes available short-circuit to less than 1000 amperesand clear in less than 1/ 2 cycle.
PROTECTIVEDEVICE
2' #12 Cu12,000Aavailablefault current
Short-Circuit
PROTECTIVEDEVICE(20A, 1 cycle opening time;not current limiting)
110.10 Proper Protection of System Components from Short-CircuitsConductor Short-Circuit Protection Using Fuse "Worst Case" Let-Thru Values Modern current limiting fuses provide excellent protection ofconductors. In some applications, conductors are permitted to beprotected by overcurrent protective devices that are sized greaterthan the conductor ampacity. Examples are control circuitconductors and equipment grounding conductors. In theseconditions, conductor protection analysis is even more critical. Also,the smaller the wire size, the more difficult it is to provide properprotection; especially for 14 AWG and smaller. The reason is that theshort circuit withstand for these smaller conductors is extremely low.
The table below illustrates the inherent protection ability of currentlimiting fuses. This table shows, for short circuit conditions only, thesmallest conductor that is protected by specific class and ampererated fuses. The conductors are copper with 75 degree thermoplasticinsulation. The conductor withstand is based on the Insulated CableEngineers Association, Inc. insulated cable damage charts inPublication 32-382. The fuse protection criterion is based on themaximum clearing I2t limits permitted in UL/CSA/ANCE 248, the Tri-national Standards for fuses. These fuse I2t limits are establishedunder worst case test conditions at the fuse rated voltage. It shouldbe noted that commercially available fuses have I2t let-thru valuesless than these limits. Also, in most applications, the fuses are appliedin systems where the voltage is less than the fuse rating and the shortcircuit conditions (actual short circuit current, short circuit powerfactor and point on voltage wave where short is initiated) may notapproach the worst case test conditions required in UL/CSA/ANCE248. This means that using commercially available fuses in practicalapplications, the actual fuse I2t let through values will be considerablyless than the maximum limits established in the standard. So specificfuses may be able to provide short circuit protection to even smallerconductor sizes than shown in these tables.
Smallest Conductor Size Protected By Specific Fuse for Short-CircuitProtection OnlyBased on: (1) ICEA Publication 32-382 for Conductor Withstand,(2) Copper Conductors, 75 Degree Thermoplastic Insulation, (3)Fuse Maximum Clearing I2t limits allowed per UL/CSA/ANCE 248Fuse Standards, (4) 100,000 RMS Amperes Short-Circuit Current
Protection of Motor Controllers, Contacts and Relays
In this circuit, what kind of fuse must be used to provide adequateprotection of the starter?
A current-l imit ing fuse, such as the Buss® LOW-PEAK®
YELLOW™ or FUSETRON® dual-element fuse. Such a fuse mustlimit fault currents to a value below the withstand rating of thestarter and clear the fault in less than 1/ 2 cycle.
25,000Aavailablefault current
Size 1 Starter(Tested by UL with 5000A available)
Short-Circuit
12
110.10 Proper Protection of System Components from Short-CircuitsWhat is Type 2, motor starter protection?UL 508E and IEC 947-4-1 have test procedures designed to verifythat motor controllers will not be a safety hazard and will not causea fire.
These standards offer guidance in evaluating the level ofdamage likely to occur during a short-circuit with various branchcircuit protective devices. They address the fault protectioncoordination between the branch circuit protective device and themotor starter. They also provide a method to measure theperformance of these devices should a short-circuit occur. Theydefine two levels of protection (coordination) for the motor starter:
Type 1. Considerable damage to the contactor and overloadrelay is acceptable. Replacement of components or a completelynew starter may be needed. There must be no discharge of partsbeyond the enclosure (door closed).
Type 2. No damage is allowed to either the contactor oroverload relay. Light contact welding is allowed, but must beeasily separable. Also, the overload protection must retaincalibration.
Where Type 2 protection is desired, the controller manufacturermust verify that Type 2 protection can be achieved by using aspecified protective device. Many U.S. manufacturers have boththeir NEMA and IEC motor controllers verified to meet the Type 2requirements. Only current-limiting devices have been able toprovide the current-limitation necessary to provide verified Type 2protection. In many cases, Class J, Class RK1, or Class CC fusesare required, because most Class RK5 fuses and circuit breakersaren't fast enough under short-circuit conditions to provide Type 2protection.
Type 2 protection is defined and suggested in the notes toTable 1 of NFPA 79 (Electrical Standard for Industrial Machinery).
Type 2 Starter Protection TablesBussmann® publishes tables by starter manufacturers of the fusetype and size that provide Type 2 protection. Visit the Application/ Publications section of www.bussmann.com for these tables bystarter manufacturer. Table below illustrates part of one of thesetables.
Protection of Circuit Breakers
There are several key concepts about the protection of circuitbreakers that need to be understood.
1. The user should be aware of the potential problemsassociated with series-rated circuit breakers. The engineercan not always "engineer" the installation as beforebecause,
2. A molded case circuit breaker's interrupting capacity maybe substantially less than its interrupting rating, seediscussion in 110.9 of booklet and
3. Some molded case circuit breakers exhibit "dynamic"operation that begins in less than 1/ 2 cycle. This makesthem more difficult to protect than other static electricalcircuit components.
The most practical and reliable solution is to specify a fully-rated fusible system.
Series Rated Systems for Circuit Breaker ProtectionIf the available short-circuit current exceeds a circuit breaker’sinterrupting rating, it may be possible to use a series rated systemwhere current-limiting fuses are utilized to protect the circuitbreakers in specific panelboards and switchboards. Seediscussion in this booklet under 240.86 Series Ratings and 110.22Series Rating Labeling Requirements.
MOTOR CONTROLLER AND FUSE SELECTION TABLE FOR TYPE 2 PROTECTION (UL & CSA VERIFIED)
THREE PHASE MOTOR HORSEPOWER200V(FLC) 230V(FLC) 460V(FLC) 575V(FLC)
STARTERSIZE CATALOG # HEATER
MAXIMUM
LPN/LPS LPJ LP-CCCLASS RK1 CLASS J CLASS CC
Example of Type 2 Starter/Fuse selection table (partial) by starter manufacturers.Complete tables for several controller manufactures available on www.bussmann.com.
13
110.10 Proper Protection of System Components from Short-Circuits
Protection of Bus Structures
NEMA Standards Publication No. BU1-1999 Busways recognizesthat current-limiting devices can be used to protect busway whenthe available short-circuit current exceeds the busway short-circuitcurrent rating. In BU1-1999 under 5.5 Short Circuit Current itreads “Busway may be used on circuits having available short-circuit currents greater than the 3 cycle rating of the busway ratingwhen properly coordinated with current-limiting devices.”
In the circuit below, what must be the busway short-circuit bracing?
100,000 amperes, because the overcurrent device is not current-limiting.
In this circuit, what would the busway short-circuit bracing have to be?
36,000 amperes (as shown in the Minimum Bracing Table). Withan available short-circuit current of 100,000 amperes, the LOW-PEAK® YELLOW™ KRP-C1600SP fuse will only let-through anequivalent of 36,000 amperes, RMS symmetrical.
Minimum Bracing Required for Bus Structures at 480V.(Amperes RMS Symmetrical)Rating*Busway Fuse Available Short-Circuit Amperes RMS Sym.
Transfers switches are utilized to transfer power from the normalsource to an emergency power source in order to feed anemergency system or critical loads.
The short-circuit current withstand rating of the transfer switchmust be equal to or greater than the available short-circuit currentat the location where the device is installed. The required ratingwill be the sum of the available short-circuit current from both thenormal and emergency sources if the transfer switch uses aclosed transition (parallels both sources). If the transfer switchuses an open transition, the larger fault current, from the normal oremergency source, will be used. Typically the larger fault currentwill be from the normal source.
These devices are typically tested per UL 1008 to meet basicshort-circuit testing requirements. In this testing, the short-circuitwithstand rating is typically dependent upon the type of upstreamovercurrent protective device. The table below, shows anexample of a typical manufacturers’ short-circuit withstand rating.Note that the transfer switch withstand rating is usuallysubstantially higher if current-limiting fuses are used upstream. Forspecific short-circuit current ratings of transfer switches basedupon the overcurrent protective device selected, the manufacturerof the transfer switch must be consulted.
Withstand Rating with Withstand Rating WithATS Size (Amps) Class J or L Fuse (Max Size) CB (Max Size)
Can cable limiters protect service entrance equipment from short-circuitcurrents?
Current-limiting cable limiters not only can be used to isolate a“faulted” service cable, but also can help to protect utility meterswith low withstand ratings against high short-circuit currents. (SeeSection 230.82).
It should be noted that while meter sockets have short-circuitcurrent ratings, the short-circuit current rating of the meter itself isnot typically included, resulting in a potentially significant safetyhazard.
CABLE LIMITER
UNDERGROUND CABLE(Residential and lightcommercial buildings)
METER
14
110.16 Flash Protection Field Marking
110.16 Flash Protection. Switchboards, panelboards, industrial control panels, and motorcontrol centers in other than dwelling occupancies, that are likelyto require examination, adjustment, servicing, or maintenancewhile energized, shall be field marked to warn qualified persons ofpotential electric arc flash hazards. The marking shall be locatedso as to be clearly visible to qualified persons before examination,adjustment, servicing, or maintenance of the equipment.
FPN No. 1: NFPA 70E-2000, Electrical Safety Requirements forEmployee Workplaces, provides assistance in determiningseverity of potential exposure, planning safe work practices,and selecting personal protective equipment.
FPN No. 2: ANSI Z535.4-1998, Product Safety Signs andLabels, provides guidelines for the design of safety signs andlabels for application to products.
Reprinted from NEC® 2002
Example of warning label – this label warns of both arc flash and shockhazards plus reminds workers to use proper PPE (Personal ProtectiveEquipment).
What is the importance of 110.16?This new requirement is intended to reduce the occurrence ofserious injury or death due to arcing faults to workers who work onor near energized electrical equipment. The warning label shouldremind a qualified worker who intends to open the equipment foranalysis or work that a serious hazard exists and that the workershould follow appropriate work practices and wear appropriatepersonal protection equipment (PPE) for the specific hazard (anon qualified worker must not be opening the equipment).
What is an arc flash hazard?An arcing fault is the flow of current through the air between phaseconductors or phase conductors and neutral or ground. An arcingfault can release tremendous amounts of energy at the point of thearc in a small fraction of a second. The result can be extremelyhigh temperatures, a tremendous pressure blast and shrapnel(equipment parts) hurling at high velocity (in excess of 700 milesper hour). An accidental slip of a tool or a loose part tumblingacross live parts can initiate an arcing fault in the equipment. If aperson is in the proximity of an arcing fault, the flash can causeserious injury or death.
Figure 1 shows sequential photos of one of many staged teststhat helped to understand and quantify the effects of arcing faultson workers. In this test, mannequins with temperature andpressure sensors were placed in the test cell. This was a 480 volt,three phase system with an available three phase short-circuitcurrent of 22,600 symmetrical rms amperes. A non current-limitingovercurrent protective device was the nearest upstream protectivedevice. An arcing fault was initiated in a combination motorcontroller enclosure. The arcing fault quickly escalated into a threephase arcing fault in the enclosure. The current flowed for 6cycles (1/10 second). The temperature recorders (with maximumtemperature l imit of 437°F) on the neck and hand of themannequin closest to the arcing fault were pegged (beyond 437°Flimit) (threshold for incurable burn for skin is 205°F for 1/10second). The pressure sensor on this mannequin’s chest pegged
the recorder at over 2160 lbs/ft2 (the threshold for severe lungdamage is 2160 lbs/ft2). This test and others are detailed in “StagedTests Increase Awareness of Arc-Fault Hazards in ElectricalEquipment”, IEEE Petroleum and Chemical Industry ConferenceRecord, September, 1997, pp. 313-322. This paper can be foundon www.bussmann.com under Services/Safety Basics. One findingof this IEEE paper is that current-limiting overcurrent protectivedevices reduce damage and arc-fault energy (provided the faultcurrent is within the current-limiting range).
What are the labeling requirements?The type of equipment specified in 110.16 that is likely to beworked on as described is required to have a field affixed arc flashwarning label. This will serve as a reminder to qualified workersthat a serious hazard exists, that they or their management mustassess the risk prior to approaching the hazard and that they mustfollow the work practices for the level of hazard they may beworking on or near.
110.16 only requires that this label state the existence of an arcflash hazard. It is suggested that the party responsible for the labelinclude more information on the specific parameters of the hazard.In this way the qualified worker and his/her management can morereadily assess the risk and better insure proper work practices,PPE and tools. The specific additional information that should beadded to the label includes:
Available Short- Circuit CurrentFlash Protection BoundaryIncident energy at 18 inches expressed in cal/cm2
This example label includes more of the vital information that fosters saferwork practices.
Are there restrictions on working on live equipment?OSHA regulations state in 1910.333 (a) that workers should notwork on live equipment (greater than 50 volts) except for one oftwo reasons (NFPA 70E–2000 Electrical Safety Requirements forEmployee Workplaces, Part II 2-1.1.1 states essentially the samerequirement): 1. Deenergizing introduces additional or increased hazards(such as cutting ventilation to a hazardous location) or 2. Infeasible due to equipment design or operationallimitations (such as when voltage testing is required fordiagnostics ).
However, when it is necessary to work on equipment "live", it isnecessary to follow safe work practices, which include assessingthe risks, wearing adequate personal protective equipment andusing the proper tools.
Until equipment is put into a “safe work condition”, (there areprocedural steps provided in NFPA 70E-2000 Part II 2-1.1.3), theequipment is considered to be “live”. One of the latter steps in thisprocedure is a voltage test of each phase conductor to verify theyare deenergized. The worker performing this voltage testing mustassume the equipment is live and therefore must wear appropriatePPE for the hazard assessed for the specific equipment and circuitparameters.
How is the flash hazard assessed?The arc flash hazard can be assessed prior to working onequipment. Knowing the available bolted short circuit current, theminimum sustainable arcing fault current, and the time duration forthe equipment supply overcurrent protective device to open, it ispossible to calculate the Flash Protection Boundary (FPB) andIncident Energy Exposure level. NFPA 70E provides the formulasfor this critical information as well as other important information onsafe work practices, appropriate personal protective equipmentand appropriate tools to use. A qualified worker should not enterthe flash protection boundary to work on live parts unless he/she iswearing the appropriate PPE for the level of hazard that couldoccur. Figure 2 depicts the flash protection boundary and the threeshock boundaries that shall be observed per NFPA 70E.
Figure 2 - Graphic illustrating the flash protection boundary and the threeshock protection boundaries. The flash protection boundary canbe greater than limited shock boundary.
How can the risks associated with this hazard be reduced?There are viable means to reduce the risks of the shock and flashhazards. Use finger safe products that will reduce the chance thata shock or arcing fault can occur. Use current-limiting fuses orcurrent-limiting circuit breakers. Current-limiting fuses or current-limiting circuit breakers can reduce the risks associated with arcflash hazards by limiting the magnitude of the fault currents(provided the fault current is within the current-limiting range) andreducing the time duration of the fault. Figure 3 is the same testsetup as shown in Figure 1 except that the arcing fault is clearedby 601 ampere current-limiting fuses. Consequently the arc flashwas greatly reduced.
Compare Figure 3 to Figure 1, which is the same test setup, butwith noncurrent-limiting protection (Figure 1) versus current-limitingprotection (Figure 3)!
Are there resources to learn more?To learn more about electrical hazards and safety requirementssee Bussmann® Safety Basics™ Handbook for Electrical Safety.Bussmann® also offers a trainers kit for electrical safety trainingwhich includes a video, handbook, electronic presentations andmore – order Safety Basics™ Kit Part # SBK from your localBussmann® distributor. For more information about Safety Basics™visit www.bussmann.com.
110.16 Flash Protection Field Marking
Limited Shock Boundary: Qualified or Unqualified Persons* * Only if accompanied by Qualified Person
Prohibited Shock Boundary: Qualified Persons Only. PPE as if direct contact with live part
Restricted Shock Boundary: Qualified Persons Only
Note: shock boundaries dependent on system voltage level
Flash Protection Boundary (FPB)Must wear appropriate PPEFPB dependent on fault level and time duration.
Equ
ipm
ent
16
110.16 Flash Protection Field Marking
110.22 Field Marking of Series Combination RatingsWhat are the labeling requirements for the installer?110.22 and 240.86(A) require marking when a series combinationrating is utilized. 110.22 places responsibility on the installer(electrical contractor) to field install labels on the equipmentenclosures which note the short-circuit rating of the seriescombination and call out the specific replacement overcurrentprotective devices to be utilized. If the upstream overcurrentprotective device protecting the downstream circuit breaker is in adifferent enclosure, then both enclosures need to have field-installed labels affixed. This field marking is critical to ensuring thatproper devices are installed as initially intended and in the future.It becomes absolutely necessary when replacement of fuses orcircuit breakers is needed; this field marking helps ensure that theoriginal system design is maintained. If the wrong replacementcircuit breaker is used on the loadside or lineside or the wrongfuse is used on the lineside, the series rating is no longer valid.This could result in a serious fire or safety hazard.
Are there other labeling requirements for series rated systems?See discussion in this book on 240.86(A) for additional seriesrated labeling requirements that are the responsibility of theequipment manufacturer. Those requirements are meant to ensurethat the switchboard, panelboard, or loadcenter is tested, listedand marked for use with the acceptable combination of devicesbeing utilized. Also refer to the section in this book on 110.16concerning field labeling for arc flash hazards
Short-circuit calculations must be performed at panellocations where series rated combinations systems are utilized.This is necessary to assure that the series combination rating issufficient for the short-circuit current available at the specificinstallation point.
Where is more information on series rated systems?For more information on series combination ratings and theavailable fuse / circuit breaker combinations, see the discussion inthis bulletin for 240.86 or visit series rated systems underApplication Information at www.bussmann.com.
Contractor Installed Label
CAUTIONSeries Rated Combination System
with LPJ-200SP fuses in MDP1Rated 100,000 Amperes
Replace with XXX Circuit Breakers Only
CAUTIONSeries Rated Combination System
with panel LDP1Rated 100,000 AmperesReplace with Bussmann LPJ-200SP Fuses Only
Panel LDP1
PanelMDP1
Contractor Installed Label
NRTL Listing of SeriesCombination Rating of100,000 amperes whenXXX Circuit BreakerProtected by Maximumof 400 A Class J Fuse
Panel Mfr’s Label
Field labeling requirement (110.22) and manufacturer’s labelingrequirement (240.86)
Figure 3 – Staged Test with Current-Limiting Protection (KRP-C 601 SP Fuses)
1 2 3
4 5 6
17
215.10 Requirements for Ground-Fault Protection of Equipment on FeedersWhat is the importance of this Section?
Equipment classified as a feeder disconnect, as shown in theseexamples, must have ground fault protection as specified in230.95.
G.F.P. is not required on feeder equipment when it is provided onthe supply side of the feeder (except for certain Health CareFacilities requirements, Article 517).
Additionally, the requirements of this section do not apply to firepumps or to a continuous industrial process where a nonorderlyshutdown will introduce additional or increased hazards.
See 230.95 for an in-depth discussion of Ground FaultProtection.
Ground fault protection without current-limitation may not protect systemcomponents. See Section 110.10.
VIOLATION
High VoltageService 4160V
480Y/277V
Feeder W/OG.F.P.
1000Aor Greater
COMPLIANCE
High VoltageService 4160V
480Y/277V
FeederProvidedw/G.F.P.
1000Aor Greater
COMPLIANCE Feeder of any ratingno G.F.P. Required(Except Per Article 517)
480Y/277V
G.F.P.
1000AorGreater
What is the importance of these Sections?The overcurrent protective device provided for branch circuits,must not be less than the total non-continuous load, plus 125% ofthe continuous load (defined as a load that continues for 3 hoursor more).
Rating not less than = [(10A) x 1.0] + [(8A) x 1.25]= 20A
EXAMPLE
The branch circuit rating shall not be less than 20 amperes.
An exception is given in 210.20(A) and 215.3 that reads:
Where the assembly, including the fuse protecting the branchcircuit(s), is listed for operation at 100% of its rating, the ampererating of the overcurrent device must not be less than the sum ofthe continuous load plus the non-continuous load.
Many bolted pressure switches are listed at 100% rating withClass L fuses.
Other requirements in 210.20 are:210.20(B) requires protection of conductors in accordance with240.4. Flexible cords and fixture wires are required to beprotected in accordance with 240.5.
210.20(C) requires the ampere rating of the fuse to not exceedthat specified in applicable NEC® Articles listed in 240.3 forspecific equipment. For instance, it references Article 430 formotors. Note: See 430.52 and Table 430.52 for maximum sizingof fuses for motor branch circuits.
Note for feeders: See 430.62 for increased sizing of feeders withall motor loads. See 430.63 for increased sizing of feeder withmotor loads and additional lighting and appliance loads. See430.94 for the sizing requirements feeders supplying motor controlcenters
210.20(A) and 215.3 Ratings of Overcurrent Devices on Branch Circuits andFeeders Serving Continuous and Non-Continuous Loads
20A Rating
Non-Continuous10A
Continuous Load8A
18
230.82 Equipment Allowed to be Connected on the Line Side of the Service Disconnect
230.95 Ground Fault Protection for Services
What are the advantages of using cable limiters on the supply side of theservice disconnect as permitted by 230.82(1)?Typical cable installations are shown in the illustration below. Thebenefits of cable limiters are several:1. The isolation of a faulted cable permits the convenientscheduling of repair service.2. Continuity of service is sustained even though one or morecables are faulted.3. The possibility of severe equipment damage or burn down as aresult of a fault is greatly reduced. (Typically, without cablelimiters, the circuit from the transformer to the service equipment isafforded little or no protection.).4. Their current-limiting feature can be used to provide protectionagainst high short-circuit currents for utility meters and providecompliance with 110.10.
COMMERCIAL/INDUSTRIAL SERVICE ENTRANCE(Multiple cables per phase)
RESIDENTIAL SERVICE ENTRANCE(Single cable per phase)
What do 230.82(6) and (7) mean?The control circuit for power operable service disconnectingmeans and ground fault protection must have a means fordisconnection and adequate overcurrent protection–interruptingrating and component protection.
Why is this important?An unprotected control circuit (without overcurrent protection)ahead of the service disconnecting means could incur anovercurrent that could cause a fault involving the serviceconductors on the lineside of the service disconnect. Then theprotection may be provided by only the overcurrent protectivedevices on the transformer primary which may be slow to respondto such a condition.
