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LEARNING MODULE 10:
LOADCENTERS
101BASICS
SERIE
S
Cutler-Hammer
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LOADCENTERS
Welcome to Module 10, which is about loadcenters. A loadcenter is a device that
delivers electricity from a supply source to loads in residential and lightcommercial/industrial applications.
FIGURE 1. TYPICAL LOADCENTER
Like the other modules in this series, this one presents small, manageable sectionsof new material followed by a series of questions about that material. Study the
material carefully, then answer the questions without referring back to what youvejust read.
You are the best judge of how well you grasp the material. Review the material asoften as you think necessary. The most important thing is establishing a solidfoundation to build on as you move from topic to topic and module to module.
Key points are in bold.
Glossary terms are underlined and italicized the first time they appear.
You may view definitions of glossary items by clicking on terms and words that are
underlined and italicized in the text. You may also browse the Glossary by clickingon the Glossary bookmark in the left-hand margin.
WELCOME
A Note On Font
Styles
Viewing the
Glossary
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LOADCENTERS
Well step through each of these topics in detail:
Section Title Page Number
What Does a Loadcenter Do? 3
Applications 4
Basic Circuitry And Wiring 5
Residential Service Entrance Panel 5
Residential Distribution Panel 6
Commercial/ Industrial Service Entrance Panel 7
Commercial/ Industrial Distribution Panel 7
Grounding Issues 8
Grounding the Service Entrance Panel 8
Equipment Grounding Bus 9
Grounding Downstream Loadcenters 10 Ground Fault 12
Review 1 13
Loadcenter Components and Installation 14
Anatomy of a Loadcenter 14
Enclosure 15
Installation and Mounting 16
Circuit Breakers 18
Installing Circuit Breakers 18
How Many Circuit Breakers Fit in a Loadcenter? 20
Interchangeable vs. Non-InterchangeableBreakers
21
Ratings 21
Loadcenter Types 23
Main Breaker Loadcenter 23
Main Lug Only Loadcenter 24
Convertible Loadcenter 24
Selecting a Loadcenter 25
Required Application Information 25
Sample Application 26
Review 2 27
Glossary 28
Review Answers 31
WHAT YOU
WILL LEARN
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LOADCENTERS
Every residential and light commercial/industrial building utilizes electricity forlighting, receptacles, and/or appliance loads. Conductors are used to bring power
from the power lines to the building. Then a device is needed to divide this power
for branch circuits and to protect these branch circuits from overloads and
short circuits. This device is called a loadcenter.
Electricity moves from the supply source into the building to the loadcenter, and isthen distributed through the buildings branch circuits to the loads. Each branch
circuit is connected, orterminated, at the loadcenter.
Each branch circuit is protected by a circuit breakerhoused in the loadcenter. In the
event of a short circuit or an overload on a branch circuit, the circuit breaker
cuts the power before any property damage or personal injury can occur.
NOTE: New electrical installations use circuit breakers in the loadcenter, so we willnot discuss fuses in this training module.
SERVICEENTRANCE
GROUNDING
LOADCENTER
SWITCHES
LIGHTING
RECEPTACLES
APPLIANCES
FIGURE 2. ELECTRICAL DISTRIBUTION TO LOADS
WHAT
DOES A
LOADCENTER
DO?
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LOADCENTERS
There are two main applications for a loadcenter:
As a service entrance panel.This is the term for a loadcenter used at the
point at where the power enters a building. There can only be one service
entrance panel per building.
As a distribution panel.A distribution panel is simply a loadcenter used at apoint beyond the buildings service entrance. This can be useful when
adding additional electrical service to an existing building. The power to thedistribution panel usually comes from a branch circuit of the service entrancepanel.