ServiceDisconnect
(Open) (Open)
Faulted cable isolated; only the cablelimiters in faulted cable open; othersremain in operation
OpenFaulted cable isolated; the otherservices continue in operationwithout being disturbed
RESIDENCES
#4
#3
#2
#1
What is the importance of this section?This means that 480Y/277 volt, solidly grounded “wye” onlyconnected service disconnects, 1000 amperes and larger, musthave ground fault protection in addition to conventional over-current protection. Ground fault protection, however, is notrequired on a fire pump or a service disconnect for a continuousprocess where its opening will increase hazards. All deltaconnected services are not required to have ground faultprotection. The maximum setting for the ground fault relay (orsensor) must be set to pick up ground-faults which are 1200amperes or more and actuate the main switch or circuit breaker todisconnect all phase conductors. A ground fault relay with adeliberate time delay characteristic of up to 3000 amperes for 1second can be used. (The use of such a relay greatly enhancessystem coordination and minimizes power outages).
Under short-circuit conditions, unlike current-limiting fuses,ground fault protection in itself will not limit the line-to-ground orphase-to-phase short-circuit current. When mechanical protectivedevices such as conventional circuit breakers are used withG.F.P., all of the available short-circuit current will flow to the pointof fault l imited only by circuit impedance. Therefore, it isrecommended that current-limiting overcurrent protective devicesbe used in conjunction with G.F.P. relays.
In this circuit, what protection does the fuse provide in addition to thatprovided by the ground fault equipment?
Current limitation under short-circuit conditions and high-levelground-faults.
In this circuit, is protection provided against high magnitude ground-faults as well as low level faults?
No, it is not. There is no current-limitation.
Is G.F.P. required on all services?No. The following do not require G.F.P.:1. Continuous industrial process where non-orderly shutdownwould increase hazard.2. All services where disconnect is less than 1000 amperes.3. All 120/208 volts, 3Ø, 4W (wye) services.4. All single-phase services including 120/240 volt, 1Ø, 3W.5. High or medium voltage services. (See NEC® 240.13 and215.10 for equipment and feeder requirements.)6. All services on delta systems (grounded or ungrounded) suchas: 240 volt, 3Ø, 3W Delta, 480 volt, 3Ø, 3W Delta, or 240 volt, 3Ø,4W Delta with midpoint tap.7. Service with 6 disconnects or less (230.71) where eachdisconnect is less than 1000 amperes. A 4000 ampere servicecould be split into five 800 ampere switches.8. Resistance or impedance grounded systems.
1000 ampereswitch & fuseor larger
480Y/2773Ø, 4WService
Ground faultprotectionrequired
SWBD
1000 amperecircuit breakeror larger
480Y/2773Ø, 4WService
Ground faultprotectionrequired
SWBD
19
230.95 Ground Fault Protection for Services
240.1 Scope of Article 240 on Overcurrent Protection
What are some of the problems associated with G.F.P.?Incorrect settings, false tripping and eventually, disconnection.(The knocking-out of the total building service or large feeders asa result of minor faults or nuisance tripping cannot be tolerated inmany facilities). Unnecessary plant down time is often morecritical, or even more dangerous, than a minor ground fault.
Note: G.F.P. without current limitation may not protect systemcomponents. See 110.10 and 250.4(B)(4).
How can ground faults be minimized?1. To prevent blackouts, make sure that all overcurrent protectivedevices throughout the overall system are selectively coordinated.When maximum continuity of electrical service is necessary,ground fault protective equipment should be incorporated infeeders and branch circuits. [Per 230.95 (FPN No. 2).]2. Insulating bus structures can greatly minimize the possibility offaults. The hazard of personnel exposure to energized electricalequipment is also reduced with insulated bus structures.3. Specify switchboards and other equipment with adequateclearance between phase conductors and ground. Ground faultsare rare on 120/208 volt systems because equipment manufactur-ers provide ample spacing for this voltage. Insist on greaterspacing for 277/480 volt equipment and the likelihood of groundfaults will be greatly reduced.4. Avoid unusually large services; split the service wheneverpossible.5. Adequately bond all metallic parts of the system to enhanceground fault current flow. Then, if a ground fault does occur, it ismore likely to be sensed by fuses or circuit breakers.
To respond properly to a line-to-ground feeder or branch-circuit typefault, what should be the setting of a ground fault relay located on themain disconnect?The setting should allow the feeder circuit (or preferably thebranch) overcurrent protective devices to function withoutdisturbing the G.F.P. relay.
How is a G.F.P. setting determined?By making a coordination study. Such a study requires the plottingof the time-current curves of the protective devices.
A simple solution to the problem of coordinating ground faultrelays with overcurrent protective devices is shown in the systemrepresented in the graph at right. The G.F.P. relay coordinates withthe feeder fuses KTS-R 250. The G.F.P. relay with a degree ofinverse time characteristics provides coordination with feederfuses in order to avoid outages. (230.95 permits an inverse time-delay relay with a delay of up to 1 second at 3000 amperes.)
devices often do not permit a selectively coordinated system* andBLACKOUTS can occur. For ground faults (and phase-to-phaseshort-circuits as well) of current magnitude above theinstantaneous trip setting on the main circuit breaker’s overcurrentelement, the main will nuisance trip (open) causing a blackouteven though the fault is on a feeder or branch circuit. Appropriateselection of current-limiting fuses with proper G.F.P. settings canprovide the highest degree of coordination and prevent blackouts.
Reference the Bussmann® bulletin SPD Electrical ProtectionHandbook for more in-depth discussion on ground fault protectionand coordination of ground fault relays and overcurrent protectivedevices in electrical systems.
A system wherein only the protective device nearest the fault operates andnone of the other protective devices in the system are disturbed.
6
43
2
1.8.6
.4
.3
.2
.1.08.06
.04
.03
.02
.01
.05
.5
5
100
200
300
400
500
600
800
1,00
0
2,00
0
3,00
04,
000
5,00
06,
000
8,00
010
,000
20,0
00
30,0
00
108
20
3040506080
100
200
300
KRP-C1600SP
KTS-R250
KTS-R125
KTS-R250
GFPsetat1200 AMPSPICK UP &0.5 SEC.
KRP-C1600SP
KTS-R125
CURRENT IN AMPERES
TIM
E IN
SE
CO
ND
S
*
What is the importance of this Section?The basic purpose of overcurrent protection is to open a circuitbefore conductors or conductor insulation are damaged when anovercurrent condition exists. An overcurrent condition can be theresult of an overload or a short-circuit. It must be removed beforethe damage point of conductor insulation is reached. Conductorinsulation damage points can be established from availableengineering information, i.e., Publication P-32-382, “Short-CircuitCharacteristics of Cable”, ICEA, (Insulated Cable EngineersAssociation, Inc.), IEEE Color Books, Canadian Electrical Code,and IEC Wiring Regulations.
When selecting an overcurrent protective device to protect a conductor,is it adequate to simply match the ampere rating of the device to theampacity of the conductor?No. Although conductors do have maximum allowable ampacityratings, they also have maximum allowable short-circuit currentwithstand ratings. Damage ranging from slight degradation ofinsulation to violent vaporization of the conductor metal can resultif the short-circuit withstand is exceeded. (See 110.10.)
Why, in the circuit below, is the 10 AWG wire protected even though theavailable short-circuit current exceeds the wire withstand? The 10 AWGconductor can withstand 4300 amperes for one cycle and 6020 amperesfor one-half cycle.****Footnote—From ICEA tables and formula.
Under short-circuits, the LOW-PEAK® YELLOW™ Dual-Element
fuse (30 ampere) is fast acting. It will clear and limit (cut off) short-circuit current before it can build up to a level higher than the wirewithstand. The opening time of the fuse is less than one-half cycle(less than 0.008 seconds). In this particular example, theprospective current let-thru by the fuse is less than 1850 amperes.Thus, opening time and current let-through of the fuse is far lower
30ALow-Peak YellowClass RK1 Dual-ElementFuse
10 AWG THW COPPER WIRE40,000Aavailable
Short-Circuit
20
240.1 Scope of Article 240 on Overcurrent Protection
than the wire withstand. (Conductor protection is not a problemwhen the conductor is protected by current-limiting fuses whichhave an ampere rating that is the same as the conductor ampacityrating. In the case of short-circuit protection only, fuses can oftenbe sized many times higher than the wire ampacity rating,depending upon the current-limiting characteristics of the fuse.)
Does the circuit below represent a misapplication? (10 AWG THWinsulated copper wire can withstand 4300 amperes for one cycle and6020 amperes for one-half cycle).
Yes. The 40,000 ampere short-circuit current far exceeds the with-stand of the 10 AWG THW wire. Note the table and chart whichfollow.
What can be done to correct the above misapplication?There are two possible solutions:1. Use a larger size conductor (i.e., 1/0 AWG), one with a withstandgreater than the short-circuit for one cycle (see chart below).2. Use an overcurrent protective device which is current-limitingsuch as that shown in the previous question.
The following table is based on Insulated Cable EngineersAssociation, Inc. (ICEA) insulated cable damage charts inPublication 32-382. This table assumes that the conductor ispreloaded to its ampacity before a short-circuit is incurred. Theformula that was used to develop the ICEA Damage Charts isgiven following the table. This formula can be used to extrapolatewithstand data for wire sizes or time durations not furnished in theICEA Publication 32-382 charts. A sample chart is shown at right.
The mechanical overcurrent protective device opening timeand any impedance (choking) effect should be known along withthe available short-circuit current and cable withstand data todetermine the proper conductor that must be used.
Where:I = Short-Circuit Current—AmperesA = Conductor Area—Circular Milst = Time of Short-Circuit—SecondsT1 = Maximum Operating Temperature—75°CT2 = Maximum Short-Circuit Temperature—150°C
Note: ICEA (Insulated Cable Engineers Association) is the most widely acceptedauthority on conductor short-circuit withstand ratings.
Conductor mustbe protected forits entire length
30A MECHANICAL OVERCURRENTPROTECTIVE DEVICE(Clearing time 1 cycle;not current-limiting)
40,000Aavailable
Short-Circuit
10 AWG COPPER WIRE(THW insulation)
I 2t = 0.0297 log T2 + 234
A T1 + 234 I 2
t = 0.0125 log T2 + 228 A T1 + 228
1 CYCLE -
0.016
7 SECOND
40,000 Amps - 1 Cycle
4,300Amps - 1Cycle
Conductor-CopperInsulation-ThermoplasticCurves Based on Formula
IA
2
t = .0297 logT2 + 234T1 + 234
IAt
T1
T2
= Short-Circuit Current - Amperes= Conductor Area - Circular Mils= Time of Short-Circuit - Seconds= Maximum Operating Temperature - 75°C= Maximum Short-Circuit Temperature - 150°C
Where
SH
OR
T C
IRC
UIT
CU
RR
EN
T –
TH
OU
SA
ND
S O
F A
MP
ER
ES
100
80
6050
40
30
20
10
8
65
4
3
2
1
.8
.6
.5
.4
.3
.2
.1
10 8 6 4 2 1
1/0
2/0
3/0
4/0
AWG
250M
CM 500
1000
CONDUCTOR SIZE
2 CYCLE -
0.033
3 SECOND
4 CYCLE -
0.066
7 SECOND
8 CYCLE -
0.133
3 SECOND
16 C
YCLE - 0.2
667 S
ECOND
30 C
YCLE - 0.5
000 S
ECOND
60 C
YCLE - 1.0
000 S
ECOND
100 C
YCLE - 1.6
667 S
ECOND
21
240.2 Definitions: Coordination, Current-limiting Overcurrent Protective Device,and Tap Conductors
To further appreciate current-limitation, assume for example,that the available prospective short-circuit current in a circuit is50,000 amperes. If a 200 ampere LOW-PEAK® YELLOW™ fuse isused to protect the circuit, the current let-through by the fuse willbe only 6500 amperes instead of 50,000 amperes. Peak currentwill be only 15,000 amperes instead of a possible 115,000amperes. Thus, in this particular example, currents are limited toonly 13% of the available short-circuit values.
As is true of fuse application in general, the application ofcurrent-l imiting fuses in respect to current-l imitation andcomponent protection (110.10) is quite simple. Graphs or tablessuch as the one shown below permit easy determination of the“let-thru” currents that a fuse will pass for various levels ofprospective short-circuit currents. For example, the table belowshows that the 200 ampere LOW-PEAK® YELLOW™ fuse will let-through 6500 amperes when prospective short-circuit current is50,000 amperes.
For the above circuit, the Size 1 Starter has a short-circuitwithstand rating of 5000 amperes.* The question is, with the25,000 ampere available short-circuit current, will a LOW-PEAK®
YELLOW™ fuse provide adequate protection of the starter? Byreferring to the table below, it can easily be seen that for aprospective short-circuit current of 25,000 amperes, fuses withratings of 100 amperes or less will limit fault currents to below the5000 ampere withstand of the starter and, thus, provide adequateprotection.
*Footnote: See discussion on Section 110.10 in this bulletin.
The reader should note that much of the current-limitationclaimed by small ampere circuit breakers is actually the result ofthe significant impedance added to the circuit breaker test circuitafter the circuit has been calibrated. Refer to the circuit breakerprotection portion of 110.9 for further information on circuit breakertest circuits.
What is the definition of tap conductors?A tap conductor is defined as a conductor, other than a serviceconductor that has overcurrent protection ahead of its point ofsupply that exceeds the value permitted for similar conductorsthat are protected as described elsewhere in 240.4.
Repinted from NEC® 2002
What is the meaning of coordination as used in 240?Coordination is defined as properly localizing a fault condition sothat only the affected equipment is removed from the system. Thiscan be achieved by understanding the characteristics ofovercurrent protective devices and selecting the appropriatetypes and sizes. See discussion in this booklet on 240.12 and anindepth discussion in Bussmann® publication SPD ElectricalProtection Handbook.
What is the meaning of current-limiting overcurrent protective device?
†
What is the importance of this Section?
ACTION OF NON-CURRENT-LIMITING CIRCUIT BREAKER
ACTION OF CURRENT-LIMITING FUSE.
Simply stated, a current-limiting protective device is one whichcuts off a fault current in less than one-half cycle†. It thus preventsshort-circuit currents from building up to their full available values.
The greatest damage done to components by a fault currentoccurs in the first half-cycle (or more precisely, “the first majorloop” of the sinewave). Heating of components to very hightemperatures can cause deterioration of insulation, or evenexplosion. Tremendous magnetic forces between conductors cancrack insulators and loosen or rupture bracing structures.
The levels of both thermal energy and magnetic forces areproportionate to the square of current. Thermal energy isproportionate to the square of “RMS” current; maximum magneticfields to the square of “peak” current. If a fault current is 100 timeshigher than normal load current, its increased heating effectequals (100)2 or 10,000 times higher than that of the normalcurrent. Thus, to prevent circuit component damage, the use ofcurrent-limiting protective devices is extremely important,particularly since present-day distribution systems are capable ofdelivering high level fault currents.
Footnote: The more technical definition of a current-limiting protective device isexpressed by 240.2.
Circuit breaker tripsand opens short-circuitin about 11/2 cyclesInitiation of
short-circuit current
Normalload current
Areas within waveformloops represent destructiveenergy impressed uponcircuit components
Fuse opens and clearsshort-circuit in lessthan 1/2 cycle
25,000Aavailablefault current
Size 1 Starter(Tested with 5000A available)
Short-Circuit
22
What is the importance of this section?Flexible cords and extension cords shall have overcurrent protec-tion rated at their ampacities. Supplementary fuse protection is anacceptable method of protection. For 18 AWG fixture wire 50 feetor over, a 6 ampere fuse would provide necessary protection, andfor 16 AWG fixture wire 100 feet or over, an 8 ampere fuse wouldprovide the necessary protection. 18 AWG extension cords mustbe protected by a 7 ampere fuse.
Also, 760.23, covering special non-power-limited fire alarmcircuits, requires 7 ampere protection for 18 AWG conductors and10 ampere protection for 16 AWG conductors.
Violation(EXTENSION CORD)
Receptacle
20ABranch Circuits
18 AWG Extension Cord
16 AWG Fixture Wire100 ft. or over
Violation(FIXTURE WIRE)
20A FuseTo load
BRANCHCIRCUIT
16 AWG Fixture Wire100 ft. or over
Compliance(FIXTURE WIRE)
8A FuseTo load
BRANCHCIRCUIT
Receptacle
20ABranch Circuits
Compliance(EXTENSION CORD)
18 AWG Extension Cord7 AmpFuse
What does 240.4(F) mean?Conductors fed from single-phase, 2-wire secondary transformersand three phase, delta-delta connected transformers with three-wire (single-voltage) secondaries can be considered protected bythe primary side fuses if the transformer is properly protected inaccordance with Section 450.3. The primary fuse must be less thanor equal to the secondary conductor ampacity t imes thesecondary-to-primary transformer voltage ratio.
240.4 Protection of Conductors Other Than Flexible Cords and Fixture WiresWhat is the meaning of 240.4(B) and 240.4(C)?Where the ampacity of a conductor does not correspond with astandard rating (240.6) of a fuse, the next standard rating may beused as long as the fuse is not above 800 amps and theconductors are not part of a multi-outlet branch circuit supplyingreceptacles for cord and plug-connected portable loads.
What does 240.4(D) mean?It requires the fuse to not exceed 15A for 14 AWG, 20A for 12AWG, and 30A for 10 AWG copper; or 15A for 12 AWG and 25Afor 10 AWG aluminum and copper-clad aluminum after anycorrection factors for ambient temperature and number ofconductors have been applied. This is required unlessspecifically permitted in 240.4(E) through (G).
12 AWG copper conductor, 75°C, has ampacity perTable 310.16 of 25A. But, fuse must be sized 20Aor less, unless otherwise allowed.
12 AWG Conductor, 75°C
240.5 Protection Flexible Cords, Fixture Cables and Fixture Wires
18 AWG Fixture Wire50 ft. or over
Violation(FIXTURE WIRE)
20A FuseTo load
BRANCHCIRCUIT
18 AWG Fixture Wire50 ft. or over
Compliance(FIXTURE WIRE)
6A FuseTo load
BRANCHCIRCUIT
240.6 Standard Ampere Ratings
What is the importance of this section?In addition to the standard ratings of fuses and circuit breakers,this section states that the rating of an adjustable trip circuitbreaker is considered to be the highest possible setting. Thisbecomes important when protecting conductors or motor circuits.For example, if a copper 75°C conductor is required to carry 200amperes continuously, a 250 kcmil cable might be chosen. If acircuit breaker were chosen to protect this cable with an externaladjustable trip from 225 through 400 amperes, the rating of thebreaker would be 400 amperes, and 500 kcmil cable wouldtherefore be required, increasing costs significantly. However, ifthis adjusting means is “restricted”, such as behind a bolted
equipment door, behind locked doors accessible only by aqualified person, or if a removable and sealable cover is over theadjusting means, then the rating can be considered to be equal tothe adjusted setting.Note: Standard ampere ratings for fuses and inverse time circuitbreakers are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100,110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600,700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000 and6000 amperes.
In addition, standard fuse ratings are 1, 3, 6, 10 and 601.The use of nonstandard ampere ratings of fuse and circuit
breakers is permitted.
23
240.8 Protective Devices Used in Parallel and 404.17 Fused SwitchesWhat do these sections mean?There are cases in which an original equipment manufacturer, forvarious reasons, must parallel fuses and receive an appropriateequipment listing. For example, this would be the case of some
solid-state power conversion equipment. However, for thestandard safety switch, conventional branch circuit applications,switch-boards, and panelboards, the use of parallel fuses is notallowed.
What is the importance of this section?Supplementary fuses, often used to provide protection for lightingfixtures, cannot be used where branch circuit protection isrequired.
What are the advantages of supplementary protection?The use of supplementary protection for many types of appliances,fixtures, cords, decorator lighting (Christmas tree lights. . .)*, etc.,is often well advised. There are several advantages:1. Provides superior protection of the individual equipment bypermitting close fuse sizing.2. With an occurrence of an overcurrent, the equipment protectedby the supplementary protective device is isolated; the branchcircuit overcurrent device is not disturbed. For instance, the in-line-fuse and holder combination, such as the Type HLR fuseholderwith Type GLR or GMF fuses, protects and isolates fluorescentlighting fixtures in the event of an overcurrent.3. It is easier to locate equipment in which a malfunction hasoccurred. Also, direct access to the fuse of the equipment ispossible.
Footnote–Supplementary protection for series connected decorator lighting sets andparallel sets (Christmas tree string lights) was required in 1982. Manufacturers haveimplemented this requirement.
What is a supplementary overcurrent protective device?A fuse or mechanical overcurrent protective device that is limited inapplication, due to the wide range of permissible ratings andperformance. Supplementary overcurrent protective devices maybe incomplete in construction or restricted in performancecapabilities. Such a device is not suitable for and can not be usedwhere branch circuit protection is required. They can only be usedwhere specifically permitted in the NEC®. The Tri-NationalStandard for supplementary fuses is UL/CSA/ANCE 248-14.
What do I need to know about supplementary overcurrent protectivedevices?All too often supplementary overcurrent protective devices arebeing misapplied where branch circuit rated fuses or circuitbreakers are required. This can be a serious safety issue. Onemust be sure to read the devices’ label, instruction sheets, andmanufacturers’ literature for proper usage to meet the NorthAmerican electrical system (NEC®, NRTL & Inspection). Be surenot to confuse the device ratings and terminology pertinent to the
North American electrical system with that pertinent to IECapplications. Many devices on the market meet specific IECrequirements and specific North American electrical systemrequirements but the ratings are neither universal norinterchangeable. It is very easy to read the IEC ratings thinkingthey apply to the North American electrical system ratings. Theliterature from many manufacturers is easily misinterpreted orconfusing concerning North American ratings and IEC ratings.
Supplementary overcurrent protective devices are not general usedevices, as are branch circuit devices, and must be evaluated forappropriate application in their particular use. One example of thedifference and limitations is that a supplementary protector(mechanical device) recognized to UL 1077 has spacings,creepage and clearance, that are considerably less than that of abranch circuit rated circuit breaker listed to UL 489.
Example:• A supplemental protector, UL1077, has spacings that are 3/8
inch through air and 1/2 inch over surface.• A branch circuit rated UL489 molded case circuit breaker
has spacings that are 3/4 inch through air and 1 1/4 inchover surface.