FIGURE 3. A SERVICE ENTRANCE PANEL AND A DISTRIBUTION PANEL
Applications
SERVICE
ENTRANCE
PANEL
DISTRIBUTIONPANEL
POWERFROM
UTILITY
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LOADCENTERS
Figure 4 shows a typical residential application. Loadcenters are typically rated 225
amps or less, and a maximum of 240 volts. Most homes have 200 amp service
and a 120/240-volt, single-phase, three-wire system. Three conductors run from
the distribution transformerto the service entrance panel. This transformer converts
the utility voltage into a voltage and current supply suitable for use in mostresidential applications. Two of the main service conductors (ormains) are
ungrounded (hot), and the third is the neutral. The hots are connected to themain circuit breakerin the loadcenter. The neutral is attached to the neutral bar.
If a voltmeterreading is taken between the two hot conductors (A and B), it will
measure 240 volts. If a reading is taken between a hot conductor and the neutral(N), it will measure 120 volts.
FIGURE 4. TYPICAL SINGLE-PHASE, THREE-WIRE LOADCENTER
This is convenient because 240 volts are needed to power central air conditioners,electric clothes dryers, and electric range tops. At the same time, 120 volts arerequired to service lighting, small appliances, and receptacle loads.
BASIC
CIRCUITRY
AND WIRING
Residential
Service EntrancePanel
DISTRIBUTIONVOLTAGE
DISTRIBUTION
TRANSFORMER
A N B
GROUND
120120
240
NEUTRAL
L
M
LOADCENTER
LIGHTING
LOAD
MOTOR
LOAD
BUS BARS
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LOADCENTERS
Now, lets consider the case of a residential distribution panel. Remember that adistribution panel is a panel used at a point beyond the service entrance. Thismeans that the distribution panel is typically fed by a service entrance panel.
You can add on to a loadcenter in two ways. The first way is to use a branch circuitbreaker. The second way is to use feed-through lugs.
MAINBREAKER
FEED-THROUGHLUGS
TRANSFORMER
MAIN LUGS
DISTRIBUTION PANEL
SERVICEENTRANCE
PANEL
N A B
N
N
A
A
B
B
FIGURE 5. A SERVICE ENTRANCE PANEL FEEDING A DISTRIBUTION PANEL
In the example shown here, the service conductors (the two hots and the neutral)run from the feed-through lugs of the service entrance panel to the main lugs of the
distribution panel. In this way, the distribution panel itself is protected by the
main circuit breaker in the service entrance panel. The neutral conductor is
isolated. The importance of this will be discussed later.
Usually, a residential distribution panel is used to supply a washer, dryer, air-conditioner and/or hot tub.
Residential
Distribution
Panel
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LOADCENTERS
The commercial/industrial service entrance panel is supplied with a different power
system than the residential one. A distribution transformer provides a 120/208
volt, three-phase, four-wire system to the panel. Four conductors go to the
service entrance. The three mains are ungrounded (hot), and the fourth is theneutral. The hots are connected to the main breaker in the loadcenter. The neutralis attached to the neutral bar and is connected the same way as in the residentialapplication.
If a voltmeter reading is taken between any two hot conductors (A, B and C), itwill measure 208 volts. If a reading between any hot conductor and the neutral (N)is taken, it will measure 120 volts.
FIGURE 6. TYPICAL THREE-PHASE, FOUR-WIRE LOADCENTER
This system can handle single-phase or three-phase applications. The 120/208
volt single-phase power is used the same way as in the residential application. Thethree-phase, 208-volt power is rarely used. For this reason, we will not discussthree-phase any further in this module.
The distribution panel is treated the same as the residential distribution panel. It istypically fed by the service entrance panel and it can be single phase or threephase. Again, it is important to remember that the neutral is isolated.
Commercial/
Industrial
Service Entrance
Panel
Commercial/
Industrial
Distribution
Panel
DISTRIBUTION
VOLTAGE
DISTRIBUTION
TRANSFORMER
A N C
GROUND
120120
NEUT
L
M
LIGHTING
LOAD
MOTOR
LOAD
BUS BARS
B
120
208 208
208
LOADCENTER
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LOADCENTERS
Grounding is an important aspect of any electrical equipment and must be
considered carefully. The National Electrical Code (NEC) defines ground as a
conducting connection, whether intentional or accidental, between an electricalcircuit or equipment and the earth, or to some conducting body that serves in placeof the earth. There are two objectives to the intentional grounding of electricalequipment:
Keep voltage differentials between different parts of a system at a minimumwhich reduces shock hazard.