Branch circuit overcurrent protective devices’ have standardoverload characteristics to protect branch circuits and feederconductors. Supplementary overcurrent protective devices do nothave standard overload characteristics and may differ fromstandard branch circuit overload characteristics.
The interrupting ratings or short-circuit current ratings ofsupplementary overcurrent protective devices can ranges from lessthan 32 amps to 100,000 amps. When supplementary overcurrentprotective devices are considered for proper use, it is important tobe sure that the device’s interrupting rating equals or exceeds theavailable short-circuit current (see the discussion for 110.9 in thisbooklet).
Verify the supplementary overcurrent protective device’s voltagerating and that the device has the proper voltage for theinstallation, including compliance with slash voltage ratingrequirements, if applicable. For discussion on slash voltage ratingsee 240.85 in this booklet.
What does this section mean?Thermal overload devices generally can neither withstand openinga circuit under short-circuit conditions nor even carry short-circuitcurrents of higher magnitudes. When using thermal overload
protective devices, the use of a current-limiting fuse will not onlyprovide short circuit protection for the circuit but for the thermaloverload device as well.
240.9 Thermal Devices
240.10 Requirements for Supplementary Overcurrent Protection
*
240.12 System Coordination or SelectivityWhat is the importance of this section?Whenever a partial or total building blackout could causehazard(s) to personnel or equipment, the fuses and/or circuitbreakers must be coordinated in the short-circuit range. It isacceptable for a monitoring system to be used to indicate anoverload condition, if the overcurrent protective devices cannot becoordinated in the overload region. However, in the vast majorityof cases, both circuit breakers and fuses will be able to becoordinated in the overload range, so the monitoring systems willseldom be required. Typical installations where selectivecoordination would be required include elevator circuits, hospitals,industrial plants, office buildings, schools, government buildings,military installations, high-rise buildings, or any installation wherecontinuity of service is essential.*
*Footnote: See also Section 4-5.1 of NFPA 110 (Emergency and Standby PowerSystems) and Sections 3-3.2.1.2(4) & 3-4.1.1.1 of NFPA 99 (Health Care Facilities) foradditional information on selective coordination.
VIOLATION
Fault exceeding the instantaneous trip setting of all 3 circuit breakers in series willopen all 3. This will blackout the entire system.
Fault opens the nearest upstream fuse, localizing the fault to the equipmentaffected. Service to the rest of the system remains energized.
If the ampere rating of a feeder overcurrent device is larger than therating of the branch circuit device, are the two selectively coordinated?No. A difference in rating does not in itself assure coordination. Forexample, a feeder circuit breaker may have a rating of 400amperes and the branch breaker 90 amperes. Under overloadconditions in the branch circuit, the 90 ampere breaker will openbefore, and without, the 400 ampere breaker opening. However,under short-circuit conditions, not only will the 90 ampere deviceopen, the 400 ampere may also open. In order to determinewhether the two devices will coordinate, it is necessary to plot theirtime-current curves as shown. For a short-circuit of 4000 amperes:
1. The 90 ampere breaker will unlatch (Point A) and free thebreaker mechanism to start the actual opening process.2. The 400 ampere breaker will unlatch (Point B) and it, too, wouldbegin the opening process. Once a breaker unlatches, it will open.The process at the unlatching point is irreversible.3. At Point C, the contacts of the 90 ampere breaker finally openand interrupt the fault current.4. At Point D, the contacts of the 400 ampere breaker open. . .theentire feeder is “blacked out”!
Example of Non-Selective System.
Now, let’s take the case of fuse coordination. When selectivecoordination of current-limiting fuses is desired, the SelectivityRatio Guide (next page) provides the sizing information necessary.In other words, it is not necessary to draw and compare curves.Current-l imit ing fuses can be selectively coordinated bymaintaining at least a minimum ampere rating ratio between themain fuse and feeder fuses and between the feeder fuse andbranch circuit fuses.
These ratios are based on the fact that the smaller downstreamfuses will clear the overcurrent before the larger upstream fusesmelt. An example of ratios of fuse ampere ratings which provideselective coordination is shown in the one-line circuit diagram.
1000A I.T.=10x
225AI.T.=8x
Opens
Opens
20AI.T.=8x Opens
22,000 AmpShort-Circuit
22,000 AmpShort-Circuit
Opens20A
225A NotOpen
1000ANotOpen
19-B
.001
.002
.003
.004
.006
.008
.08
.01
.02
.03
.04
.06
.1
.2
.3
.4
.6
.81
2
34
68
10
20
3040
8060
100
200
300400
600800
1,000
100
200
300
600
400
800
1,00
0
2,00
0
3,00
04,
000
6,00
08,
000
10,0
00
20,0
00
30,0
0030
,000
TIM
E IN
SE
CO
ND
S
CURRENT IN AMPERES
POINT D
POINT C
POINT B
POINT A
400 AMPCircuit BreakerI.T. = 5X
90 AMPCircuitBreaker
90A
Short-Circuit
400A
2:1 (or more)
LPS-RK400SPLPS-RK90SP
Short-Circuit
COMPLIANCE
24
25
made to a switchboard bus for an adjacent panel, such as anemergency panel, the use of cable limiters is recommended assupplementary short-circuit protection of the tapped conductor.These current-limiting cable limiters are available in sizes designedfor short-circuit protection of conductors from 12 AWG to 1000kcmil. They provide current-limiting short-circuit protection but notoverload protection.
The most common use of tap conductors for feeders, 240(B), andtransformer secondary conductors, 240(C), are the 10 foot, 25 footand outside tap conductor rules. It is important to realize thatalthough they allow for unprotected lengths of conductors, inalmost all cases, termination in a single or group of overcurrentprotection devices is required. In addition, it may be necessary tomeet other requirements for panelboards, 408.16, andtransformers, 450.3.
240.12 System Coordination or Selectivity
240.13 Ground Fault Protection of Equipment on Buildings or Remote Structures
240.21 Location Requirements for Overcurrent Devices and Tap Conductors
*Selectivity Ratio Guide (Line-Side to Load-Side) for Blackout Prevention.Circuit Load-Side Fuse
Note: At some values of fault current, specified ratios may be lowered to permit closer fuse sizing. Plot fuse curves or consult with Bussmann.General Notes: Ratios given in this table apply only to Buss® fuses. When fuses are within the same case size, consult Bussmann.Consult Bussmann for latest LPJ—SP ratios..
Line
-Sid
e Fu
se
*
**
What does this section require?Equipment ground fault protection of the type required in 230.95 isrequired for each disconnect rated 1000 amperes or more in480Y/277V solidly grounded systems that will serve as a maindisconnect for a separate building or structure. Refer to 215.10and 230.95.
Note: G.F.P. that is not current-limiting may not protect systemcomponents. See 110.10 and 250.1 (FPN).
High VoltageService
Building A
800A480Y/277V
Building B
1000A or Greater480Y/277V
G.F.P. NotRequired
G.F.P. NotRequired
G.F.P. Required
240.21 Location in CircuitRequires overcurrent protection to be provided in eachungrounded circuit conductor and be located at the point wherethe conductors receive their supply except as specified in240.21(A)-(G). No conductor supplied per 240.21(A)-(G) shallsupply another conductor, except through an overcurrent devicemeeting the requirements of 240.4. In other words, “you can’t tapa tap!”
Note: Smaller conductors tapped to larger conductors can be aserious hazard. If not adequately protected against short-circuitconditions (as required in 110.10 and 240.1), theseunderprotected conductors can vaporize or incur severeinsulation damage. Molten metal and ionized gas created by avaporized conductor can envelop other conductors (such as barebus), causing equipment burn-down. Adequate short-circuitprotection is recommended for all conductors. When a tap is
26
240.21 Location Requirements for Overcurrent Devices and Tap ConductorsWhat are requirements for 240.21(B)(1) Taps Not Over 3m (10 ft) Long?Fuses are not required at the conductor supply if a feeder tapconductor is not over ten feet long and:
(1) Has an ampacity not less than the combined computed loadssupplied. Not less than the rating of the device supplied,unless the tap conductors are terminated in a overcurrentprotective device not exceeding the tap conductor’sampacity, and
(2) Does not extend beyond the switchboard, panelboard orcontrol device which it supplies, and
(3) Is enclosed in raceway, except at the point of connection tothe feeder, and
(4) For field installed taps, where the tap conductors leave theenclosure, the ampacity of the tap conductor must be at least10% of the overcurrent device rating.
Note: For overcurrent protection requirements for lighting andappliance branch-circuit panelboards and certain powerpanelboards, see 408.16(A), (B), and (E).
In Figure 1, the feeder fuse (600A) is sized per the NEC® for theloads served. All field installed taps to the motors are 10 foot inlength.
The three smaller motors (FLC rating per Table 430.150) include a1 HP (FLC = 4.2A), a 5HP (FLC = 15.2A), and a 7.5 HP motor (FLC = 22A). Per NEC® 430.22, the 1 HP motor requires aconductor with a minimum ampacity of 1.25 X 4.2 = 5.25A. The 5 HP motor requires a conductor with a minimum ampacity of 1.25 X 15.2 = 19A. The 7.5 HP motor requires a conductor with aminimum ampacity of 1.25 X 22 = 27.5A. Based upon this and310.15, the 1, 5 and 7.5 HP motors are required to have a 14AWG, 14 AWG and 10 AWG (75° C, Cu) conductor, respectively.This assumes all lugs /terminals are rated 60°/75° C.
Figure 1
240.21(B)(1)(4) requires that the maximum fuse for f ieldinstallations shall not exceed 10 times the ampacity of the tapconductor, for example: — 14 AWG, 75° C, Cu conductor (20A ampacity), max feeder
overcurrent device is 200A.— 10 AWG, 75° C, Cu conductor (35A ampacity), max feeder
overcurrent device is 350A.
To field tap the conductors of the 1, 5, and 7.5 HP motors to a600A feeder overcurrent device would be a violation of240.21(B)(1)(4). To comply with 240.21(B)(1)(4), the smallermotors must be fed from a feeder fuse that does not exceed the10 times rating of the tap conductor’s ampacity (200A as shown inthe Figure 2).
Figure 2
The smallest of the three larger motors is a 15 HP motor with a FLCof 42A per Table 430.150. Per 430.22, the motor conductorampacity is sized at 1.25 X 42A = 52.5 amperes. Per 310.15, thiswould require a 6 AWG conductor (75°C, Cu, 65A ampacity,assuming 60/75°C lugs). Per 240.21(B)(1)(4), this could betapped to the 600A feeder since the ampacity of the 6 AWG,75°C, Cu conductor (65A) is more than 10% of the feeder fuse.
Since, the tap conductors to the other motors (20 and 25 HP) arerequired to have an ampacity greater than the 15 HP motor per430.22 and 310.15, the 20 and 25 HP motors could be tapped to600A feeder fuses as well.
What are requirements for 240.21(B)(2) Taps Not Over 7.5 m (25 ft)Long?Fuses are not required at the conductor supply if a feeder tapconductor, is not over 25 feet long, and:
(1) Has an ampacity not less than 1/3 that of the feeder fusesfrom which the tap conductors receive their supply, and
(2) Terminate in a single set of fuses sized not more than the tapconductor ampacity, and
(3) Is suitably protected from physical damage.
Example in Figure 3 complies with 240.21(B)(2):— Taps are not over 25 ft — 400A Feeder fuses
1. Minimum ampacity of 1/3 the feeder OCPD. — 100 HP FLA = 124A (Table 430.150)— Tap conductors to motor = 124 (1.25) = 155A (430.22) Æ 2/0 Conductor, 75°C, Cu (175A) (310.15)
— Maximum fuse would be 175 X 3 = 525A (400A ok) 2. Terminate in single set of fuses not more than the tap
conductor ampacity (OCPD ( 175A). Optimum Fuse size: = 1.25 X FLA (or next standard size) = 1.25 X 124 = 155A (LPS-RK 175SP (175A) fuses (430.52).
3. Protected from physical damage.
Figure 3
M M M M M M
ComplianceViolation240V, 3Ø600A
1HP
5HP
7.5HP
15HP
20HP
25HP
10'TAP
M M M
Wireway
200A
10'TAP
1HP
5HP
7.5HP
M
100 HP
480V, 3Φ400A
25’
LPS-RK 175SPFuses
2/075 ° CTaps
In Conduit
M
100 HP
500 kcmil
27
What are requirements for 240.21(B)(3) Taps Supplying a Transformer(Primary Plus Secondary Not Over 7.5 m (25 ft) Long)?Fuses are not required at the conductor supply where theconductor feeds a transformer if:
(1) The primary conductor ampacity is at least 1/3 of the rating ofthe fuses protecting the feeder, and
(2) The secondary conductor ampacity when multiplied by thesecondary to primary voltage ratio is at least 1/3 of the ratingof the fuses protecting the feeder, and
(3) The primary plus secondary is not over 25 ft. long. (Anyportion of the primary conductor that is protected at itsampacity is excluded in the 25 feet), and
(4) The primary and secondary conductors is protected fromphysical damage, and
(5) The secondary conductors terminate in a single set of fusesthat will limit the load to the ampacity of the secondaryconductors (per 310.15).
See Figure 4.
Figure 4
What are requirements for 240.21(B)(4) Taps Over 7.5 m (25 ft) Long?Fuses are not required at the conductor supply in high baymanufacturing buildings (over 35’ high at walls) if:
(1) Only qualified persons will service such a system, and(2) Feeder tap is not over 25 feet long horizontally and not over
100 feet long, total length, and(3) The ampacity of the tap conductors is not less than 1/3 of the
fuse rating from which they are supplied, and (4) Taps terminate in a single set of fuses that limits the load to
the ampacity of the tap conductor, and(5) Taps are protected from physical damage, and(6) Taps are not spliced, and (7) Taps are sized at least 6 AWG copper or 4 AWG aluminum,
and(8) Taps do not penetrate walls, floors, or ceilings, and(9) Taps are made no less than 30 feet from the floor.
What are requirements for 240.21(B)(5) Outside Taps of UnlimitedLength?Fuses are not required at the supply for tap conductors locatedoutside of a building or structure and where all of the following aremet:
(1) The conductors are protected from physical damage, and (2) The conductors terminate in a single set of fuses that limit the
load to the ampacity of the conductors, and (3) The fuses are a part of or immediately adjacent to the
disconnecting means, and (4) The disconnecting means is readily accessible and is
installed outside, inside nearest the point of entrance, orinstalled per 230.6, nearest the point of entrance of theconductors.
240.21(C) Transformer Secondary Conductors.Conductors may be tapped to a transformer secondary, withoutfuses at the secondary, under several conditions as specified in240.21(C)(1) through 240.21(C)(6). See Figure 5.
Note: Refer to 450.3 for transformer overcurrent protectionrequirements and 408.16 for panelboard protection.
Figure 5
What are requirements for 240.21(C)(1) Protection by PrimaryOvercurrent Device?Fuses are not required on the secondary of a single-phase 2-wire(single-voltage) or three-phase, three wire (single-voltage), delta-delta transformer to provide conductor protection if:
— The transformer primary (supply) side is protected inaccordance with 450.3, and
— The fuses on the primary of the transformer does not exceedthe ampacity of the secondary conductor multiplied by thesecondary to primary voltage ratio.
Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are notconsidered to be protected by the primary fuses. This eliminatesthe use of this tap rule on delta-wye transformers (480-208/120).
What are requirements for 240.21(C)(2) Transformer SecondaryConductors Not Over 3 m (10 ft) Long?Fuses are not required on the secondary of a transformer ifsecondary conductors are not over 10 ft long, and:
(1a) Secondary conductor ampacity is not less than thecombined computed loads, and
(1b) Secondary conductor ampacity is not less than the rating ofthe device they supply or the rating of the fuses at theirtermination, and
(2) Secondary conductors do not extend beyond theenclosure(s) of the equipment they supply, and
(3) Secondary conductors are enclosed in a raceway.
240.21 Location Requirements for Overcurrent Devices and Tap Conductors
480V
150 AmpFeederFuse
TRANSFORMER2:1 RATIO 100 Amp
Fuse
100 AmpRatedConductor
50 AmpRatedConductor
150 AmpRatedConductor 240V
480V
300 AmpFeederFuse
TRANSFORMER1:1 RATIO 100 Amp
Fuse
100 AmpRatedConductor
100 AmpRatedConductor
300 AmpRatedConductor 480V
25 Feet or Less
25 Feet or Less Service
Feeder – Transformer Primary OCPD
Must meet one of the following: 240.21(C)(1) through 240.21(C)(6)
No overcurrent protection at beginning of tap.
Conductor tapped to secondaryof transformer.
28
Note: For overcurrent protection requirements for lighting andappliance branch-circuit panelboards and certain powerpanelboards, see Sections 408.16(A), (B), and (E). Refer to 450.3for transformer protection.
Example (Figure 6):— Transformer Primary OCPD (450.3)
o 70A Fuse (same as previous example)— Secondary Conductors (3 AWG - 100A):
o Less than or equal to 10 ft, ando Not less than computed load (80A), ando Not less than the rating of the device they supply/terminate in
(100A Main), ando Do not extend beyond the enclosure(s) of the equipment they
supply and they are enclosed in a raceway and— Panelboard complies with 408.16(A) (lighting panel)
Figure 6
What are requirements for 240.21(C)(3) Industrial Installation SecondaryConductors Not Over 7.5 m (25ft)?Transformer secondary conductors, if 25 ft or less, do not requirefuses at the transformer terminals, for industrial installations only, if:
(1) Secondary conductor ampacity is not less than the secondaryfull load current of the transformer and not less than the sumof the ratings of the fuses, and
(2) All overcurrent devices are grouped, and(3) Secondary conductors are protected from physical damage.
Note: Refer to 408.16 for panelboard protection requirements and450.3 for transformer protection.
What are requirements for 240.21(C)(4) Outside Secondary Conductors ofUnlimited Length?Fuses are not required on the secondary of a transformer toprovide conductor protection where the secondary conductors arelocated outdoors, except at the point of termination, and all of thefollowing are met:
(1) The secondary conductors are protected from physicaldamage, and
(2) The secondary conductors terminate in one set of fuses thatlimits the load to the ampacity of the conductors, and
(3) The fuse is part of or immediately adjacent to thedisconnecting means, and
(5a) The disconnecting means for the conductors is readilyaccessible and outside of a building or structure, or
(5b) Inside nearest the point of entrance of the conductors, or(4c) Installed in accordance with 230.6, nearest the point of
entrance of the conductors.
Note: Refer to 408.16 for panelboard protection requirements and450.3 for transformer protection.
What are requirements for 240.21(C)(5) Secondary Conductors from aFeeder Tapped Transformer?Transformer secondary conductors installed in accordance with240.21(B)(3) are permitted to be protected by fuses as specified inthat section. 240.21(B)(3) really covers tap conductors on both theprimary and secondary of the transformer, so it covers240.21(C)(5) as well.
What are requirements for 240.21(C)(6) Secondary Conductors Not Over7.5 m (25 ft) Long?Where the length of the secondary conductors does not exceed 25ft and complies with all of the following.
(1) Secondary conductors have an ampacity that, whenmultiplied by the ratio of the secondary-to-primary voltage, isat least 1/3 the rating of the transformer primary fuses, and
(2) Secondary conductors terminate in a single set of fuses thatlimit the load current to not more than the conductor ampacitypermitted by 310.15, and
(3) Secondary conductors are suitably protected from physicaldamage.
Example in Figure 7 conforms to 240.21(C)(6). Larger secondary conductors are possible. For example, 1 AWG,75°C, Cu (130A) conductors could have been used on secondarywith 125A fuses in the fusible disconnect switch.
Figure 7
What are requirements for 240.21(D) Service Conductors?Service-entrance conductors are permitted to be protected byfuses in accordance with 230.91. 230.91 requires the servicefuses to be an integral part of the service disconnecting means orlocated immediately adjacent to the disconnecting means.
What are requirements for 240.21(E) Busway Taps?Busways and busway taps are permitted to be protected againstovercurrent in accordance with 368.10 through 368.13. See 368.11and 368.12 in this booklet.
240.21 Location Requirements for Overcurrent Devices and Tap Conductors
Fusible Disc Switch withfuses sized to conductorampacity (60A fuses)
What are requirements for 240.21(F) Motor Circuit Taps?Motor-feeder and branch-circuit conductors are permitted to beprotected against overcurrent in accordance with section 430.28and 430.53. NEC® 430.28 covers motor feeder taps that terminatein branch-circuit fuses. NEC® 430.53 covers group motorinstallations that do not require termination in a branch-circuitovercurrent protective device.
What are requirements for 240.21(G) Conductors from GeneratorTerminals?Conductors from generator terminals that meet the sizerequirement in 445.5 are permitted to be protected by thegenerator overload protective device required by 445.4.
240.40 Disconnecting Means for Fuses
What does the section require?A line side disconnecting means must be provided for all cartridgefuses where accessible to other than qualified persons and for anyfuse in circuits over 150 volts to ground. This section does notrequire a disconnecting means for the typical 120/240V singlephase residential plug fuse application.
There is no requirement for a disconnecting means ahead of acurrent-limiting cable limiter or other current-limiting fuse ahead ofthe service disconnecting means.
One disconnect is allowed for multiple sets of fuses asprovided in 430.112 for group motor applications and 424.22 forfixed electric space-heating equipment.
240.21 Location Requirements for Overcurrent Devices and Tap Conductors
30
240.50 Plug Fuses, Fuseholders, and AdaptersWhat does this section mean?Normally, plug fuses are applied in 125 volt circuits for appliances,small motors, machines, etc. They may be used on 240/120 voltssingle-phase circuits, and 208/120 volt three-phase circuits, wherethe neutral is solidly grounded.
240.51 Edison-Base FusesWhat are these fuse types?These are generally referred to as branch circuit listed fuses whichare NOT size rejecting. They can provide protection for appliancesand small motors in residential, commercial, and industrialapplications. For branch circuit protection, these fuses may onlybe used for replacements in existing installations where there is noevidence of overfusing or tampering.
240.53 and 254 Type S Fuses, Adapters and FuseholdersWhat are these fuse types?These are branch circuit listed fuses that are size (ampere)rejecting. They become size rejecting when a special Type Sholder or Type S adapter is used. For example, when a 20 ampereadaptor is installed, a 25 or 30 ampere fuse can not be used.