Keep impedance of the ground path to a minimum. The lower the impedancethe greater the current is in the event of a fault. The greater the current thefaster an overcurrent device will open.
In the service entrance panel shown in Figure 7, the neutral from the transformer isconnected to the neutral bar in the loadcenter. The neutral bar is then grounded tothe earth.
TRANSFORMER
N A B
NEUTRALBUS
NEUTRALBONDEDTO BOX
GROUNDINGROD
FIGURE 7. GROUNDING A SERVICE ENTRANCE PANEL
GROUNDING
ISSUES
Grounding the
Service Entrance
Panel
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LOADCENTERS
This is accomplished by running a conductor from the loadcenters neutral bar to acold water pipe or copper rod buried in the ground (or other suitable groundingelectrode.) Then, a bondingscrew or strap is used to physically bond the neutral
bar to the metal enclosure.
In this way, the neutral bar, the neutral of the transformer, and the enclosure are
grounded. When the neutral is grounded in this manner, the equipment ground fromthe branch circuits is sometimes allowed to connect to the neutral.
The equipment grounding bus. is connected directly to the loadcenter enclosure.
For safety, all equipment (on both the feeder and branch circuits) is connected tothe equipment grounding bus. This requirement keeps the connected equipment atthe same potential as the enclosure itself.
EQUIPMENTGROUNDING BUS
FIGURE 8. EQUIPMENT GROUNDING BUS
One additional point for service entrance applications: this bonded and groundedneutral can also be used as the point of connection for equipment grounding.
Grounding the
Service Entrance
Panel
(continued)
Equipment
Grounding Bus
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LOADCENTERS
It is important to remember that the service entrance panel is the only ground
connection point for the neutral. The neutral is both insulated and isolated at any
downstream panels.
As shown in Figure 9, the downstream panels enclosure is grounded with agrounding conductor running back to the service entrance panel.
TRANSFORMER
INSULATEDNEUTRAL
DISTRIBUTIONPANEL
SERVICEENTRANCE
PANEL
EQUIPMENTGROUND
N A B
N
N
A
A
B
B
FIGURE 9. GROUNDING THE DOWNSTREAM LOADCENTER
Grounding
Downstream
Loadcenters
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LOADCENTERS
In Figure 10, the table lamp has a short circuit. If you trace the thick line back, youwill see how fault current is returned to the source. This is why the downstreamloadcenter contains a branch circuit breaker. It trips, disconnecting power from theload.
TRANSFORMER
DISTRIBUTIONPANEL
SERVICEENTRANCE
PANEL
N A B
N
N
A
A
B
B
INSULATEDNEUTRAL
EQUIPMENTGROUND
FIGURE 10. A SHORT IN A DOWNSTREAM LOAD
Grounding
Downstream
Loadcenters
(continued)
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LOADCENTERS
In most cases, NEC also requires equipment to have an additional grounding
conductor. This conductor connects the metal housing of an appliance to the
system ground to reduce the possibility of a shock due to a ground fault.
A ground fault is simply current leakage from an ungrounded conductor to thegrounding path in an electrical system. Consider Figure 11. A piece of insulation
covering a conductor at an electrical outlet has flaked off. This could allow currentto pass from the conductor to the metal outlet box. The box is part of the system
ground. If properly grounded, the current should follow the grounding path to
ground, instead of an unfortunate persons finger that touched the box.
FIGURE 11. GROUND FAULT
Since a distribution panel takes its power from the service entrance panel (Figure9), the neutral needs to be isolated from the ground. It is considered a branch circuitof the main service panel, and needs to have its own ground to prevent a ground
fault.
If all the loads in a panel are connected evenly between A-N and B-N, no currentwill flow in the neutral. However, loads are rarely balanced. As a result, somecurrent usually flows through the neutral. The amount current flowing in the neutralconductor at any one time is the difference between the current flowing through leg
A and leg B. This is why a path to ground is needed.
Ground Fault
BARE CONDUCTOR IN
CONTACT WITH
METAL OUTLET BOX
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LOADCENTERS
Answer the following questions without referring to the material just presented.
Begin the next section when you are confident that you understand what youve
already read.
1. Loadcenters have two main applications. These are:
_________________________________________________________
_________________________________________________________
2. In which two market segments are loadcenters typically used?
_________________________________________________________
________________________ / _______________________________
3. For safety reasons, the National Electrical Code (NEC) requires the serviceentrance panels __________ to be grounded. Beyond the service entranceequipment, it is always ____________ and ___________.
4. In your own words, explain under what conditions no current will flow throughthe neutral. Then explain how to figure the amount of current flowing throughthe neutral.
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
5. In your own words, explain the main purpose of a loadcenter.
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
REVIEW 1
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LOADCENTERS
Before we go any further in our exploration of the loadcenter, lets take a moment toconsider the main components and features of a typical loadcenter.
FIGURE 12. ANATOMY OF A TYPICAL LOADCENTER
1. Front Cover Provides a neat, protective face for the product. Usually includes ahinged door to allow access to the circuit breaker handles.
2. Inner Cover orDead Front
Serves as a protective barrier to prevent contact with energized (hot)parts within the unit.
3/4. Inner CoverAdjustmentScrews
Used to secure the inner cover tightly against the tops of the circuitbreakers. This eliminates any space that might allow access to hotparts inside.
5. Twistouts Removable perforated metal spaces that can be twisted out, to allowthe protrusion of the circuit breaker handles.
6. Wiring Diagram Lists the specifics of the loadcenter, including: circuit breakerapplication, UL information, short circuit ratings, series ratings, wireconnection torque values, and unit accessories.
7. Sliding Latch Provided on indoor loadcenters to allow easy opening of the loadcenterdoor.
8. Door The hinged part of the front cover which allows access to the circuit
breaker handles.
9/19. Backpan Topand BottomMountingScrews
Allows for easy removal of the loadcenter interior assembly.
LOADCENTER
COMPO-
NENTS AND
INSTALL-
ATION
Anatomy of a
Loadcenter
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LOADCENTERS
10/18.
EnclosureMountingKeyholes
Located at top center and bottom center. Whether surface-mountedright-side up or inverted, the unit can be easily centered on the wall byhanging the enclosure with the uppermost keyhole.
11. Drywall ScribeMarkings
Serve as a guide for mounting the loadcenter flush with the drywall andbetween the wall studs.
12. SurfaceMounting Holes(4)
Once centered for surface mounting, the loadcenter is affixed withscrews through these four holes.
13. Main Lugs orMain Breaker
Provide the means for the termination of the power cables coming fromthe electric meter. Main lugs simply provide a safe mechanicalconnection to the bus bars. A main breaker provides thermal magneticprotection for the loadcenter.
14. Backpan The solid steel piece that supports the loadcenter bus bar assembly. Italso provides the means to affix branch circuit breakers.
15. Bonding Strap Used to bond the neutral to the enclosure (when required by code).
16. Bus Bars (2) An extension of the incoming power cables. Provide a mechanicalmeans of affixing branch circuit breakers. Notches are added to (oromitted from) the bus bars to limit the number of branch circuitsallowed.
17. Knockouts Circular indentations in the top, bottom, sides, and back of theenclosure. When removed, the electrician can bring wire into and out ofthe loadcenter without compromising safety.
20. Neutral Bar Provides the termination point for the neutral wires from both theincoming service and the load circuits. Most loadcenters have a twinneutral design, positioning terminations on both sides of the unit. Thisis a wiring convenience for the electrician.
The loadcenter enclosure is typically constructed of cold rolled or galvanized steel.It serves to house the other loadcenter components. It is designed to providecomponent and personnel protection.