What are the advantages of Type S Fuses?Type S fuses are size rejecting to prevent overfusing. They areused with special adapters that cannot easily be removed.
Type S fuses are required for new installation where plug fuses areto be used as the branch circuit protection.
Edison-base fuses can be used for supplementary overcurrentprotection in new installations.
M
120 V
240 V
M208 V120 V
Fuse Adapter
31
The interrupting rating must be marked on all branch circuit circuitbreakers with interrupting ratings other than 5000 amperes.(Caution: do not confuse supplementary protectors with branchcircuit circuit breakers. Supplementary protectors can only beused for supplementary protection and have many limitations.Supplementary protectors may have very low interrupting ratings.See 240.10 in this booklet.)
What does this section mean?300 volt rated fuses can be used to protect single-phase line-neutral loads when supplied from three-phase solidly grounded480/277 volt circuits, where the single-phase line-to-neutral voltageis 277 volts.
What does this section mean?All low voltage branch circuit fuses have a voltage ratingassociated with them. They can be properly applied at systemvoltages up to that rating.
240.61 Classification of Fuses and Fuseholders
240.83 Circuit Breaker – Markings
240.60 Cartridge Fuses and Fuseholders
480/277V
600 VoltFuses
277V 1Ø Load
300 Volt Fuse
Branch circuit listed fuses are designed so that it is verydifficult to replace an installed fuse with one of lesser capability.This is based on voltage, current, or current-limiting vs. non-current-limiting ratings.
The interrupting rating must be marked on all branch circuitfuses with interrupting ratings other than 10,000 amperes.
300,000 amperes INTERRUPTING RATING
35,000 amperes INTERRUPTING RATING at 480 volts
400 amp NORMAL CURRENT RATING
225 amp NORMAL CURRENT RATING
New CUBEFuse™. Shown is first finger-safe fuse, TCF 60 CUBEFuse™, beinginserted into a TCFH60 CUBEFuse™ base. The fuse shown has a 60 amperenormal current rating, 300,000 ampere interrupting rating and is rated 600 volts.It is dual-element, time delay fuse that is very current-limiting (Class Jcharacteristics). For more information visit www.cubefuse.com
32
Typical plant electrical systems use three-phase distributionschemes. As an industry practice, short-circuit calculations lead tothe selection of overcurrent protective devices based on availablethree-phase fault currents. If the overcurrent devices have anadequate three-phase interrupting rating, engineers are generallysatisfied that the system complies with NEC® 110.9.
Figure 1
How often, however, do three-phase faults occur? Commonlyreferred to as "three-phase bolted faults", these shorts require allthree legs to be electrically connected (Figure 1). Though boltedfaults do occur, far more common is the mishap of a slippedscrewdriver, dropped wrench, or worn insulation that shorts onephase to ground, creating a single-pole short-circuit (Figure 2).These phase-to-ground faults affect the performance of circuitbreakers in different ways, depending upon the groundingscheme. Two of these performance areas were addressed bychanges to the 2002 NEC®. They are the proper usage of slashratings and individual pole interrupting capabilities. The followingparagraphs explain the reasons behind these 2002 Codechanges.
Figure 2 - Single pole short-circuit
What is a slash rating? A slash-rated circuit breaker is one with two voltage ratingsseparated by a slash, such as 480Y/277 volt. The smaller of thetwo ratings is for overcurrents at line-to-ground voltages, meant tobe cleared by one pole of the device. The larger of the two ratingsis for overcurrents at line-to-line voltages, meant to be cleared bytwo or three poles of the circuit breaker. See Figure 3.
Figure 3
Slash-rated circuit breakers are not intended to open phase-to-phase voltages across only one pole. Where it is possible for fullphase-to-phase voltage to appear across only one pole, a fullyrated circuit breaker must be utilized. A fully rated circuit breaker isone that has only one voltage rating, such as a 480 volt circuitbreaker. For example, a 480 volt circuit breaker can open anovercurrent at 480 volts with only one pole, such as might occurwhen Phase A shorts to ground on a 480 volt B-Phase groundeddelta system.
What is the NEC® change for slash ratings?240.85 of the 2002 NEC® was changed to read:
240.85 Applications. A circuit breaker with a slash rating, suchas 120/240V or 480Y/277, shall be permitted to be applied in asolidly grounded circuit where the nominal voltage of anyconductor to ground does not exceed the lower of the two valuesof the circuit breaker’s voltage rating and the nominal voltagebetween any two conductors does not exceed the higher value ofthe circuit breaker’s voltage rating…”
The change was the addition of the words “solidly grounded”*.This was needed to emphasize that slash-rated devices were notappropriate on resistance-grounded and ungrounded systems.The following paragraphs explain why slash-rated devices cannotbe utilized on these types of systems.
* Solidly grounded is defined in 230.95 of the NEC® as“Connection of the grounded conductor to ground without insertingany resistance or impedance devices.”
240.85 Clarifies Requirements for the Use of Slash-Rated Circuit Breakers andApplication of Individual Pole Interrupting Capabilities for Various GroundingSchemes
Three phase bolted fault test has all three phase
conductors connected electrically together.
Grounded Equipment
480Y/277 Volt
Three phase
Four wire
Solidly grounded
wye system
Circuit breaker
480Y/277 slash voltage rating 480 volts
line to line
277 volts
line to ground
A
B
C
Ground
480Y/277 VoltThree phaseFour wireSolidly groundedwye system
Yes No
Yes No
Yes No
Yes No
Slash Rated Exercise 1
*480Delta
Ungrounded480
480480
Corner
Grounded
B Phase
480
277480Resistance
Grounded WYE480
277480Solidly
Grounded WYE480Y/277
Can 480Y/277
Circuit Breaker
Be Used?
L-G
Volt
L-L
Volt
Secondary
System Type
System
Voltage
* Ungrounded delta systems - phase conductors are capacitively coupled to ground
Can you get the correct answers to this exercise? (Answers at bottom ofpage)
Answers to Slash Rated Exercisea)Yesb)No –not solidly grounded systemc)No –Line to ground voltage is 480 volts which exceeds 277 volt ratingd)No –not solidly grounded
33
Do fuses have slash ratings?No. Fuses, by their design, are rated for full voltage; thereforeslash rating concerns are not an issue. For instance Class J fusesare rated at 600 volts. These fuses could be utilized on 600 volt orless systems irrespective if the system is solidly grounded,ungrounded or resistance grounded.
What was added about circuit breakers’ single pole interruptingcapabilities? A Fine Print Note was added to 240.85 of the 2002 NEC® to alertusers that circuit breakers have single-pole interrupting capabilitiesthat must be considered for proper application. It states:
240.85 FPN: Proper application of molded case circuit breakerson 3-phase systems, other than solidly grounded wye,particularly on corner grounded delta systems, considers thecircuit breakers’ individual pole interrupting capability.
The following paragraphs will also explain why this FPN was addedto the 2002 NEC®.
What is a single pole interrupting capability?The single-pole interrupting capability of a circuit breaker is itsability to open an overcurrent at a specified voltage utilizing onlyone pole of the circuit breaker.
What are the single-pole interrupting capabilities for overcurrentdevices? Per ANSI C37.13 and C37.16, an airframe/power circuit breakerhas a single-pole interrupting rating of 87% of its three-pole rating.Listed three-pole molded case circuit breakers have minimum
single-pole interrupting capabilities according to Table 7.1.7.2 ofUL 489. Table 1 on this page indicates the single-pole ratings ofvarious three-pole molded-case circuit breakers taken from Table7.1.7.2 of UL 489. A similar table is shown on page 54 of the IEEE“Blue Book”, Recommended Practice for Applying Low-VoltageCircuit Breakers Used in Industrial and Commercial PowerSystems, (Std 1015-1997). Molded-case circuit breakers may notbe able to safely interrupt single-pole faults above these valuessince they are typically not tested beyond these values. Forcurrent-limiting fuses, the marked interrupting rating is the testedsingle-pole interrupting rating.
If the ratings shown in Table 1 are too low for the application, theactual single-pole rating for the breaker must be ascertained toinsure proper application.
TABLE 1
Single-Pole Interrupting Ratings for Three-Pole Molded CaseCircuit Breakers (ANY I.R.)
FRAME RATING 240V 480/277V 480V 600/347V 600V
100A Maximum250V Maximum 4,330 -- -- -- --
100A Maximum251-600V -- 10,000 8,660 10,000 8,660
101 – 800 8,660 10,000 8,660 10,000 8,660
801 – 1200 12,120 14,000 12,120 14,000 12,120
1201 – 2000 14,000 14,000 14,000 14,000 14,000
2001 – 2500 20,000 20,000 20,000 20,000 20,000
2501 – 3000 25,000 25,000 25,000 25,000 25,000
3001 – 4000 30,000 30,000 30,000 30,000 30,000
4001 – 5000 40,000 40,000 40,000 40,000 40,000
5001 – 6000 50,000 50,000 50,000 50,000 50,000
How about an actual example?As an example of single-pole interrupting capability in a typicalinstallation, consider a common three-pole, 20 amp, 480 volt circuitbreaker with a three-pole interrupting rating of 65,000 amperes.Referring to Table 1, this breaker has an 8,660 ampere single-poleinterrupting capability for 480 volt faults across one pole. If theavailable line-to-ground fault current exceeds 8,660 amps at 480volts, such as might occur on the secondary of a 1000 KVA, 480volt, corner-grounded delta transformer, the circuit breaker may bemisapplied. In this case, the circuit breaker manufacturer must beconsulted to verify interrupting ratings and proper application.
CALCULATING GROUND FAULT CURRENTS
How much short-circuit current will flow in a ground fault condition?The answer is dependent upon the location of the fault with respectto the transformer secondary. Referring to Figure 5, the groundfault current flows through one coil of the wye transformersecondary and through the phase conductor to the point of thefault. The return path is through the enclosure and conduit to thebonding jumper and back to the secondary through the groundedneutral. Unlike three-phase faults, the impedance of the return pathmust be used in determining the magnitude of ground fault current.This ground return impedance is usually difficult to calculate. If theground return path is relatively short (i.e. close to the center tap ofthe transformer), the ground fault current will approach the three-phase short-circuit current.
240.85 Clarify Requirements for the Use of Slash-Rated Circuit Breakers andApplication of Individual Pole Interrupting Capabilities for Various GroundingSchemes
Yes No
Yes No
Yes No
Yes No
Slash Rated Exercise 2
*480Delta
Ungrounded480
480480
Corner
Grounded
B Phase
480
277480Resistance
Grounded WYE480
277480Solidly
Grounded WYE480Y/277
Can 600 Volt
Fuses
Be Used?
L-G
Volt
L-L
Volt
Secondary
System Type
System
Voltage
* Ungrounded delta systems - phase conductors are capacitively coupled to ground
Answers to Slash Rated Exercise 2 a)Yesb)Yesc)Yesd)Yes
Single Pole Interrupting Capabilities
A circuit breaker’s ability to open an overcurrent at
a specified voltage utilizing only one pole of the
circuit breaker
Can you get the correct answers to this exercise? (Answers at bottom ofpage)
34
Theoretically, a bolted line-to-ground fault may be higher than athree-phase bolted fault since the zero-sequence impedance canbe less than the positive sequence impedance. The ground faultlocation will determine the level of short-circuit current available.The prudent design engineer assumes that the ground faultcurrent equals at least the available three-phase bolted faultcurrent and makes sure that the overcurrent devices are ratedaccordingly.
SOLIDLY GROUNDED WYE SYSTEMSThe Solidly Grounded Wye system shown in Figure 4 is by far themost common type of electrical system. This system is typicallydelta connected on the primary and has an intentional solidconnection between the ground and the center of the wyeconnected secondary (neutral). The grounded neutral conductorcarries single-phase or unbalanced three-phase current. Thissystem lends itself well to industrial applications where 480V(L-L-L)three-phase motor loads and 277V(L-N) lighting is needed.
Figure 4 - Solidly Grounded WYE System
If a fault occurs between any phase conductor and ground (Figure5), the available short-circuit current is limited only by thecombined impedance of the transformer winding, the phaseconductor and the equipment ground path from the point of thefault back to the source. Some current (typically 5%) will flow in theparallel earth ground path. Since the earth impedance is typicallymuch greater than the equipment ground path, current flowthrough earth ground is generally negligible.
Figure 5 - Single-Pole Fault to Ground in Solidly Grounded WyeSystem
In solidly grounded wye systems, the first low impedance fault toground is generally sufficient to open the overcurrent device on thefaulted leg. In Figure 5, this fault current causes the branch circuitovercurrent device to clear the 277 volt fault. Because the branchcircuit device will clear the fault with only 277 volts across onepole, a slash-rated 480Y/277 volt circuit breaker is perfectlyacceptable.
This system requires compliance with single-pole interruptingcapability for 277 volt faults on one pole. If the overcurrent deviceshave a single-pole interrupting capability adequate for theavailable short-circuit current, then the system meets NEC® 110.9.
Although not as common as the solidly grounded wye connection,the following three systems are typically found in industrialinstallations where continuous operation is essential. Wheneverthese systems are encountered, it is absolutely essential that theproper application of slash ratings and single-pole interruptingcapabilities be assured. This is due to the fact that full phase-to-phase voltage can appear across just one pole. Phase-to-phasevoltage across one pole is much more difficult for an overcurrentdevice to clear than the line-to-neutral voltage associated with thesolidly grounded wye systems.
CORNER-GROUNDED-DELTA SYSTEMS (SOLIDLY GROUNDED)
The system of Figure 6 has a delta-connected secondary and issolidly grounded on the B-phase. If the B-phase should short toground, no fault current will flow because it is already solidlygrounded.
Figure 6 – Corner-Grounded Delta System (Solidly Grounded)
If either Phase A or C is shorted to ground, only one pole of thebranch-circuit overcurrent device will see the 480V fault as shownin Figure 7. A slash rated 480Y/277 volt circuit breaker could not beutilized on this 480 volt corner-grounded delta circuit because thevoltage to ground (480 volts), exceeds the lower of the two ratings(277 volts). This system also requires compliance with single-poleinterrupting capabilities for 480 volt faults on one pole because thebranch-circuit circuit breaker would be required to interrupt 480volts with only one pole.
240.85 Clarify Requirements for the Use of Slash-Rated Circuit Breakers andApplication of Individual Pole Interrupting Capabilities for Various GroundingSchemes
B
A
C
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
NN
480V
480V
Solidly Grounded WYE System
277V
277V277V
Solidly Grounded WYE System
B
A
C
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
N N
480V
480V
Fault to
conduit
Single pole must
interrupt fault current
Corner Grounded Delta System
B
A
480V
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
C
480V
480V
35
Figure 7 – Fault to Ground on a Corner-Grounded Delta System
A disadvantage of Corner-Grounded Delta systems is the inabilityto readily supply voltage levels for fluorescent or HID lighting(277V). Installations with this system require a 480-120Vtransformer to supply 120V lighting. Another disadvantage, asgiven on page 33 of IEEE Std 142-1991, Section 1.5.1(4) (GreenBook) is “the possibility of exceeding interrupting capabilities ofmarginally applied circuit breakers, because for a ground fault, theinterrupting duty on the affected circuit breaker pole exceeds thethree-phase fault duty.”
RESISTANCE GROUNDED SYSTEM“Low or High” resistance grounding schemes are found primarilyin industrial installations. These systems are used to limit, tovarying degrees, the amount of current that will flow in a phase toground fault.
“Low” resistance grounding is used to limit ground fault current tovalues acceptable for relaying schemes. This type of grounding isused mainly in medium voltage systems and is not widely installedin low voltage applications (600V or below).
The “High” Resistance Grounded System offers the advantage thatthe first fault to ground will not draw enough current to cause theovercurrent device to open. This system will reduce the stresses,voltage dips, heating effects, etc. normally associated with highshort-circuit current. Referring to Figure 8, High ResistanceGrounded Systems have a resistor between the center tap of thewye transformer and ground. High resistance grounding systemsare used in low voltage systems (600V or less).
With high resistance grounded systems, line-to-neutral loads arenot permitted per National Electrical Code®, 250.36(4).
Figure 8 - Resistance Grounded System
When the first fault occurs from phase to ground as shown inFigure 9, the current path is through the grounding resistor.Because of this inserted resistance, the fault current is not highenough to open protective devices. This allows the plant tocontinue “on line”. NEC® 250.36(3) requires ground detectors to beinstalled on these systems, so that the first fault can be found andfixed before a second fault occurs on another phase.
Figure 9 - First Fault in Resistance Grounded System
Even though the system is equipped with a ground alarm, theexact location of the ground fault may be difficult to determine. Thefirst fault to ground MUST be removed before a second phasegoes to ground, creating a 480 volt fault across only one pole ofthe affected branch circuit device. Figure 10 shows how the 480volt fault can occur across one pole of the branch circuit device. Itis exactly because of this possibility that a slash rated 480Y/277volt device can not be used in this system. 480 volts would beimpressed across one pole of the branch circuit device, eventhough it had been tested for only 277 volts.
Figure 10 - Second fault in Resistance Grounded System
The magnitude of this fault current can approach 87% of the L-L-Lshort-circuit current. Because of the possibility that a second faultwill occur, single-pole interrupting capability must be investigated.The IEEE “Red Book”, Std 141-1993, page 367, supports thisrequirement, “One final consideration for resistance-groundedsystems is the necessity to apply overcurrent devices based upontheir ”single-pole” short-circuit interrupting rating, which can beequal to or in some cases less than their ‘normal rating’.”
240.85 Clarify Requirements for the Use of Slash-Rated Circuit Breakers andApplication of Individual Pole Interrupting Capabilities for Various GroundingSchemes
B
A
480V
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
C
Single pole must
interrupt fault current
Fault to
conduit
Corner Grounded Delta System
Resistor
B
A
C
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
480V
480V
High Resistance Grounded System
B
A
C
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
480V
480VFirst fault
to steel
conduit
Resistor keeps first
fault current low:
5 Amps or so
Low Value of Fault Current
Because of Ground Resistor in
Short-Circuit Path
High Resistance Grounded System
B
A
C
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
480V
First fault
to steel
conduit
Second Fault
To Enclosure
High Value of Fault
Current Because
Ground Resistor No
Longer in Path
High Resistance Grounded System
Single pole must
interrupt fault current
480V
36
UNGROUNDED SYSTEMS
The Ungrounded System of Figure 11 offers the same advantagefor continuity of service that is characteristic of high resistancegrounded systems.
Figure 11 – Ungrounded System
Although not physically connected, the phase conductors arecapacitively coupled to ground. The first fault to ground is limitedby the large impedance through which the current has to flow(Figure 12). Since the fault current is reduced to such a low level,the overcurrent devices do not open and the plant continues to“run”.
Figure 12 - First Fault to Conduit in Ungrounded System
As with High Resistance Grounded Systems, ground detectorsshould (but are not required by the 2002 NEC®) be installed, towarn the maintenance crew to find and fix the fault before asecond fault from another phase also goes to ground (Figure 13).
Figure 13 - Second Fault to Conduit in Ungrounded System
The second fault from Phase B to ground (in Figure 13) will createa 480 volt fault across only one pole at the branch circuitovercurrent device. It is because of this possibility that a slash-rated device cannot be used on this type of system. A pole thatwas tested for 277 volts might see an overcurrent and try to open480 volts.
Again, the values from Table 1 for single pole interruptingcapabilities must be used for molded case circuit breaker systemsas the tradeoff for the increased continuity of service. The IEEE“Red Book”, Std 141-1993, page 366, supports this requirement,“One final consideration for ungrounded systems is the necessityto apply overcurrent devices based upon their “single-pole” short-circuit interrupting rating, which can be equal to or in some casesless than their normal rating.” In 250.4(B) Ungrounded Systems (4)Path for Fault Current of the 2002 NEC®, it is required that theimpedance path through the equipment be low so that the faultcurrent is high when a second fault occurs on an ungroundedsystem.
What are fuses’ single pole interrupting capabilities?By their inherent design a fuse’s interrupting rating is its single poleinterrupting rating. Modern current-limiting fuses are available thathave tested single pole interrupting ratings of 200,000 or 300,000amperes. The Low-Peak® LPJ_SP, KRP-C_SP, LPS-RK_SP andLPN-RK_SP Fuses all have UL Listed 300,000 ampere single poleinterrupting ratings. This is a simple solution to assure adequateinterrupting ratings for present and future systems no matter whatthe grounding scheme.
CONCLUSIONS
Two significant additions to NEC 240.85 were included in the 2002NEC®. They cover voltage ratings of slash-rated circuit breakersand single-pole interrupting capabilities of circuit breakers. Theproper application of both of these ratings is dependent upon thetype of grounding scheme utilized.
Slash-rated devices must be utilized only on solidly groundedsystems. This automatically eliminates their usage on resistance-grounded and ungrounded systems. They can be properly utilizedon solidly grounded wye systems, where the voltage to grounddoes not exceed the smaller of the circuit breaker’s two values andthe voltage between any two conductors does not exceed thelarger of the circuit breaker’s two values. Slash-rated devices cannot be used on corner-grounded delta systems whenever thevoltage to ground exceeds the smaller of the two ratings. Whereslash-rated devices will not meet these requirements, fully rateddevices are required.
An overcurrent protective device must have an interrupting ratingequal to or greater than the current available at its line terminals forboth three-phase bolted faults and for one or more phase-to-ground faults. Although most electrical systems are designed withovercurrent devices having adequate three-phase interruptingratings, the single-pole interrupting capabilities are easilyoverlooked.
Simple solutions exist to provide adequate interrupting ratings ifmolded case circuit breaker single-pole interrupting capabilities asshown in Table 1 are not sufficient. First, the manufacturer can beconsulted to see if single-pole interrupting capabilities are incompliance for the specific manufacterer’s circuit breaker.Second, air frame/power circuit breakers have tested single-poleinterrupting ratings that are 87% of the published three-pole rating.And third, current-limiting fuses are available that have testedsingle-pole interrupting ratings of 200,000 and 300,000 amps.