The National Electrical Manufacturers Association (NEMA) and UL have establishedguidelines for electrical enclosures. They are as follows:
NEMA Type 1
General
Purpose
This enclosure type is for general purpose, indoor use. It issuitable for most applications where unusual service conditionsdo not exist. It provides protection from accidental contact withenclosed equipment
NEMA Type 3R
Raintight
This enclosure type is intended for outdoor use. It providesprotection against falling rain and sleet, and damage fromexternal ice formation. It has a gasket cover.
Enclosure
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LOADCENTERS
A loadcenter can be flush-mountedorsurface-mountedagainst a wall. A flush-
mounting is recessed into a hole in the wall. A surface-mounting is attached to (andprojects out from) the wall.
Practically all loadcenters are flush mounted, which offers some important
benefits such as:
Space savings Since the box is behind the wall, the room is not robbed ofspace
Appearance This option is more attractive, as the wiring and box areconcealed
Safety Except for the trim, the panel does not jut out to possibly catch clothingor other objects
FIGURE 13. FLUSH-MOUNT VS. SURFACE-MOUNT
Surface-mounted loadcenters are generally used in industrial buildings, and in
basements of office and commercial buildings. Since most of these areas have
walls made of poured concrete, flush-mounting is impractical. Steel columns areoften used for surface mounting.
Installation and
Mounting
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LOADCENTERS
The NEC specifies clearances around loadcenters. This is to provide access andworking space. There are three basic rules:
Headroom in the location must be a minimum of 6-1/2 feet.
For systems up to 150 volts, the minimum distance from the loadcenter to the
ground shall be 3 feet.
For access, there must be 30 inches minimum space in front of the loadcenter,and sufficient space to allow the hinged door to open and rotate 90 degrees.
6 FEETHEADROOM
30 INCHES IN FRONTOF DOOR
SPACE ALLOWANCEOF FOR
DOOR TO OPEN90 DEGREES
FIGURE 14. LOADCENTER CLEARANCE REQUIREMENTS
Installation and
Mounting
(continued)
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LOADCENTERS
Now, lets take some time to look at one of the main components of a loadcenter:the circuit breaker.
Each branch circuit in the loadcenter is protected by a branch circuit breaker(better
known as a miniature circuit breaker). Miniature circuit breakers are covered in
much more detail in Module 9, Miniature Circuit Breakers.
Miniature circuit breakers are also calledplug-in breakers because they are
connected by plugging them into the bus barstabs.
The bus bars provide a convenient means of providing electricity to the various
loads in a building. The circuit breakers attach to the bus bars on vertical stabs
that alternate from each hot source.
Single pole and double pole circuit breakers are the most commonly used breakers
in a loadcenter. If a double pole breaker is plugged onto two adjacent stabs,
240 volts are drawn. A single pole breaker is plugged onto a single bus stab
and feeds a 120-volt circuit. To understand how this is possible, consider the
physical shape of the bus bars.
FIGURE 15. GETTING 120V AND 240V POWER FROM THE BUS BARS
CIRCUIT
BREAKERS
Installing Circuit
Breakers
DOUBLE
POLE
BREAKER
SINGLE
POLE
BREAKER
DOUBLE POLE BREAKERPLUGGED ONTO A STAB
FROM EACH BUS BAR
DRAWS 240 VOLTS SINGLE POLE BREAKER
PLUGGED ONTO ONLY ONE
STAB
DRAWS 120 VOLTS
BUS BARS
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LOADCENTERS
Now, to reinforce the point, consider Figure 16.
FIGURE 16. TYPICAL SINGLE-PHASE, THREE-WIRE LOADCENTER
Trace the 240-volt M circuit. Current comes in on conductor A, flows through theleft bus bar and branch circuit breaker, and enters the motor loads branch circuit.From there, current passes through another branch breaker and out on conductorB. The circuit is made across both ungrounded legs of the transformer.
Trace the 120-volt L circuit. Current comes in on conductor A, flows through theleft bus bar and branch circuit breaker, and enters the lighting loads branch circuit.From there, current passes out on the neutral N. The circuit is made from anungrounded leg of the transformer to the transformers grounded leg.