240.85 Clarify Requirements for the Use of Slash-Rated Circuit Breakers andApplication of Individual Pole Interrupting Capabilities for Various GroundingSchemes
Ungrounded System
B
A
SERVICE
PANEL
BRANCH
PANEL Steel
Conduit
A
B
C
C
480V
480V
Low Value of Fault Current
Because of Large Capacitively
Coupled Impedance to Ground
Ungrounded System
B
A
SERVICE
PANEL
BRANCH
PANEL Steel
Conduit
A
B
C
C
480V
480V
First fault
to steel
conduit
Ungrounded System
B
A
SERVICE
PANEL
BRANCH
PANEL Steel
Conduit
A
B
C
C
480V
480V
Second Fault
To Enclosure
High Value of Fault
Current Because
Large Impedance is
No Longer in Path
First fault
to steel
conduit
Single pole must
interrupt fault current
37
110.9 requires that overcurrent devices be able to safely interruptwhatever overcurrents they are apt to encounter. For branchcircuit fuses and circuit breakers, this means that they must safelyinterrupt both overloads and short-circuits, up to the maximumavailable short-circuit current. There are two ways that overcurrentprotective devices can meet these short-circuit requirements.They can be fully rated or they can be series rated.
Fully Rated SystemA fully rated system is one in which all of the overcurrent protectivedevices have an individual interrupting rating at least as great asthe available short-circuit current at their point of application. Fullyrated systems can consist of all fuses, all circuit breakers, or acombination of fuses and circuit breakers. See 110.9 discussion inthis booklet.
Figure 1 - Fully Rated Fuse System
What is a series rated system?A series rated system is a combination of circuit breakers, or fusesand circuit breakers, that can be applied at available short-circuitlevels above the interrupting rating of the load side circuitbreakers, but not above that of the main or line-side device. Seriesrated systems can consist of fuses protecting circuit breakers, orcircuit breakers protecting circuit breakers. Figure 2 illustrates afuse/circuit breaker series rated system.
Figure 2 - Series Rated Fuse Protecting Circuit Breaker System
Fully rated systems can be used everywhere, as long as individualinterrupting ratings are adequate. On the other hand, series ratedsystems have l imited applications and have extra NEC®
requirements that must be followed. 240.86 covers requirementsfor series rated systems.
What are the labeling requirements for series ratings?Factory labeling Requirement 240.86(A) requires that, when a series rated combination is used,the switchboards, panelboards, and loadcenters be tested, listedand factory marked for use with the series rated combinations tobe utilized. It is the responsibility of the panelboard, switchboardand loadcenter manufacturers to have a Nationally RecognizedTesting Laboratory listing for the complete package, whichincludes the series rated devices to be used in the specific gear.This is evidenced by a factory marked label affixed to theequipment - Figure 3. Because there is often not enough room inthe equipment to show all of the legit imate series ratedcombinations, UL 67 (Panelboards) allows for a bulletin to bereferenced and supplied with the panelboard. The bulletin is to beaffixed to the panelboard. These bulletins typically provide all ofthe acceptable combinations.
Field Labeling RequirementBesides the factory labeling requirement of 240.86(A) mentioned inthe previous paragraph,110.22 requires the installer to place labelsin the field which note the short-circuit rating of the seriescombination and call out for specific replacement overcurrentdevices to be utilized - Figure 3. See the 110.22 discussion in thisbooklet on this requirement.
Figure 3 - Field labeling requirement 110.22 and factory labelingrequirement 240.86(A)
What fuses protect which circuit breakers?Unfortunately, it is often difficult to determine which combinationsgo with which panelboards. In order to clear the confusion,Bussmann® has researched the major manufacturers’ applicationliterature and published the tables. These tables show, bymanufacturer, the various combinations of fuses and circuitbreakers that are acceptable by panelboard type. These tablesare published on www.bussmann.com under ApplicationInformation/Publications. Table 1 is a partial reprinting of one ofthese tables.
240.86 Series Ratings
ISC=300,000 Amp
Available Short Circuit
ISC=300,000 Amp
Available Short Circuit
LPJ-200SP Fuse
300,000 A Interrupting Rating
LPJ-20SP Fuse
300,000 A Interrupting Rating
Contractor Installed Label
CAUTIONSeries Rated Combination System
with LPJ-200SP fuses in MDP1
Rated 100,000 Amperes
Replace with XXX
Circuit Breakers Only
CAUTIONSeries Rated Combination System
with panel LDP1
Rated 100,000 Amperes
Replace with Bussmann
LPJ-200SP Fuses Only
Panel LDP1
Panel
MDP1
Contractor Installed Label
NRTL Listing of Series
Combination Rating of
100,000 amperes when
XXX Circuit Breaker
Protected by Maximum
of 400 A Class J Fuse
Panel Mfr’s Label
ISC=200,000 Amp
Available Short Circuit
ISC=300,000 Amp
Available Short Circuit
LPJ 400 SP Fuse
300,000 A Interrupting Rating
20A Circuit Breaker
10,000 A IR
Series Rated Combination200,000 A. IR
38
Table 1 Example of Available Fuse / Circuit Breaker SeriesRating Tables by Manufacturer (partial table)
I
What are the series rated system motor contribution limitations?One critical requirement limits the use of series rated systemswhere motors are connected between the line-side (protecting)device and the load-side (protected) circuit breaker. 240.86(B)requires that series ratings shall not be used where the sum ofmotor full load currents exceeds 1% of the interrupting rating of theload-side (protected) circuit breaker. An application of this typewould provide added short circuit current, via the motorscontributing to a fault, in excess of what the load side (protected)circuit breaker was tested to handle. Example in Figure 4.
Figure 4 - Example of violation of 240.86(B) due to motorcontributions.
What are other series rated system limitations?The biggest disadvantage of a series rated system is that, bydefinition, the line side (protecting) device must open at the sametime, and in conjunction with the load side (protected) circuitbreaker. This means that the entire panel loses power because thedevice feeding the panel must open under medium to high-levelshort circuit conditions - Figure 5. As a result, series rated systemsshould not be used in health care facilities (517.17), continuousprocess industrials, computer rooms, emergency circuits (700.25FPN), elevator circuits (620.62), main switchgear, or criticaldistribution panels. On the other hand, fully rated systems can beselectively coordinated so that only the device closest to the shortcircuit opens, leaving the rest of the system up and running.
Figure 5 - Example of lack of selective coordination inherent inseries rated systems
Another disadvantage of the series rated system is the likelypossibility of future expansions or system upgrades, where the newavailable short-circuit current exceeds the series rating. Thetypical solution at that point is to tear out the existing series ratedpanel and replace it with a new, properly rated one.
Series Rating Inspection Form Check ListIn order to help in meeting the multitude of NEC® requirementssurrounding the use of series rated combinations, Bussmann® hascreated an inspection form check list. This form can be filled out bythe installer and verified by the inspector. The form provides acompliance checklist and background information, on the reverseside, on the various NEC® requirements. This form is available onwww.bussmann.com and the last two pages of this booklet.
Where is more information on series rated systems?A more complete discussion of series rated systems and the fuse /circuit breaker series rated tables by manufacturer are onwww.bussmann.com under Application Information/Publications.
240.86 Series Ratings
Load Side Circuit Breaker
Series Rated Combination ChartLine Side Fuse
Series Rated Systems
10,000 A. I.R.
Series Rated
Combination
22,000 A. I.R.
Motor F.L.A. > 100A (1% IR)
This does not
comply with NEC®
240.86(B)
Motor Contribution
OPENS
NOT AFFECTED
UNNECESSARY
POWER LOSS
Fault
Series Rated
Lack of Selective Coordination
Maximum Maximum Line Side Max FuseSystem Voltage SCCR Fuse Current Rating Circuit Breaker Amps Poles
QO, QOB ALL 1, 2, 3QO, QOB (AS) ALL 1, 2, 3QO, QOB (GF I) ALL 1, 2, 3QO, QOB ALL 1, 2, 3QO, QOB (AS) ALL 1, 2, 3QO, QOB (GF I) ALL 1, 2, 3
Note for NQOD Panelboards: 1P for use at 120V Only
Maximum Maximum Line Side Max FuseSystem Voltage SCCR Fuse Current Rating Circuit Breaker Amps Poles
Note for NEHB Panelboards: 1P for use at 277V Only
Maximum Maximum Line Side Max FuseSystem Voltage SCCR Fuse Current Rating Circuit Breaker Amps Poles
100kA JJS, LPJ 400200kA JJS, LPJ 200
Note for NF Panelboards: 1P for use at 277V Only
EDB, EGB, EJB ALL480Y/277 Vac 1, 2, 3
Load Side
Load Side
NEHB Panelboards
240 Vac 200kA
JJS, LPJ
NQOD Panelboards
Load Side
NF Panelboard
200
400JJN
(See Notes Below)
(See Notes Below)
(See Notes Below)
NOTE (1): The data in these charts was compiled from information in Square D, Series Rating Data Bulletin No. 2700DB9901 and Square D Digest 171. Cooper Bussmann assumes no responsibility for the accuracy or reliability of the information. The information contained in the tables may change without notice due to equipment design modifications.
NOTE (2): The line-side fused switch may be in a separate enclosure or in the same enclosure as the load-side circuit breaker. A line-side fused switch may be integral or remote.
NOTE (3): Max fuse current rating denotes the largest amperage fuse that may be used for that series rated combination. A lower amperage fuse may be substituted for the listed fuse.
39
240.90 and 240.2 Supervised Industrial InstallationsWhat is the intent of Sections 240.90 and 240.2?The special provisions of Part VIII of Article 240 apply only to theprocess and manufacturing portions of an industrial installation.The intent of Part VIII is to limit its use to large industrial locations.
240.2 defines a supervised industrial installation and thiscriteria must be met: (1) the maintenance crew must be qualifiedand under engineering supervision, (2) Total load must be 2500KVA or greater as calculated in accordance with Article 220, (3)and, there must be at least one service at 277/480 or 480 volts orhigher.
240.92(B)(1) Short-Circuit and Ground-Fault Protection (Supervised Industrial Installations only)What are the requirements of 240.92(B)(1)?The cable can be 100' or less as long as the primary overcurrentdevice is no larger than 150% of the ampacity of the secondaryconductor multiplied by the secondary to primary voltage ratio, or
The conductors are protected by a differential relay with a tripsetting not greater than the secondary conductor ampacity, or
The conductors are shown to be protected under short-circuitconditions by engineering calculations. Typical methods are foundin IEEE Color Books, Canadial Electrical Code, IEC WiringRegulations, ICEA (Insulated Cable Engineers Association), andmanufacturers’ literature.
What does this section mean?Conductors may be connected directly to the secondary terminalsof a transformer of a separately derived system, withoutovercurrent protection at the connection if the conductors meetspecial requirements for short-circuit, overload, and physicalprotection per 240.92(B)(1), (2), and (3).
240.92(B)(2) Overload Protection (Supervised Industrial Installations only)What are the requirements of 240.92(B)(2)?Overload protection can be achieved by terminating in oneovercurrent device, or in up to six overcurrent devices, grouped inone location, that add up to no more than the ampacity of theconductor. Engineering calculations can be used to demonstrateoverload protection. Finally, relays can be used to limit the load tothe ampacity by opening devices on the line or load side.
40
What are the requirements of this section?This specifies the location of the overcurrent protective device
for circuits of over 600 volts. It requires that protection be providedat the beginning of the feeder or branch-circuit unless anotherlocation is determined under engineering supervision. Animportant change to this 2002 section requires the engineering
supervision to consider the available fault current, overcurrentprotective devices’ short circuit current characteristics and theconductor withstand characteristics, as found in IEEE Color Books,Canadian Electrical Code, IEC Regulations, ICEA (Insulated CableEngineers Association), and manufacturers’ literature.
240.100 Feeder and Branch Circuit Protection Over 600 Volts Nominal
240.100(C) Conductor Protection
What is the meaning of 240.100(C)?This requires that the short-circuit ratings of the cable not beexceeded. These ratings can be found in the IEEE Color books,
ICEA (Insulated Cable Engineers Association), IEC WiringRegulations, Canadian Electrical Code, and manufacturers’literature.
240.92(C) Outside Feeder Taps (Supervised Industrial Installations only)What are the requirements for Outside Feeder Taps?Outdoor conductors may be tapped to a feeder or connected tothe secondary of a transformer without overcurrent protection atthe tap or connection if all 5 of the following conditions are met.
1) The conductors must be protected from physical damage.2) The sum of the one to six grouped overcurrent devices at thetermination of the outdoor conductor must limit the load to theampacity of the conductor.3) The conductors are outside except at the point of loadtermination.4) The overcurrent device must be a part of the disconnectingmeans or immediately adjacent to it.5) The disconnecting means are readily accessible and locatedoutside or inside nearest the point of entrance.
240.100(B) Protective Devices
What does 240.100(B) require?This section is for applications over 600 volts. It requiresovercurrent protective devices to be capable of detecting andinterrupting all currents that can occur in excess of their tripsetting or melting point.
An electrical system with proper grounding is designed to providefor personnel safety and equipment protection. A system that canlose its grounding integrity or failure to provide a system withproper grounding is a serious safety hazard. Two importantaspects of systems with proper grounding are:
(1) Low impedance ground path
When a ground fault occurs this reduces the electric shock hazardif a person comes in contact with any of the metalic electricalenclosures or conduit. A poor ground return path may result inenclosures or conduit having a lethal potential. A personunknowingly coming in contact may be electrocuted.
The lower the impedance of the ground return path, the higher thefault current if a line to ground fault occurs; the higher the faultcurrent the more likely the overcurrent protective device willoperate to clear the fault.
(2) The ground fault current path must “safely carry the maximumground fault current”.
This means that the equipment grounding conductors that mightcarry a ground fault must be selected by engineering analysis. Thisanalysis should take into account maximum possible fault current,the operating characteristics of the fuses or circuit breakers, andthe damage characteristics of these conductors. All to often, thisaspect is overlooked or not properly analyzed. The result can besituations where the equipment grounding conductors changecharacteristics due to a fault and then the grounding systemintegrity is no longer as intended.
Merely sizing the equipment grounding conductors per Table250.122 may not provide a conductor size that can “safely carrythe maximum ground fault current”. The solution may be toincrease the size of the equipment ground conductor or to use thewire size in Table 250.122 in conjunction with current limiting fusesfor the circuit protection. Be sure to read and comply with thefollowing grounding requirements of Article 250.
41
250 Grounding
250.2 Definitions (Grounding)
250.4(A)(4) & (5) General Requirements for Grounded Systems
What is the definition for “effective ground-fault current path”?The NEC® definition is:
An intentionally constructed, permanent, low-impedanceelectrically conductive path designed and intended to carrycurrent under ground fault conditions from the point of a groundfault on a wiring system to the electrical supply source.
250.4 covers the “General Requirements for Grounding andBonding”. (A) covers these general requirements for groundedsystems.
What is the meaning of 250.4(A)(4)?All the electrically conductive materials that could becomeenergized are to be connected together in such a manner thatthere is an “effective ground-fault current path”. In this way, if aline to ground fault occurs there is an adequate path to completethe circuit.
What is the requirement in 250.4(A)(5) for “Effective Ground-FaultCurrent Path”?This NEC® section states:
Electrical equipment and wiring and other electrically conductivematerial likely to become energized shall be installed in amanner that creates a permanent, low impedance circuitcapable of safely carrying the maximum ground-fault currentlikely to be imposed on it from any point on the wiring systemwhere a ground fault may occur to the electrical supply source.The earth shall not be used as the sole equipment groundingconductor or effective ground-fault current path.
This means that the ground-fault current path must be designed(not left to chance) to be low-impedance and capable ofadequately carrying the current that could flow under groundfault conditions. For instance, this means the equipmentgrounding conductors must be designed to be sure they do notbecome damaged or annealed due to possible ground faultcurrents. It is not enough to merely select equipment groundingconductors based on Table 250.122. See the discussion in thisbooklet on 250.122
250.90 Bonding Requirements and Short-Circuit Current Rating
250.4(B)(4) General Requirements for Ungrounded Systems
250.96(A) Bonding Other Enclosures and Short-Circuit Current Requirements
What does this section mean?All bonding provided must have the capacity to conduct safely anyfault current it is likely to see.
What do these sections require?All materials used in the grounding and bonding of equipmentshall be capable of safely carrying the short-circuit current thatcould flow through the ground path. This will, in many cases,
require the use of a current-limiting fuse to protect the equipmentfrom damage. See Section 110.10 for more on componentprotection.
(B) covers “General Requirements for Grounding and Bonding” ofungrounded systems.
What is the requirement in 250.4(B)(4) for “Path for Fault Current”?
Electrical equipment, wiring, and other electrically conductivematerial likely to become energized shall be installed in a mannerthat creates a permanent, low-impedance circuit from any pointon the wiring system to the electrical supply source to facilitatethe operation of overcurrent devices should a second fault occuron the wiring system. The earth shall not be used as the soleequipment grounding conductor or effective fault-current path.
Ungrounded systems are designed so that the first line to groundfault does not cause a fault current of sufficient magnitude to openthe overcurrent protective devices. However, if a second fault toground occurs (involving another phase) it is imperative to clearthe fault current as quickly as possible. The low-impedance pathof likely ground faults will facilitate the operation of the overcurrentprotective devices. To better understand this; see the discussionin this booklet for ungrounded systems under 240.85. Thediscussion includes diagrams for the first and second ground faultsin ungrounded systems.
Although 250.4(B)(4) does not explicitly mention resistancegrounded systems, the same conditions for the first and secondfaults occur with these systems too.
42
43
1/0 AWG CopperGroundingElectrodeConductor
50,000A RMS
400A Non-Current-Limiting Device
Service EquipmentMetal Enclosure
Non-MetallicRaceway
GroundedServiceNeutral
3 AWG CopperEquipmentGroundingConductor
VIOLATION
Would need to increaseEquipment GroundingConductor to 2/0.
3ØLoad
Metal Enclosure
500 kcmil Copper
1/0 AWG CopperGroundingElectrodeConductor
50,000A RMS
400A Current-Limiting Device
Service EquipmentMetal Enclosure
Non-MetallicRaceway
GroundedServiceNeutral
3 AWG CopperEquipmentGroundingConductor
3ØLoad
Metal Enclosure
500 kcmil CopperCOMPLIANCE
Conforms to Section 110.10,Table 250.122, and 250.4(A)(5) or250.4(B)(4)
250.122 Sizing of Equipment Grounding ConductorsWhat are the ramifications of 250.122 and especially the note at thebottom of Table 250.122?The integrity of the grounding path is essential for safety; itfacilitates the operation of the overcurrent protective devices.Improper sizing of the grounding conductors can result in theirannealing, melting or vaporizing before the protective deviceclears the circuit. Generally, the grounding electrode conductorand the equipment grounding conductors are smaller than thecircuit conductors and their ampere rating is less than that of theovercurrent protective device. The protective device may be tooslow to protect an undersized conductor against high faultcurrents (see Section 240.1 of this Bulletin). Consideration mustbe given to the size of the grounding conductors, their withstand,the magnitude of ground fault currents, and the operatingcharacteristics of circuit overcurrent devices. Where the protectivedevice is not fast enough to protect the undersized equipmentgrounding conductor, the conductor size may need to beincreased, or a different overcurrent device could be chosenwhich could provide adequate protection for the conductor. Thissection of the NEC® requires this analysis.
Caution: Table 250.122 in the NEC® gives “Minimum SizeEquipment Grounding Conductors for Grounding Raceway andEquipment” This table has a note that reads:
Note: Where necessary to comply with 250.4(A)(5) or250.4(B)(4), the equipment grounding conductor shall be sizedlarger than given in this table.
For example, Table 250.122 allows a circuit protected by a 400ampere overcurrent device to have a 3 AWG copper equipmentgrounding conductor. If the 400 ampere overcurrent device takesone cycle to open in a circuit where 50,000 amperes are available,typical cable manufacturer’s withstand charts show that the 3AWG conductor would be damaged. One solution would be toinstall a 2/0 AWG copper equipment grounding conductor whichwould be able to withstand the 50,000 amperes for one cycle. Theother alternative is to limit the 50,000 amperes to within the 22,000ampere for one cycle limit of the 3 AWG conductor. This can beaccomplished easily with the use of current-limiting fuses.
What is the importance of 250.122(D)?Since instantaneous only circuit breakers (MCP’s) can be set
as high as 1700% of motor full-load current, the equipmentgrounding conductor shall be sized based on the motor overloadrelay.
What is the problem with 250.122(F)(2)?This allows for protection of a parallelled equipment groundingconductor in a multiconductor cable with equipment ground faultprotection.
However, ground fault protection is not a substitute forovercurrent protection. It is designed to prevent the burn down ofswitchboards. It was not designed for, nor is it fast enough toprotect equipment grounding conductors from annealing undershort-circuit conditions.
Take a 4,000 ampere circuit with 50,000 amperes available, asan example. Nine 750 kcmil/phase with one 500 kcmil as an EGCcould be used. However, if 9 conduits are utilized, the 1996 coderequired a 500 kcmil EGC in each conduit. The requirementsstarting in the 1999 code would allow for a 2 AWG EGC in eachconduit. (75°C, 750 kcmil is rated for 475 amps, and EGCassociated with a 500 ampere overcurrent device is a 2 AWG.)The I2t required to anneal a 2 AWG copper equipment groundingconductor (Soares’ validity rating) is 24.5 x 106 ampere squaredseconds. The I2t let-through for GFP, set at 475 amperes for atypical delay of .3 seconds at 50,000 amperes, is 50,000 x 50,000x .3 = 750 x 106. That’s more than 30 times the I2t needed to annealthe copper. After a fault, the equipment grounding conductorwould not be “tight” under the lug. In other words, there would nolonger be an effective ground fault path. The 500 kcmil required bythe previous NEC® has an I2t rating of 1,389 x 106 ampere squaredseconds, more than enough to stay tight under a lug after a faultoccurs. For more detailed explanation of these concepts, reviewthe latest edition of Soares Book on Grounding, now published byIAEI.
Another serious conductor damage level is the Onderdonkconductor melting point. This is the I2t at which the metal of agiven size conductor melts. In the example of the previousparagraph the Onderdonk I2t melting point for 2 AWG copper is83.1 x 106 and 500 kcmil is 4,721. x 106. This means in theexample given with the GFP I2t let-thru of 750 x 106, that the 2 AWGcopper equipment grounding conductor could melt. So after aground fault there would no longer be an effective ground faultcurrent path. If this did occur, the electricians repairing the faultmay not even notice that the equipment grounding conductor is nolonger effective unless they ran specific tests for this condition.
What does 368.12 require?368.12 requires busway used as a feeder to have devices or plug-in connections for tapping off feeder or branch circuits from thebusway which contain fuses for the feeder or branch-circuits.Exceptions are permitted as follows:
1. As permitted in 240.212. For fixed or semifixed light fixtures (luminaires), where the
branch-circuit overcurrent device is part of the fixture(luminaire) cord plug on cord-connected fixtures (luminaires).
3. Where fixtures (luminaires) without cords are plugged directlyinto the busway and the overcurrent device is mounted on thefixture (luminaires).
368.11 and 368.12 Busway Reduction and Feeders or Branch Circuits
408.16 Panelboard Overcurrent ProtectionWhat is the meaning of 408.16(A)?Lighting and appliance branch circuit panelboards must beprotected by a main overcurrent device (up to two sets of fuses, aslong as their combined ratings do not exceed that of thepanelboard), unless the feeder has overcurrent protection notgreater than the rating of the panelboard.
What is the meaning of 408.16(B)?A Power Panelboard having supply conductors which include aneutral and having more than 10% of its overcurrent devicesprotecting branch circuits of 30 amperes or less, shall haveindividual protection on the line side not greater than the rating of
What is the importance of this section?It offers an overview of protection for motors, motor circuits, motorcontrollers, and motor control centers.
430.1 Scope of Motor Article
What is the importance of this section?It states that conductors supplying motors shall be selected fromapplicable tables in Article 310 and Section 400.5. Thedetermination of conductor ampacity, or ampere rating ofswitches, branch circuit protection, etc., should be taken from themotor F.L.A. tables in Article 430, Tables 430.147 through 430.150.
There is an exception for listed motor-operated appliances withboth a horsepower rating and a full load current rating marked onthe nameplate. In this case, the ampere rating on the nameplate
the panelboard. Individual protection is not required when the powerpanel is used as service equipment in accordance with 230.71.
General Comment—The service entrance split bus load center orpanelboard having up to 6 main disconnects is no longer permittedon new installations.
The tap rules found in 240.21 do not remove theserequirements for lighting and appliance branch circuit panelboardprotection, nor do they remove the requirements for transformerprotection found in 450.3.
430.6 Ampacity of Conductors for Motor Branch Circuits and Feedersshould be used to determine the ampacity or rating of the motorcircuit conductors, disconnecting means, controller, and thebranch circuit short-circuit and ground fault protection. Similarexceptions exist for multispeed motors (Exc. 1) and equipmentemploying shaded pole or permanent-split capacitor-type fan orblower motor.
The separate overload device should always be based on thenameplate current rating.
What does 368.11 section mean?Overcurrent protection is required whenever busway is reduced inampacity unless all of the following conditions are met:1. Industrial establishment only.2. Length of smaller bus does not exceed 50 feet.3. Ampacity of smaller bus must be at least 1/3 that of the
upstream overcurrent device.4. Smaller bus must not contact combustible material.
44
What is the purpose of the fine print note in this section?The fine print note is intended to point out the need for conductorderating at high ambient temperatures. It also directs the user tobe aware of other information, such as conductor size andnumber, to assure proper application.
310.10 Temperature Limitation of Conductors
35°CEnvironment
(3) 12 AWG 75°C Copper Conductorsin a Raceway
This fuse is sized at 25 (amperes) x .94 (temperature deratingfactor) = 23.5 amperes. The next standard size is 25 amperes, but the obelisk for 12 AWG copper, 75°C in Table 310.16 directs the reader to Section 240.4(D) where the maximumovercurrent device is given as 20 amperes.
35°CEnvironment
(9) 12 AWG 75°C C Copper in a Raceway
This fuse is sized at 25 (amperes) x .94 (temperature deratingfactor) x .70 (9 conductors in a raceway derating factor fromTable 310.15(B)(2)(a) to ampacity tables) = 16.45 amperes.The next standard size is a 20 ampere Fuse.
45
430.32 Motor Overload Protection
430.8 Marking on Controllers
What does this section require?This section clarifies the need for overload protection in all threephases of a 3-phase, 3-wire system, where one phase also servesas the grounded conductor.
430.36 Fuses Used to Provide Overload and Single-Phasing Protection
What are the typical ways of providing motor overload protection externalto the motor?Generally, motor starters with overload relays and/or Class RK1and RK5 dual-element fuses are used to provide motor runningprotection.
Typically, how are the devices selected for protection of motors?With starters and overload relays, the proper heater element isselected from manufacturers’ tables based on the motornameplate full-load current rating. The level of protection reachedin this selection process complies with Article 430.
When employing only dual-element Class RK1 and RK5 fusesfor motor running overload protection, the rating of the fuse shouldbe as follows:
M
M
LOW-PEAK YELLOWClass RK1 Dual-ElementFuse
Do fuses sized as above also provide branch circuit protectionrequirements?Yes. Sizing FUSETRON® Class RK5 and LOW-PEAK® YELLOW™Class RK1 Dual-Element fuses for motor running overloadprotection also provides the necessary short-circuit protection per430.52. The use of these dual-element fuses permits close sizing.Thus, fuse case sizes often can be smaller, thereby permitting theuse of smaller switches.
Can circuit breakers and fuses other than Class RK1 and RK5 dual-element fuses be used to give motor overload protection?Not generally. The conventional circuit breakers usually must besized at 250% of the motor full-load amperes to avoid tripping onmotor starting current, and thus cannot provide overloadprotection. Instantaneous only circuit breakers or motor short-circuit protectors are only equipped with a short-circuit trippingelement and, therefore, are incapable of providing overloadprotection. For motor applications, the non-time-delay fuses suchas the LIMITRON® KTS-R fuses normally have to be sized at 300%of a motor full-load current rating to avoid opening on motor start-up and, therefore, do not provide overload protection.
When single-phasing occurs on a 3-phase motor circuit,unbalanced currents flow through the motor, which can damagethe motor if not taken off-line. Class RK1 and RK5 dual-element,time-delay fuses, sized for motor overload protection, can providesingle-phase damage protection. See 430.36.
Footnote–Abnormal Motor Operation: The application of motors under certain abnormaloperating conditions often requires the use of larger size fuses than would normally berequired. The use of oversize fuses limits protection to short-circuit or branch circuitprotection only. The types of abnormal motor installations that may be encountered includethe following: (a) Fuses in high ambient temperature locations. (b) Motors having a highCode Letter (or possibly no Code Letter) with full-voltage start. (c) Motors driving highinertial loads or motors which must be frequently cycled off-and-on. Typical high inertialloads are machines such as punch presses having large mass flywheels, or machinessuch as centrifugal extractors and pulverizers, or large fans which cannot be brought up tospeed quickly. (d) High efficiency motors with high inrush currents.M M
Size at 115%or less of motorfull-load amps
S.F. less than 1.15ortemp. rise over 40°C.
S.F. 1.15 or higherortemp. rise 40°C.or less
Size at 125%or less of motorfull-load amps
LOW-PEAK YELLOW Class RK1 orFUSETRON ClassClass RK5Dual-Element Fuse
LOW-PEAK YELLOW Class RK1 orFUSETRON ClassClass RK5Dual-Element Fuse
460 Volts10HPF.L.A. = 14A
LPS-RK171/2SP
B
A
C
LPS-RK171/2SP
LPS-RK171/2SP
M
What is the purpose of this FPN?This FPN was added to warn the user about the delicate nature ofsmall contacts and overload relays which can easily be damagedunder short circuit conditions unless properly protected bycurrent-limiting protective devices.
46
What is the basic content of this section?This Section deals with the protection of motor branch circuitsagainst short-circuit damage. It establishes the maximumpermissible settings for overcurrent protective devices. (Branchcircuits include all the circuit components–wire, switches, motorstarters, etc.) As is apparent in Code Table 430.52, maximumsettings vary with different types of motors, each type havingunique starting characteristics. Motors to which the maximumpermissible settings or ratings apply (shown in the condensedTable below) include all types of single-phase, three-phasesquirrel cage and three-phase synchronous motors. The tablebelow does not apply to Design E, Wound Rotor, and dc motors.
These maximum values do not preclude the application oflower sizes. Also, compliance with Section 110.10 must beanalyzed. Motor starters have relatively low short-circuit currentwithstands. Refer to Buss® bulletin SPD for specific fuserecommendations.
Maximum Rating or Setting of Protective Devices†
Fuse Circuit Breaker*Non-Time-Delay Dual-Element Instantaneous InverseAll Class CC Time-Delay Type Only Time Type300% 175% 800% 250%†See Article 430, 430.52.*For latest information, check manufacturer’s data and/or Underwriters’ Laboratories U.L.Standard #508 for damage and warning label requirements.
What about starter withstandability and Section 110.10 requirements forcomponent protection?
Under short-circuit conditions, the branch circuit protective devicemust protect the circuit components from extensive damage.Therefore, the following factors should be analyzed: availableshort-circuit current, let-through characteristics of the overcurrentprotective device, and starter withstandability.
As an Example, this Size 1 Starter has been tested by U.L. with22,000 ampere available short-circuit current per U.L. Standard508. Thus, in the example above, the available short-circuitcurrents should not exceed 22,000 amperes since the circuitbreaker is not current-limiting.
Additionally an MCP, if used in a combination controller, mustbe listed for that specific combination. The MCP cannot be used asa separate motor branch circuit short-circuit protective device toprotect a motor controller. Applications of MCP’s on many motors,i.e., high efficiency or high Code Letter, may cause the MCP tooperate needlessly, even when sized at 1700% of motor current.
In the circuit below using a Buss® LOW-PEAK® YELLOW™ dual-elementtime-delay fuse, can available short-circuit current exceed 22000amperes?
Yes. Because the LOW-PEAK® YELLOW™ fuse is “current-limiting,” excellent short-circuit protection is provided, even thoughavailable short-circuit current greatly exceeds 22,000 amperes.(Specifically, the LOW-PEAK® YELLOW™ fuse would giveprotection against fault currents through 200,000 amperes.) It isalso significant to note that because the Class RK1 LOW-PEAK®
YELLOW™ fuse is a time-delay fuse, it actually could be sized at125% of full-load current or the next larger size (30 amperes) withthe advantage of permitting the use of a smaller disconnect switch,and providing backup overload protection and even better short-circuit protection.
These maximum sizing allowances are all overridden if amanufacturer's label shows overcurrent protection values lowerthan what 430.52 allows.
The overload relay heater elements of a motor controller oftenhave relatively low short-circuit current withstand ratings. Themaximum ratings of protective devices given in Table 430.52, thus,do not necessarily apply since they may be too large to provideadequate protection. Consequently, the starter manufacturer oftenincludes an overload relay table within the starter enclosure. If thetable states the maximum fuse size ratings to be used which willadequately protect the overload relays, the protective device mustbe a fuse.
TYPICAL EXAMPLE: The chart shown below is typical for startermanufacturers and may be found on the inside of the door of thestarter enclosure. (See starter manufacturer for specific recom-mendation.)
240.6 has an exception listing additional standard fuse ampereratings of 1, 3, 6 and 10 amperes. The lower ratings were added toprovide more effective protection for circuits with small motors, inaccordance with 430.52 and 430.40 and requirements forprotecting the overload relays in controllers for very small motors.Fuse manufacturers have available other intermediate fuse ampereratings to provide closer circuit protection (such as sizing ClassRK1 and RK5 dual-element fuses at 125% of motor current) or tocomply with “Maximum Fuse” sizes specified in controllermanufacturer’s overload relay tables.
430.52 Sizing of Various Overcurrent Devices for Motor Branch Circuit Protection
M
NON-CURRENT-LIMITINGCIRCUIT BREAKER
SIZE 1 STARTER LISTED FOR 22,000AMPS WITH THE 50A BREAKER
Short-circuit currentshould not exceed22,000 amperes
71/2 HP(22A)
SIZE 1 STARTER LISTED FOR 200,000AMPS WITH A 40A CLASS R FUSE
LOW-PEAK DUAL-ELEMENT CLASS RK1 FUSEMax. size: 175% x 22 = 38.5. Go to nextstandard size of 40A.
230V3Ø M
71/2 HP(22A)
Heater Full Load Current Max.Code of Motor (Amperes) FuseMarking (40°C Ambient)XX03 .25- .27 1XX04 .28- .31 3XX05 .32- .34 3XX06 .35- .38 3
Is there a way to be sure a motor starter has legitimate short circuitprotection?
Yes, specify Type 2 motor starter protectionMotor starters typically have low short circuit withstand ratings. Thedamage level permissible under short circuit testing per UL 508 formotor starters may not meet the expectations of many designersand users; the heaters are allowed to disintegrate and the contactsallowed to weld or disintegrate. This level of protection is referredto as Type 1. There is another alternative, which is Type 2protection. Type 2 is “no damage” protection; heater elements arenot permitted to be damaged and the contacts must be able to beeasily parted (if slightly welded). Motor starters in conjunction withspecific type and size current-limiting fuses that are tested to Type2 protection are available. Bussmann® publishes tables by startermanufacturers of the fuse type and size that provide Type 2protection. Visit the “Application / Publications” section ofwww.bussmann.com for these tables by starter manufacturer.
Also, see the discussion in this booklet under 110.10. For a morein-depth discussion see the Bussmann® SPD, Electrical ProtectionHandbook.
430.52(C)(5) allows other fuses to be used in place of those allowed inTable 430.52. Why is this Code provision necessary?
Some “solid-state” motor starters and drives require fusesspecifically designed to protect semiconductor components. TheCode provision was necessary in order to give branch circuit,short-circuit and ground fault “status” to these fuses.
What is the significance of 430.52(C)(3) Exc. 1, (C)(6) & (C)(7)?Design B energy efficient motors are included with Design Emotors as far as protection with instantaneous trip circuit breakers(MCP’s), self-protected combination controllers, and motor short-circuit protectors (MSCP’s) are concerned. These branch-circuitdevices may be set as high as 1700% of the motor full load currentas shown in Tables 430.147 through 430.150. Motor controllersmay have difficulty opening at current levels just below the 1700%rating.
430.53 Connecting Several Motors or Loads on One Branch CircuitWhat does this section mean?Simply stated, branch circuit protection for group motorinstallations must be testing agency and factory listed for suchinstallations. This listing can be accomplished as a factoryinstalled assembly with specified marking, or field installed asseparate assemblies l isted for use with each other, withinstructions provided by the manufacturer. For the best protectionof group motor installations, the branch circuit protective devicemust be current-limiting. The Fine Print Note reference to 110.10emphasizes the necessity to comply with the component short-circuit withstand ratings.
If the equipment nameplate specifies “MAX” fuse for a multimotorcircuit, what must the branch circuit device be?It must be a fuse, rated at not more than what is specified on thenameplate. The best type of fuse to use is a current-limiting fuse.
If the equipment nameplate specifies “MAX” circuit breaker of a certainmanufacturer and part number, what must be used?Only that specific type and manufacturer may be used. In otherwords, that controller has been tested and listed with a certaincircuit breaker, with certain short-circuit characteristics. Althoughbreakers of other manufacturers and interrupting ratings may beinterchangeable, that substitution is not allowed by 430.53(C)(3).This is due in part to the fact that there is no standardization ofshort-circuit performance of circuit breakers. Also, some circuitbreakers exhibit current-limitation, to a degree, while not beingmarked current-limiting. This could prove to be a hazard if a non-current-limiting breaker of the same form and fit were to beinstalled.
M
M
BRANCHCIRCUITFUSE
Nameplate specifies max fuse asbranch circuit device.
430.52 Sizing of Various Overcurrent Devices for Motor Branch Circuit Protection
430.62 and 430.63 Sizing Fuses for Feeders with Motor Loads
What are the requirements for sizing fuses for feeders with only motorloads?Per NEC® 430.62, a fuse protecting a feeder supplying a specificfixed motor load(s), with conductor sizes based on 430.24, isrequired to have an ampere rating not greater than thepermissible largest ampere rating of the branch-circuit short-circuit and ground-fault protective device for any motor suppliedby the feeder (based on the maximum permitted value per 430.52& Table 430.52), plus the sum of the full-load current of the othermotors of the group. The ampere rating of the feeder fuses ispermitted to be based on the ampacity of the feeder conductorsthat have an ampacity greater than required by 430.24. For a fusesupplying a motor control center, the provisions of 430.94 apply.
What are the requirements for sizing fuses for feeders with motor andother loads?Per NEC® 430.63, a fuse protecting a feeder which supplies motorload and, in addition, a lighting or a lighting and appliance load, isrequired to have an ampere rating that is sufficient to carry thelighting or the lighting and appliance load as determined inaccordance with Articles 210 and 220 (125% of continuous non-motor load plus 100% of the non-continuous non-motor load) plus,for a single motor, the ampere rating permitted by 430.52 andTable 430.52, and, for two or more motors, the ampere ratingpermitted by 430.62. For a fuse supplying a motor control center,the provisions of 430.94 apply.
48
What does this section mean:
As shown in the above circuit, the motor control circuit tapped onthe load side of the motor branch circuit protective device can be
What does this section mean? 430.71 defines the control circuit of a motor controller (controlapparatus). The relationship of a control circuit to the circuitcarrying the main power current is illustrated in the circuit diagramat left.
430.71 Motor Control-Circuit Protection
430.72(A) Motor Control-Circuit Overcurrent Protection
M
CONTROL CIRCUIT
BRANCH CIRCUITFUSE
M
CONTROL CIRCUIT
BRANCH CIRCUITFUSE
Branch circuit orsupplementary-type fuse
protected by either a branch or supplementary type protectivedevice (such a control circuit is not to be considered a branchcircuit).
For motor controllers listed for available fault currents greaterthan 10,000 amperes, the control circuit fuse must be a branchcircuit fuse with a sufficient interrupting rating. (The use of Buss®
FNQ-R, KTK-R, LP-CC, LPJ_SP, JJS, or JJN fuses isrecommended; these fuses have branch circuit listing status, highinterrupting rating, current-limitation, and small size.)
430.72(B) Motor Control-Circuit Conductor ProtectionWhat does this section mean?Control circuit conductors must be protected by a fuse rated at notmore than those values shown in Column “A” of Table 430.72(B).
What if the control conductors remain within the enclosure?If the control conductors do not leave the enclosure, they can beconsidered to be protected by the branch circuit fuse, if that fusedoes not exceed the values of Table 430.72(B) Column B.
(430.72(B)(2))
M
CONTROL CIRCUIT CONDUCTORS
BRANCH CIRCUITFUSE
Branch circuit orsupplementary-type fuse
Do the circuits shown below require individual control circuit protection?
No. The LPS-RK40SP fuses are sized within the 40 ampererequirement for 16 AWG conductor within an enclosure. (See Table430.72(B).)
Yes. Individual control circuit fuses are required since the 80ampere circuit breaker has a rating in excess of the 40 ampererequirement for 16 AWG conductor within an enclosure. (See Table430.72(B).
Note: 110.10 and 240.1 require that component withstand not be exceeded. Not allovercurrent devices sized per 430.72(B) can actually protect small conductors. Seediscussion on 240.1 in this booklet.
MControl circuitwithinenclosure
The motor branch protective device is consideredto also protect the control conductors if the conductorsdo not extend beyond the enclosure and the maximumrating of the protective device is not greater thanTable 430.72(B) Column B.
16 AWG Wire Within Enclosure
LPS-RK40SP
M25 HP34A
M25 HP34A
16 AWG Wire Within Enclosur
80A
10A Required
49
Comparison By Largest HP Motor (460V) Circuit Where Branch Circuit Protective Device Is Considered To Protect The Control Conductors Per 430.72(B) (2).Protective Approx. Level Of Control Circuit Control CircuitDevice Size As Percent Protection Within Enclosure Extending Beyond Enclosure
LOW-PEAK® YELLOW™ and BranchClass RK1 Circuitor FUSETRON® Class RK5 dual-elementFuse
175% 10HP 15HP 40HP 2HP 3HP 15HPNon-Time-Delay 300% 5HP 71/2HP 20HP 1HP 11/2HP 10HPFuseThermal Magnetic 250% Branch 5HP 10HP 30HP 11/2HP 2HP 10HPCircuit Breaker CircuitInstantaneous 1000%* Only 1HP 2HP 5HP 1/4HP 1/2HP 2HPOnly CircuitBreaker*Instantaneous only circuit breakers cannot provide any overload protection. Typically to hold starting currents, instantaneous trip is set at 1000% to 1700% of motor full-load amperes.
What if the control conductors leave the enclosure?If the control conductors leave the enclosure, they can beconsidered to be protected by the branch circuit fuse, if that fusedoes not exceed the values of Table 430.72(B) Column C.
What does 430.72(B) Exception No. 2 mean?Primary fusing of a control transformer can be considered toprotect the 2-wire, secondary conductors if the fuse rating doesnot exceed the value of multiplying the appropriate rating fromTable 430.72(B) with the secondary-to-primary voltage ratio.
Maximum primary fuse shall not exceed 1.75A as determined by—120V x 7A = 1.75A480V
Therefore, .5 amp primary fuse complies.
Control conductorsextending beyond enclosure
(430.72(B)(2))
M
The motor branch circuit protective device is considered also toprotect the control conductors if it does not exceed the values ofColumn C.
M
18 AWG Copperwithinenclosure
.5 Amp120V2 WIRE
480V 100VA
BRANCH CIRCUITFUSE
Even though a fuse or circuit breaker can be sized at 300% or 400% ofthe conductor ampacity, what level of control conductor protection can beexpected?
The protective device would respond only to high level conductorovercurrents; the control conductors would not be protectedagainst lower overcurrent levels. This lack of protection couldresult in a prolonged 200% control circuit overcurrent and eventual
insulation breakdown and melting of the conductors. For example,if the control circuit run were of considerable length, the conductorimpedance might be sufficiently high to limit fault currents to 200%to 400% of the conductor ampacity. Thus, oversized overcurrentdevices would provide inadequate protection. In contrast, fusessized to the conductors ampacity would provide full-rangeovercurrent protection; their use is to be recommended.
430.72(B) Motor Control-Circuit Conductor Protection
430.72(C) Motor Control-Circuit Transformer ProtectionWhat does this section mean?†
Primary Fuse Protection Only.Transformer Primary FusePrimary AmpacityCurrent Must Not ExceedLess than 2 amperes 500% (430.72(C)(4))2 to 9 amperes 167%9 amperes or more 125%
Primary and Secondary Fuse Protection.Primary Fuse Secondary SecondaryDoes Not Exceed Current Fuse250% 9 amperes or more 125%250% Less than 9 amperes 167%
The conditions of 430.72(C)(3), permit the use of a controltransformer rated less than 50 VA* without the inclusion ofindividual protection on the primary side of the transformer in thecontrol circuit proper. Thus, protection of the transformer primaryagainst short-circuit currents is dependent upon the device used
*Control Transformers rated less than 50 VA are usually impedance protected or haveother types of protection, such as inherent protection.
50
for branch circuit protection. However, consideration should begiven to protecting the control transformer on the primary side withindividual fuses specifically sized for control transformerprotection.
Take the case, for instance, in which a short occurs in a controltransformer (such as would result from insulation deterioration andbreakdown). (See diagram above in which a 60 ampere branchcircuit fuse is shown.) Now, if the overcurrent drawn by the controlcircuit as a result of the shorted control transformer is relatively low(actually could be less than 60 amperes) compared to theresponse time of the 60 ampere branch circuit fuse or circuitbreaker, the transformer could become so hot that extensivedamage could be done to the insulation of the control conductors. . . the transformer itself could burst into flames.
*Control Transformers rated less than 50 VA are usually impedance protected or haveother types of protection, such as inherent protection.
However, inclusion of fuse protection in the primary of thecontrol transformer would minimize this type of hazard. Buss®
FNQ-R Time-Delay fuses are excellent choices. When applyingfuses, the time-current characteristics should be checked todetermine if the fuse can hold the inrush magnetizing current of thetransformer.
Fuses Commonly Used in Control Circuits.There are several fuse types which have small dimensions that areideally suited for control circuit protection. The KTK-R, FNQ-R andLP-CC fuses are listed as Class CC fuses, and JJN (JJS) fuses arelisted as Class T fuses. When used for control transformer, coil, orsolenoid protection, the fuse should be selected to withstand theinrush current for the required time.
Symbol Voltage Ampere Interrupting CommentRating Rating Class Rating
430.72(C) Motor Control-Circuit Transformer Protection
.05A normal F.L.C.(breakdown of transformer windings could causecurrent to increase many times over normal level butless than 60A) *Conductor protection is still requiredper Section 430.72(B)
*
480V 120V
(25VA)
60A
COMPLIANCE COMPLIANCE
430.94 Motor Control Center ProtectionWhat are the requirements of this section?Where motor control centers (MCC) are specified, properovercurrent protection shall be supplied in the MCC as an integralmain, or remote main. These devices should be rated based onthe common power bus rating.
LPS-RK600SP
600A Bus
600A MCC
LPS-RK600SP
600A Bus
600A MCC
LPS-RK600SP
430.83(E) Requirements for Controllers with Slash Voltage RatingsWhat are the voltage rating requirements for controllers in 430.83(E)?Controllers with slash voltage ratings, such as 480Y/277 Volts,must be applied only on solidly grounded systems where thevoltage to ground does not exceed the lower of the two ratings
and the voltage between any two conductors does not exceed thehigher of the two ratings. To understand the concepts and properapplication of slash rated devices, see the discussion for 240.85in this booklet.
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Introduction430.102 covers the requirements for the location of disconnectingmeans of motor circuits. 430.102(A) covers the requirements forthe controller disconnecting means, while 430.102(B) contains therequirements for the motor disconnecting means.
What are the old 1999 NEC® Requirements? The basic requirement from the 1999 NEC® was that adisconnecting means was required “within sight” (visible andwithin 50 feet) of every motor controller (430.102(A)). Adisconnecting means was also required “within sight” of everymotor, unless the disconnecting means for the controller wascapable of being locked in the off position (430.102(B) Exception).
The 2002 NEC® Requirements for motor disconnecting means reads(without exception):
430.102(B) Motor. A disconnecting means shall be located insight from the motor location and the driven machinery location.The disconnecting means required in accordance with430.102(A) shall be permitted to serve as the disconnectingmeans for the motor if it is located in sight from the motorlocation and the driven machinery location.
The new general rule is that a disconnecting means is requiredwithin sight of every motor, whether or not the disconnectingmeans at the controller is capable of being locked in the offposition. This is a very significant change and an enormousadvancement for improved worker safety.
An example might help. Assume an MCC, with a lockablecombination starter, feeds a 50 hp motor located 500 feet from theMCC. According to the 1999 NEC, a disconnecting means was notrequired within sight of the 50 hp motor because the disconnectingmeans for the controller, in the MCC, was capable of being lockedin the off position. A maintenance worker that was called to themotor would have to walk the 500 feet back to the MCC todisconnect and lock off the motor circuit, and then return 500 feetto work at the motor. After the work was finished, the worker mustwalk 500 feet to re-energize the circuit and then walk back to themotor to check that everything is working correctly. In situationslike this, some workers have been tempted to work the equipment“hot”, rather than walk back and forth to shut down and lock outthe circuit properly.
The 2002 NEC® requires that a disconnecting means be withinsight of that 50 hp motor. There is much less chance that theworker will attempt to work the equipment “hot”.
What does the exception under 430.102(B) permit?The exception, modified during the Comment period, makesallowances for situations where the disconnecting means would beimpractical or increase hazards, or where located in an industrialinstallation that has written safety procedures and only qualifiedpeople can work on the equipment. A Fine Print Note was added togive examples of increased hazards, such as very large motors,equipment with more than one motor (most industrial machinery),submersible motors, drives, and motors for classified areas. TheNEC® reads:
430.102(B) Exception: The disconnecting means shall not berequired to be in sight from the motor and the driven machinerylocation under either condition (1) or (2) below, provided thedisconnecting means required in accordance with 430.102(A) isindividually capable of being locked in the open position. Theprovision for locking or adding a lock to the disconnecting meansshall be permanently installed on or at the switch or circuitbreaker used as the disconnecting means. (1) Where such a location of the disconnecting means isimpracticable or introduces additional or increased hazards topersons or property.(2) In industrial installations, with written safety procedures,where conditions of maintenance and supervision ensure thatonly qualified persons will service the equipment.
FPN No. 1: Some examples of increased or additional hazardsinclude, but are not limited to: motors rated in excess of 100 hp,multi-motor equipment, submersible motors, motors associatedwith variable frequency drives and motors located in hazardous(classified) locations.”
Are permanently installed lockout provisions required?Yes, a major change was also made to the locking requirements(see the NEC® exception above). New wording mandates that thelock or provisions for locking must be permanent. This was addedto specifically eliminate the portable locking devices, which areeasily defeated, and those devices that can be overcome bysimply removing a cover. The type of lockout provision or fixture(not the lock) that is added onto the circuit breaker or switch at thetime of the lockout procedure is not permissible.
430.102 Requirements For Disconnecting Means Within Sight Of Motors
52
What are the requirements of 430.109(A)(6)?Manual motor protectors or manual motor controllers can be usedas a motor disconnecting means if they are marked “Suitable asMotor Disconnect” and located between the final branch-circuitovercurrent device and the motor. The required location would
preclude their use as the branch circuit disconnecting means.Note that these devices can not be used as the branch-circuitovercurrent device even though some of them have the ability toopen short-circuit currents.
How does this section affect the overcurrent protection requirements?If the nameplate on the equipment controller is marked with "MAXFUSE", that means a fuse must be used to protect the equipment.See Section 110.3(B) for proper installation and protection.
What are the requirements of 440.22(A)?The branch circuit protective device may be sized at the maximumvalue of 175% of the motor-compressor rated load current. If themotor cannot start due to high inrush currents, this value may beincreased to, but cannot exceed, 225% of motor rated current.
What is the importance of Note 2 found on Table 450.3(A) and 450.3(B)?The required secondary protection may be satisfied with multipleovercurrent devices that protect feeders fed from the transformersecondary. The total ampere rating of these multiple devicescannot exceed the allowed value of a single secondaryovercurrent device. If this method is chosen, dual-element, time-delay fuse protection offers much greater flexibility.
Note the following examples:This design utilized a single secondary overcurrent device. It
provides the greatest degree of selectively coordinatedtransformer protection, secondary cable protection, and switch-board/panelboard/load center protection. The transformer cannotbe overloaded to a significant degree if future loads are added(improperly) in the future.
If the single secondary overcurrent device is eliminated, muchof the protection will be reduced.
440.5 Marking Requirements on HVAC Controllers
440.22 Application and Selection of the Branch Circuit Protection for HVAC Equipment
450.3 Protection Requirements for Transformers
M M M M M
250%
150 KVA
208/120V IFLA = 417A
This fuse or circuit breaker maybe sized at 1.25 x 417A = 522A.The exception allows the next standard size of 600A to be used.
200ASwitch
LPN-RK110SP
LPN-RK110SP
LPN-RK110SP
LPN-RK110SP
LPN-RK110SP
83A 83A 83A 83A 83A
200ASwitch
200ASwitch
200ASwitch
200ASwitch
What are the requirements of 440.22(C)?440.22(C) states that if the manufacturer's heater table shows a
maximum protective device less than that allowed above, theprotective device rating shall not exceed the manufacturer's values(refer to Section 430.52 also).
430.109(A)(6) Manual Motor Controller as a Motor Disconnect
53
Using the same logic, if the single secondary main is eliminatedand thermal magnetic circuit breakers are utilized as branch circuitprotection, only three of the motors can be connected because thethermal magnetic breakers will have been sized at approximately250% of motor F.L.A. (83 x 250% = 207.5A)
Using a 200 ampere circuit breaker would allow three(600 ÷ 200) motors to be connected.
If the single secondary main is eliminated and MCP's areutilized as branch circuit protection, the transformer will beseriously underutilized because only one motor can be connected.For one motor, 1 x 700% of 83 = 581 amperes. For two motors, 2 x700% of 83 = 1162 amperes. Since the sum of the devices cannotexceed 600 amperes, only one motor can be connected when themotor circuit is protected by an MCP.
If the MCP will not hold at the 700% setting due to a highstarting current, it cannot be adjusted beyond 722% (600÷83) andtherefore it may not be able to be used.
If the single secondary main is eliminated, and dual-elementfuses are utilized as branch circuit protection, the transformer cancontinue to be loaded with the five 83 ampere motors because 5 x110 = 550 amperes (which is less than the maximum of 600amperes).
450.3 Protection Requirements for Transformers
M M M
250%
150 KVA
208/120V IFLA = 417A
No Single Secondary Device
83A 83A
200A Thermal-Magnetic CircuitBreaker
83A
200A Thermal-Magnetic CircuitBreaker
200A Thermal-Magnetic CircuitBreaker
M M M M M
250%
150 KVA
208/120V IFLA = 417A
200ASwitch
LPN-RK110SP
LPN-RK110SP
LPN-RK110SP
LPN-RK110SP
LPN-RK110SP
83A 83A 83A 83A 83A
200ASwitch
200ASwitch
200ASwitch
200ASwitch
No Single Secondary Device
450.3(A) Protection Requirements for Transformers Over 600 Volts
M
250%
150 KVA
208/120V IFLA = 417A
Only one motor can be connected when the MCP is utilized.
83A
581A MCP
No Single Secondary Device
What is the general content of this section? This part of the section sets the overcurrent protection require-ments of transformers (over 600 volts): For primary and secondaryprotection, the primary should be protected by an individualprotective device with fuse rating not in excess of 300% of theprimary's rated current. Secondary sizing (600V and below) is at125%* for any location or up to 250% for supervised installations.Secondary sizing (over 600V) can be up to 250%* for % Z ≤ 6%and 225% for % Z > 6%.
For supervised installations, secondary protection is notrequired (above, at, or below 600 volts) if the primary is at 250% (orthe next standard size if 250% does not correspond to a standardfuse size) of the primary full load amps. Note that conductorprotection and panelboard protection may still be required.
*Where this does not correspond to a standard fuse size, the next higherstandard size shall be permitted.
Fuse at300% of F.L.A. of primary
UnsupervisedLocation
Fuse at125% of F.L.A. of secondary
PRI.over600V
SEC.600Vor less
Z = 6% (or less)
54
A maximum fuse rating LPS-RK100SP will meet the 125%requirements.
What does this section require?Current-limiting cable limiters shall be used on each end of the tieconductors, specified per the size of the conductors.
What is the general content of this section?This section covers protection requirements of transformers, 600volts or less. Fusing requirements are shown in the illustratedexample below.
Where the primary FLA is ≥ 2 amps, but < 9 amps, the primaryfuse may be sized at 167% or less. If the primary FLA < 2 amps,the primary fuse may be sized at 300% or less.
*Where this does not correspond to a standard fuse size, the next standard size may beused.
Protection of circuit conductors is required per Articles 240 and310; protection of panelboards per Article 408. Specific sectionswhich should be referenced are Sections 240.4, 240.21 andSection 408.16.
Note: Transformer overload protection will not be provided by usingovercurrent protective devices sized much greater than the trans-former F.L.A. The limits of 167%, 250% and 300% will not adequatelyprotect transformers. It is suggested that for transformer overloadprotection, the fuse size should be within 125% of the transformer full-load amperes.
There is a wide range of fuse ampere ratings available toproperly protect transformers. FUSETRON® (Class RK5) and LOW-PEAK® YELLOW™ (Class RK1) dual-element fuses can often besized on the transformer primary and/or secondary, rated as low as125% of the transformer F.L.A. These dual-element fuses have timedelay to withstand the high magnetizing inrush currents oftransformers. There is a wide ampere rating selection in the 0 to 15ampere range for these dual-element fuses to provide protection foreven small control transformers.
450.3(B) Protection Requirements for Transformers 600 Volts or Less
450.6(A)(3) Tie Circuit Protection
455.7 Overcurrent Protection Requirements for Phase Converters
460.8(B) Overcurrent Protection of Capacitors
PRIMARY PROTECTION ONLY
Fuse must not belarger than 125%*of F.L.A. of primary
No secondaryprotection
PRI. & SEC.600V or less
PRIMARY AND SECONDARY PROTECTION
Fuse no larger than 250% of F.L.A.of primary whensecondary fusesare provided at 125%
125% of F.L.A.of secondary*(For secondary < 9A,the fuse may be sizedup to 167%)
PRI. & SEC.600V or less
Secondary F.L.A. ≥ 9A
What does this section mean?Phase converters supplying variable loads must be protected atnot more than 125% of the nameplate single-phase input full-loadcurrent.
For converters supplying fixed loads, the conductors shall beprotected at their ampacity, but in no case can the overcurrentprotection exceed 125% of the phase converter nameplate singlephase current.
Where the required rating does not correspond to a standardrating, sizes up to the next standard rating may be used.
LPS-RK100SP 3Ø Motor
MP/C
Nameplate =80 Amperes
What are the requirements of this section?Overcurrent protection must be provided in each ungroundedconductor supplying a capacitor bank, except for a capacitorlocated on the load side of a motor overload protective device.
The rating of this overcurrent protective device shall be as lowas practical. When energized, capacitors have a high inrushcurrent of short time duration. Generally, dual-element time-delayfuses can be sized at 150% to 175% of the capacitor rated current.
The purpose of fusing a capacitor is for short-circuit protection.When a capacitor fails, it shorts out. Proper fusing can prevent theshorted capacitor from rupturing.
55
501.6(B) Fuses for Class I, Division 2 LocationsWhat is the meaning of 501.6(B)(3)?The intent of this reference is to suggest the use of non-indicating,filled, current-limiting fuses. The following is a partial list of filled,non-indicating fuses which are current-limiting:
Data sheet 8003 on www.bussmann.com under applicationinformation/publications provides a list of fuses meeting theserequirements. Some of these are:
What does this section require?Compliance with Sections 110.9 and 110.10 is mandatory. Short-circuit ratings must be marked on the switchboard.
What does this section mean?If ground fault protection is placed on the main service or feeder ofa health care facility, ground fault protection must also be placedon the next level of feeders. The separation between ground faultrelay time bands for any feeder and main ground fault relay mustbe at least 6 cycles in order to achieve coordination between thesetwo ground fault relays. In health care facilities where no groundfault protection is placed on the main or feeder, no ground faultprotection is necessary at the next level down. Therefore, if therequirements of Sections 230.95 and 215.10 do not require groundfault protection, then no ground fault protection is required on thedownstream feeders either.
If the ground fault protection of the feeder coordinates with the mainground fault protection, will complete coordination between the main andfeeder be assured for all ground faults?No, not necessarily! Merely providing coordinated ground faultrelays does not prevent a main service blackout caused by feederground faults. The overcurrent protective devices must also beselectively coordinated. The intent of Section 517.17 is to achieve“100 percent selectivity” for all magnitudes of ground fault currentand overcurrents. 100% selectivity requires that the overcurrentprotective devices be selectively coordinated for medium and highmagnitude ground fault currents because the conventionalovercurrent devices may operate at these levels. (See discussionof Section 240.12, System Coordination, for a more detailedexplanation of selective coordination).
What is one of the most important design parameters of the powerdistribution system of a health care facility?Selective coordination. To minimize the disruption of power andblackouts in a distribution system, it is absolutely mandatory thatthe overcurrent protective devices be selectively coordinated.
What is selective coordination?A selectively coordinated system is one in which the overcurrentprotective devices have been selected so that only the overcurrentdevice protecting that circuit in which a fault occurs opens; othercircuits in the system are not disturbed. The danger of a majorpower failure in a health care facility such as a hospital is selfevident. In any facility, a power failure is at least inconvenient, ifnot quite costly; in a hospital, it can easily give rise to panic andendanger lives. Continuity of electrical service by selectivecoordination of the protection devices is a must. (See Section240.12, System Coordination, of this bulletin for a more detailedexplanation of selective coordination. Also publication SPD,Electrical Protection Handbook has a detailed explaination ofselective coordination.).
517.17 Requirements for Ground Fault Protection and Coordination in Health Care Facilities
20' 12 AWG WIRE2,000 Amperes
AvailableRow of Fluorescent
Fixtures
GLR FuseOpens
FixtureFaulted Ballast Ballasts
20ACIRCUIT
BREAKER
LightingPanel
520.53(F)(2) Protection of Portable Switchboards on Stage
CURRENT-LIMITINGFUSE
50,000Aavailable faultcurrent
Switchboard short-circuitrating 50,000A whenprotected by a current-limiting fuse
What is the importance of 501.6(B)(4)?General Comment–These fuses are used to isolate a faulted fixtureballast and maintain continuity of service. Listed or recognizedbranch circuit or supplementary fuses may be used. Additionally,the GLR fuse is used on ballasts that have a 200 ampere short-circuit withstand rating such as Class P ballasts.
56
What does this section mean?Branch circuit fuses installed in a mobile home should not exceedthe rating of the conductors they supply. These fuses should notbe more than 1.5 times the rating of an appliance rated 13.3amperes or more on a single branch, and not more than the fusesize marked on the air conditioner or other motor operatedappliance.
Do these branch circuit fuses conform to the requirements of 550.6(B)?Yes.
What does this section mean?#18 conductors can be used in control circuits of cranes andhoists if they are fused at not greater than 7 amperes.
What is an electrical contractor responsible for in an elevatorinstallation?The electrical contractor is responsible for supplying thedisconnecting means for the power to the elevator. These includethe main supply power (620.51), the car lighting (620.22(A) and620.53), and the HVAC (620.22(B) and 620.54).
What are the requirements of 620.22(A) and 620.53?620.22(A) requires the lighting, receptacle(s), auxiliary lightingpower source, and ventilation for the elevator car to be supplied bya separate branch circuit and the overcurrent protective devicemust be located in the elevator machine room or machinery space.
620.53 requires a single disconnecting means for the car lighting,receptacle(s) and ventilation that must be capable of being lockedin the open position and located in the machine room or machineryspace. It is also required to have an identifying number tocorrespond to the elevator that it supplies. This disconnect mustbe separate from the disconnecting means that supplies the mainpower to the elevator car for motion (620.51) and thedisconnecting means that supplies the heating and air-conditioning (620.22(B) and 620.54).
550.6(B) Overcurrent Protection Requirements for Mobile Homes and Parks
610.14(C) Conductor Sizes and Protection for Cranes and Hoists
What are the requirements of 620.22(B) and 620.54?620.22(B) requires the heating and air-conditioning source to besupplied by a dedicated branch circuit for each car and theovercurrent protective devices for each circuit must be located inthe machine room or machinery space.
620.54 requires a single disconnecting means for the heating andair-conditioning source that must be located in the machine roomor machinery space and must be capable of being locked in theopen position. This disconnect must also be separate from thedisconnecting means that supplies the main power to the elevatorcar for motion (620.51) and the disconnecting means that suppliesthe lighting, receptacle(s), auxiliary lighting power source, andventilation (620.22(A) and 620.53).
Note: If the nameplate on a device states “Maximum Fuse Size”, then fusesthat size or smaller must be used somewhere in the circuit.
57
What are the requirements for the main elevator power disconnect?620.51(A) requires a single disconnecting means that must be anenclosed externally operable fused motor circuit switch or circuitbreaker that is capable of being locked in the open position andmust be listed.
What other concerns are there when installing this disconnecting means?Since this is an elevator circuit, consideration needs to be madefor the interaction with the elevator code (ANSI/ASME A17.1,Safety Code for Elevators and Escalators). While the electricalcontractor does not usually get involved with the elevatorinstallation, there are requirements from the elevator code that theelectrical contractor needs to be aware of that may affect what hedoes have to install. The most influential of these requirementsbeing Rule 102.2(c)(3) from ASME A17.1 which states that ifsprinklers are installed in the elevator hoistway or machinery spacethen the elevator power must be removed prior to the applicationof water. NEC® 620.51(B) permits this removal of power on thedisconnecting means to the elevator and can be accomplishedwith a fusible shunt trip switch. The reasons for removing thispower is to reduce the hazards associated with water on liveelevator electrical equipment. This includes, but is not limited to,the braking system of the elevator, which if water were appliedmay not be able to operate properly and may cause the elevatorcar to stop between floors and leave the hoistway exposed.Another concern is that the control circuitry for the door operationthat is located on top of the elevator cab could become short-circuited when water is applied and allow the elevator to travel upand down with the doors open, again causing an exposedhoistway.
Since this removal of power is to occur prior to the sprinkleractivation, there must also be interaction with the fire alarm system,which means involvement with NFPA 72 (National Fire AlarmCode®). Shutdown is most commonly accomplished by theactivation of a heat detector located in the hoistway or machineroom. This heat detector is required to be located within two feetof each sprinkler head and is usually set at 135°F, compared to thefusible element of the sprinkler head, which is most commonly setat 165°F. (So, in theory, the system should operate as follows.First, the elevator is sent to a designated floor by the smokedetector, where the doors stay open. If the fire gets to the pointwhere it is detected by the heat detector(s), power is removedbefore it gets hot enough to release water through the sprinklerheads.) When a fire alarm system is installed in a building, section3-9.2.1 of NFPA 72-1999 requires the elevator shutdown to bemonitored for integrity. Since the fire alarm system usuallyoperates at 24V and the shunt trip coil usually operates at 120V,they cannot be directly connected. This means that the heatdetector should activate a relay (Isolation Relay), which in turn willallow power to be supplied to the shunt trip coil and cause thedisconnecting means to open the circuit. This relay can bemonitored for integrity by the fire alarm system to make sure thatthe wiring remains intact and must be located within three feet ofthe shunt trip device per 3-9.2.1. To help insure that the system isfail-safe, an additional relay (Volt Monitor Relay) is required by 3-9.4.4 of NFPA 72-1999 to monitor for presence of the shunt tripvoltage. This relay will be placed in parallel to the shunt trip coil so
620.51 Disconnecting Means (Elevators)
620.61 Overcurrent Protection (Elevators)
that it receives the same voltage. Then the contacts from this relaycan be monitored for integrity by the fire alarm system. If at anytime the voltage is not present to the relay, a trouble signal wouldbe sent to the fire alarm system and annunciate an alarm so thatsomeone will correct the problem. See Figure 1.
Figure1
If contact closure occurs by either heat detector in Loop A, PLC 1contact closes and energizes the Isolation Relay. When theIsolation Relay energizes, the IR Contact closes and the Shunt TripCoil becomes energized, causing the switch to open anddisconnect the power to the elevator. If the wiring in Loop A, LoopB, or Loop C becomes disconnected or short-circuited, the PLCwill detect it and then close PLC 2 contact and cause the TroubleSignal Alarm to be activated which will notify someone that there isa problem that needs to be examined. This system is designed tobe fail-safe so that if any problem occurs in the system, it can befixed quickly and operational when it is needed to operate.
These requirements are commonly overlooked in the biddingprocess and usually cause extreme confusion. To find out moreabout how to comply with these codes and standards with onecomplete package, contact Bussmann® for information on thePower Module™ Switch and Panel for elevator circuits. The PowerModule™ complies with these codes and standards and reducescontractor installation to a minimum in a UL listed assembly. Formore information, including data sheets, look for Power Module™,under products, at www.bussmann.com.
Where should the disconnecting means be located?620.51(C) requires the disconnecting means to be located where itis readily accessible and within sight of the elevator controller.
What requirements must be met for overload protection?Most elevators are rated for intermittent duty and 620.61(B)(1)requires the elevator motor to be protected against overloads by430.33. This section says that if a motor is rated as intermittent orsimilar duty, it is allowed to be protected against overloads by thebranch-circuit short-circuit overcurrent protective device providedit does not exceed the values specified in Table 430.52. If themotor is rated for continuous duty, such as for escalators and
moving walks, 620.61(B)(2) requires the overload protection to besized in accordance with 430.32. This means that it must have aseparate overload device sized no greater than 125% of FLA.
What about short-circuit protection?According to 620.61(D), short-circuit protection is to be providedby the branch-circuit overcurrent protective device and must besized in accordance with Article 430 Part IV.
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What does 620.62 require when it comes to coordination?When more than one driving machine is fed from a single feeder,selective coordination is required between the overcurrentprotective device in each disconnecting means and any othersupply side overcurrent protective devices. Selective coordinationfor elevator overcurrent protective devices is critical. For example,in Figure 2, if a fault were to occur that would cause overcurrentprotective device M1 to open, all of the elevators in the buildingwould lose power. One of the reasons that coordination is soimportant is because firefighters commonly use the elevator to getcloser to a fire during fire-fighting operations. See Figure 2.
Figure 2
Since F2 (Feeder 2) is a single feeder to multiple elevators, thebranch OCPDs B1, B2, and B3 must be selectively coordinatedwith F2 to comply with Section 620.62. Now look at the load sideof M1. This is also a single feeder to multiple elevators and mustalso be selectively coordinated with F2, F4, and B4 in order toeliminate the possibility of having a fault on one elevator cause theupstream OCPD to open, thereby removing the power to the otherelevators. Since B4 and F4 are usually at or near the same
ampacity, these two OCPDs cannot be coordinated together andare not required to be. The only way to meet 620.62 with B4 andF4 is to insure that both B4 and F4 are coordinated with M1. SeeFigure 3.
Figure 3
Notice that each of the feeders supplies their own elevator. Thisdoes not bypass the requirements of 620.62, which requiresselective coordination when there is more than one drivingmachine being fed from a single feeder. According to Article 100,a feeder is considered to be all circuit conductors between theservice equipment and the branch-circuit OCPD. This would meanthat the load side conductors from M1 would be a single feeder tomultiple driving machines. This would require F1, F2, F3, and F4 tobe selectively coordinated with M1 in order to comply with 620.62.These situations would require selective coordination through tothe main OCPD in the building. Otherwise, safety may becompromised if a fault occurs on the branch level that causes themain OCPD to operate. For more information on selectivecoordination see the discussion on 240.12 in this booklet or see theBussmann Electrical Protection Handbook SPD for a more in depthdiscussion.
620.62 Selective Coordination (Elevators)
620.91 Emergency and Standby Power Systems (C)Disconnecting Means (Elevators)
Figure 4 – Normal Operation w/Out Auxiliary Contact(DOES NOT COMPLY)
Elevator Controller
Elevator DisconnectWithout NCAuxiliary Contact
Drain Valve(To Lower Elevator)
Elevator Motor
Solenoid
Control Relay NC Contact(While Relay is De-energized)
Battery forLowering
+
l
M
CR
If there is a secondary source of power used for emergency purposes,how is the elevator disconnecting means affected? The disconnecting means referenced in 620.51 must be capableof disconnecting all sources of power for maintenance purposes.Hydraulic elevators have the capability of using a battery pack tolower the elevator in a loss of power situation. The batteryattachment is utilized as an extra level of safety to keep fromstranding people in the elevator for long periods of time. Undernormal operation, the main line power from the disconnectingmeans controls the raising of the elevator through a pump motorand the lowering of the elevator through a solenoid and a drainvalve. To send the cab upward, the pump motor pumps hydraulicfluid into the piston that forces the elevator upward. To return thecab back down, a drain valve at the bottom of the piston isopened by a solenoid valve and as the fluid drains back into thereservoir, the elevator lowers. If the main line power is lost, thisbattery pack attachment can supply enough power to actuate thesolenoid. See Figures 4 and 5.
59
Figure 5 – Loss Of Power w/Out Auxiliary Contact(DOES NOT COMPLY)
Please note Figure 5. By not having the auxiliary contact, themanual opening of the Elevator Disconnect will be viewed by theElevator Controller as a loss of power. A control relay is used tosense the main line power. When de-energized, this control relay’scontacts wil l become closed and allow the battery packattachment to activate the solenoid. This could be a seriousproblem if a maintenance person is the one that opened thedisconnecting means to work on the elevator. If he or she is goingto work in the pit, he or she will bring the elevator up a floor or twoto enable access. Depending upon the timing involved, this couldpotentially result in the elevator lowering with the maintenanceworker in the pit. This is why 620.91(C) requires an auxiliarycontact. See Figure 6.
Figure 6 – Normal Operation per NEC 620.91(C)(COMPLIES)
As you can see, during normal operation having the auxiliarycontact compromises nothing. See Figure 7.
Figure 7 – Loss of Power per NEC 620.91(C)(COMPLIES)
During a normal loss of power (from the utility or other devicesturned off upstream) the battery pack attachment would be able tooperate the solenoid and lower the elevator. See Figure 8.
Figure 8 – Manual Operation of Disconnecting Means(COMPLIES)
As you can see in Figure 8, by having the auxiliary contact inthe switch, the main line power and the battery pack attachmentwould both be disconnected when the switch is turned off so thatthe elevator will not be able to move. This is why 620.91(C) isimportant. Inspection plays a key part in this section in that theelectrical contractor usually installs the disconnecting means andthe elevator contractor usually installs the battery attachment. It iscritical that the wiring for this be verified at the time of installationby both the electrical inspector and the elevator inspector. Bothshould have jurisdiction over this circuit to insure the highest levelof safety.
620.91 Emergency and Standby Power Systems (C)Disconnecting Means(Elevators)
670.3 Industrial MachineryWhat does this section mean?If a main overcurrent protective device is provided on an industrialmachine, the nameplate shall state, among other things, theinterrupting rating of the device. The machine shall also bemarked “overcurrent protection provided at machine supplyterminals”.
Elevator Controller
Elevator DisconnectWithout NCAuxiliary Contact
Drain Valve(To Lower Elevator)
Elevator Motor
Solenoid
Control Relay NC Contact(While Relay is De-energized)
Battery forLowering
+
lM
CR
Elevator Controller
Elevator DisconnectWith NCAuxiliary Contact(Switch is ON)
Drain Valve(To Lower Elevator)
Elevator Motor
Solenoid
Control Relay NC Contact(While Relay is De-energized)
Battery forLowering
+
l
M
CR
Elevator Controller
Elevator DisconnectWith NCAuxiliary Contact(Switch is ON)
Drain Valve(To Lower Elevator)
Elevator Motor
Solenoid
Control Relay NC Contact(While Relay is De-energized)
Battery forLowering
+
l
M
CR
Elevator Controller
Elevator DisconnectWith NCAuxiliary Contact(Switch is OFF)
Drain Valve(To Lower Elevator)
Elevator Motor
Solenoid
Control Relay NC Contact(While Relay is De-energized)
Battery forLowering
+
l
CR
M
60
What does this section require?Emergency lighting systems cannot allow a blackout in any arearequiring emergency illumination due to the failure of any oneelement of the lighting system. Such failures could be caused bythe burning out of a light bulb or the opening of a branch circuitprotective device due to a faulted ballast. The solution to the burntout light bulb is to have additional bulb(s) in the area. The solutionto the open branch circuit protective device is to install listedsupplementary fuses on each ballast. In that way, a faulted ballastwould be taken off the line by the supplementary fuse, not by thebranch circuit protective device, allowing the rest of theemergency system to remain energized.
The fault in Fixture #3 causes the 20 ampere branch circuit overcurrentdevice to open, causing a blackout in the entire area.
The fault in Fixture #3 will open just the supplementary fuse. The 20ampere branch circuit device does not open and Fixtures 1, 2 and 4remain energized, preventing a blackout.
700.5 Emergency Systems – Their Capacity and Rating
700.16 Emergency Illumination
What does 700.5(A) require?Emergency systems and equipment must be able to handle theavailable short-circuit current at their line side. If the equipmentcannot, it may be damaged, causing additional hazards topersonnel. The use of current-limiting fuses can be a solution tothis high fault current problem.
Fixture No. 1
Fixture No. 2
Fixture No. 4
Fixture No. 3 Fault
VIOLATION
20A
Branch(Opens)
Fixture No. 3 Fault
COMPLIANCE
20A
Branch(Remainsenergized)
Fixture No. 1
Fixture No. 2
Fixture No. 4
(Open)
BLACKOUT PREVENTION!Increased Reliability
Fault opens the nearest upstream fuse, allowing other circuits to remainenergized. Reliability of the emergency system is increased.
What is the meaning of this fine print note?In order to maximize the reliability of emergency systems, theovercurrent devices must be selectively coordinated. Time-currentcurves of both fuses and circuit breakers must be examined todetermine whether or not only the overcurrent device closest to afault opens. If additional upstream devices open, the system is notselectively coordinated, causing additional sections of theemergency system to black out and therefore, reducing thereliability of that system.*
BLACKOUT!Reduced Reliability
Fault exceeding the instantaneous trip setting of all three circuitbreakers in series will open all three. This will blackout the entireemergency system.
700.25 Emergency System Overcurrent Protection Requirements (FPN)
1000A I.T.=10x
225AI.T.=8x.
Opens instantaneous at 1,800 A
Opens instantaneous at 10,000 A
20AI.T.=8A Opens
22,000 AmpShort-Circuit
22,000 AmpShort-Circuit
Opens20A
225A NotOpen
1000ANotOpen
VIOLATION
COMPLIANCE
*See also Section 4.5.1 of NFPA 110 (Emergency and Standby Power Systems) andSections 3.3.2.1.2(4) & 3.4.1.1.1 of NFPA 99 (Health Care Facilities) for additionalinformation on selective coordination.
61
What do these three sections mean?The NEC® requires that emergency and standby systems shallhave the capability of safely interrupting the available short-circuitcurrent available at the line terminals of the equipment. Refer toSections 110.9 and 110.10.
701.6 Legally Required Standby Systems – Capacity and Rating
702.5 Optional Standby Systems – Capacity and Rating
705.16 Interconnected Electric Power Production Sources – Interrupting andShort-Circuit Current Rating
725.23 Overcurrent Protection for Class 1 CircuitsWhat does this Section mean?Class 1 Control Circuit Conductors shall be protected by fuses attheir ampacities. In addition, 18 AWG and 16 AWG shall beprotected at 7 amperes and 10 amperes, respectively.
What must be added to this Control Circuit to comply with 725.23?
A 7 ampere fuse must be added to protect the #18 control wire.
18 AWG Control Wire
20 Amp
BRANCH
7 Amp Fuse
COMPLIANCE
18 AWG Control Wire
20 Amp
BRANCH
VIOLATION
760.23 Requirements for Nonpower-Limited Fire Alarm Signaling CircuitsWhat does this provision require?Fire protective signaling circuits with conductors 18 AWG andlarger must be protected at their ampacities as shown:
Fuses shall be located at the supply terminals of the conductor.
This and the facing page can be copied as a two sided sheet. This page is a check list that can be completedby the installer and verified by the inspector. The facing page provides background information on the variousNEC requirements. This form is available on the Bussmann website at www.bussmann.com.
INSPECTION FORM: Series RatingsISSUED BY: ______________________________________________________________
ESSENTIAL INFORMATION: Load Side Panel Designation ______________________Load Side Circuit Breaker Part Number ______________________Load Side Circuit Breaker Interrupting Rating ______________________Line Side Panel Designation (If applicable) ______________________Line Side Overcurrent Protective Device Part Number ______________________Line Side Overcurrent Protective Device Interrupting Rating ______________________Available Short Circuit Current ______________________Series Combination Short Circuit Rating ______________________
Compliance Checklist (For further information see discussion on reverse side for each item)
1. Manufacturer’s LabelAre both devices in use for the series rated combination marked on the end use equipment (or contained in abooklet affixed to the equipment) as required in 240.86(A)?
YES NO
2. Field Installed LabelIs the field label, required by 110.22, installed on all the end use equipment containing the devices used inthe series rated combination with proper identification of the replacement parts, panel locations, and seriescombination short circuit rating?
YES NO
3. Motor ContributionsIf motors are connected between the series rated devices, is the combined motor full load current less than1% of the downstream circuit breakers’ interrupting rating?
YES NO
4. Selective CoordinationSeries rated systems should not be used in health care facilities (NEC517.17), emergency systems (NEC 700.25 FPN), or elevator circuits which contain more than one elevator (NEC620.62). Is this seriesrated system being installed per these requirements?
YES NO
AN ANSWER OF NO TO ANY OF THESE QUESTION IS EVIDENCE OF LACK OF COMPLIANCE.LACK OF SUBMITTALS IS CONSIDERED AS EVIDENCE OF LACK OF COMPLIANCE.
Series Rated SystemsWhat is a Series Rated Combination:A combination of two devices, that have been tested under specific test conditions, that work together to clear afault. The allowed combinations are limited to those that have been selected by the circuit breaker manufacturerfor testing. Only tested combinations can be used.
Why is a Series Rated Combination used?A series rated system allows a load side circuit breaker to be applied in a system where the available shortcircuit current exceeds the interrupting rating marked on that circuit breaker.
BACKGROUND TO CHECKLIST ITEMS1) Manufacturer’s Label
Since the use of series rated systems is limited tospecific combinations that have been tested, the enduse equipment is required to be marked, by themanufacturer, per 240.86(A) of the 2002 NationalElectrical Code. Since there are hundreds ofcombinations, this marking may be in a book that isaffixed to the end use equipment, as allowed in UL67.The manufacturer’s marking is used to verify that bothdevices are part of a recognized series ratedcombination, the panelboard is listed for use with thecombination, and that the series combinationinterrupting rating is sufficient for the available shortcircuit current. This label also provides guidance forfuture upgrades as to the specific replacementdevices that are allowed.
2) Field Installed Label110.22 of the 2002 National Electrical Code requires the installer to apply a field caution label warning thata series rated combination is being used. This label must be applied on the panel containing the seriesrated combination or on both pieces of electrical equipment if the line side device is located separate fromthe load side circuit breaker to assure that the proper devices have been installed and that proper futurereplacements are made. The inspector can check the devices noted on the field label required by 110.22against the recognized combinations tested by the manufacturer and marked per 240.86.
3) Motor ContributionA series rated combination is evaluated under specifictesting conditions of which motor contribution is not apart of the criteria. If a motor is connected in themiddle of the combination, it would supply extra faultcurrent that did not exist when the combination wastested. 240.86(B) of the 2002 National Electrical Code
addresses this by restricting the use of series ratedcombinations when the sum of the full load current ofthe motors exceeds 1% of the LOAD SIDE circuitbreaker’s interrupting rating. For example, if the loadside circuit breaker is rated 10,000 A.I.R., with motorloads exceeding 100 amps, then a series ratedcombination could not be used.
4) Selective CoordinationThe biggest disadvantage of a series rated system is that, by definition, the line side (protecting) devicemust open at the same time, and in conjunction with, the load side (protected) circuit breaker. This meansthat the panel loses power because the device feeding the panel must open under medium to high levelshort circuit conditions. As a result, series rated systems should not be used in health care facilities(NEC517.17), emergency systems (NEC700.25 FPN) and elevator circuits which contain more than oneelevator (NEC620.62).
Circuit Load Ampere Fuse Symbol Voltage Interrupting RemarksRating Type Rating (AC) Class Rating
(kA)
Conventional Dimensions—Class RK1, RK5 (0-600A), L (601-6000A)
All type loads 0 LOW-PEAK® LPN-RK_SP 250V RK1†† 300 All-purpose fuses.(optimum to (dual-element, LPS-RK_SP 600V Unequaled for combinedovercurrent 600A time-delay) short-circuit andprotection). 601 to LOW-PEAK® KRP-C_SP 600V L 300 overload protection.
6000A (time-delay) (Specification grade product)
Motors, welder, 0 FUSETRON® FRN-R 250V RK5†† 200 Moderate degree oftransformers, to (dual-element, FRS-R 600V current-limitation. Time-delaycapacitor banks 600A time-delay) passes surge-currents.(circuits with heavy 0 DURA-LAGTM DLN-R 250V RK5 200inrush currents). to (dual-element, DLS-R 600V
600A time-delay)
601 to LIMITRON® KLU 600V L 200 All-purpose fuse. Time-Main, 4000A (time-delay) delay passes surge-currents.Feeder Non-motor loads KTN-R 250V RK1†† 200 Same short-circuit protectionand (circuits with no 0 KTS-R 600V as LOW-PEAK® fuses butBranch heavy inrush to must be sized larger for
currents). 600A LIMITRON® circuits with surge-currents;LIMITRON® fuses _____ (fast-acting) i.e., up to 300%.particularly suited 601 to KTU 600V L 200 A fast-acting, highfor circuit breaker 6000A performance fuse.protection.
Reduced Dimensions For Installation in Restricted Space—CUBEFuse™ (0-60A+), Class J(0-600A), T(0-1200A), CC(0-30A), G(0-60A)
All type loads 0 to 60A+ CUBEFuse™ TCF 600V J*** 300 Finger-safe. All-purpose fuses.(optimum (finger-safe, Unequaled for combinedovercurrent dual-element, short-circuit and overloadprotection). time-delay) protection. (Specification
grade product)
All type loads LOW-PEAK® LPJ_SP 600V J 300 All-purpose fuses.(optimum (dual-element, Unequaled for combinedovercurrent 0 time-delay) short-circuit and overloadprotection). to protection. (Specification __________ 600A grade product)
Non-motor loads LIMITRON® JKS 600V J 200 Very similar to KTS-R(circuits with no (quick-acting) LIMITRON®, but smaller.heavy inrush 0 to T-TRONTM JJN 300V T 200 The space saver (1⁄3 the sizecurrents). 1200A (fast-acting) JJS 600V of KTN-R/KTS-R).
Motor loads 0 LOW-PEAK® LP-CC 600V CC 200 Very compact (13⁄32˝ x 11⁄2˝);(circuits with to (time-delay) rejection feature. Excellent for motorheavy inrush 30A circuit protection.currents.)
Non-motor loads 0 LIMITRON® KTK-R 600V CC 200 Very compact (13⁄32˝ x 11⁄2˝);(circuits with no to (fast-acting) rejection feature. Excellent for outdoorheavy inrush 30A highway lighting.
Branch currents.)
Control transformer 0 TRON® FNQ-R 600V CC 200 Very compact (13⁄32˝ x 11⁄2˝);circuits and lighting to (time-delay) rejection feature. Excellent for controlballasts; etc. 30A transformer protection.
General purpose; 0 SC SC 0-20A 600V G 100 Current limiting;i.e., lighting to 21-60A 480V 13⁄32˝ dia. x varyingpanelboards. 60A lengths per amp rating.
Miscellaneous 0 ONE-TIME NON 250V H or K5† 10 Forerunners ofto NOS 600V the modern600A cartridge fuse.
General Plug fuses can FUSTAT® S 125V S 10 Base threads of Type SPurpose be used for (dual-element, differ with amp ratings. (non- branch circuits 0 time-delay) T and W have Edison base. current and small to FUSETRON®
T 125V ** 10 T & S fuses recommended limiting component 30A (dual-element, for motor circuits. W notfuses) protection. time-delay) recommended for circuits
Buss Type W W 125V ** 10with motor loads.
** U.L. Listed as Edison Base Plug Fuse.†Some ampere ratings are available as U.L. Class K5 with a 50,000A interrupting rating.
††RK1 and RK5 fuses fit standard switches, equipped for non-rejection fuses (K1, K5 and H) fuseblocks and holders; however, the rejection feature of Class R switches andfuseblocks designed specifically for rejection type fuses (RK1 and RK5) prevent the insertion of the non-rejection fuses (K1, K5, and H).
+ Higher ampering ratings planned.***Class J performance, special finger-safe dimensions.
100,
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Buss® Branch Circuit Fuse Selection Chart (600 Volts or Less)