A 240-volt circuit typically consists of four wires: two "hots", one neutral, and oneground. A 120-volt circuit consists of three wires: one hot, one neutral, and oneground. Usually, hot wires are both color-coded black, although the second hotcould be red. Neutral wires are always white. Ground wires are either green or bare(uninsulated) copper.
DISTRIBUTION
VOLTAGE
DISTRIBUTIONTRANSFORMER
A N B
NEUTRAL
120120
240
NEUTRAL
L
M
LOADCENTER
LIGHTING
LOAD
MOTOR
LOAD
BUS BARS
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LOADCENTERS
The number of single-pole circuit breakers that can be installed in a loadcenter
is limited by the number of stabs on the bus bars.
A breaker can be applied to each side (right and left) of a stab. Therefore, aloadcenter with six stabs would accommodate twelve single-pole circuit breakers.To balance the loads, the breakers should be evenly applied to each side of the
stab.
FIGURE 17. BREAKER INSTALLATION (VIEWED FROM ABOVE)
If the stab is notched, then half-size branch circuit breakers can be used. Half-sized
branch circuit breakers allow for two single-pole circuit breakers to be
installed in one space. Each unit is typically only 1/2 wide. These breakers are
every bit as functional as the full-size units and allow the added convenience of amore compact installation.
Some local inspectors will not allow the use of half-size branch circuit
breakers. This is because there is a possibility of overloading the loadcenter and/or
unbalancing the system.
FIGURE 18. SOLID AND NOTCHED AND STABS
The number of notched and unnotched stabs in a particular loadcenter andtherefore the number of breakers that may be installed is limited by ULrequirements. This is to prevent a panel from being overloaded.
Regardless of ampere ratings, a lighting and appliance type of loadcenter is
limited to 42 circuits in one enclosure. If the specifications require more than 42
circuits, two or more loadcenters will be required.
How Many
Circuit Breakers
Fit in a
Loadcenter?
BRANCH CIRCUIT
BREAKERSTAB
NOTCHED TAB ALLOWS
FOR ADDITONAL
BREAKER INSTALLATION
ON LEFT SIDE
BUS BARS
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LOADCENTERS
Branch circuit breakers are available from many manufacturers and have evolvedover the years. Those referred to as interchangeable have 1 wide frames, which
means that a single pole, interchangeable breaker is 1 wide, or takes up a 1 spacein the loadcenter.
While these units, in many cases, will physically fit in another interchangeable
manufacturers loadcenter, it is not approved by the NEC, UL, or the manufacturerof the panel.
Non-interchangeable branch circuit breakers typically have a 3/4 frame width for a
single-pole unit and are unique to a single manufacturers loadcenter. These unitscannot be installed into another manufacturers loadcenter because they will notphysically fit.
Every circuit breaker has a specific ampere, voltage, and fault current
interruption rating.
Ampere rating The ampere rating defines the maximum current a circuit breaker
can carry without tripping. The typical for miniature circuit breakers is ratings from15-125 amps.
Voltage rating In residential applications, single pole breakers protect 120 voltbranch circuits and two-pole breakers protect 240 volt branch circuits. The rating ofa circuit breaker can be higher than the circuit voltage, but never lower.
Short circuit interrupting rating This is the maximum available fault current that abreaker is designed to interrupt. Typical ratings is from 10,000 amps to 65,000amps.
Interchangeable
vs. Non-
Interchangeable
Breakers
Ratings
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LOADCENTERS
When selecting loadcenters and overcurrent protection devices, it is extremelyimportant to know both the maximum continuous amperes and available faultcurrent. There are two ways to meet this requirement: the full rating methodand the
series-rated method.
The full rating method selects circuit protection devices with ratings equal to
or greater than the available fault current.
Consider a building service entrance with has 22,000 amps of fault currentavailable. All downstream circuit protection devices must be rated at 22,000 amps.
The requirements of the series-rated method are somewhat looser. The buildings
main circuit protection device must have an interrupting rating which is at
least equal to the systems available fault current. However, the all downstream,
series-connected panels can have lower ratings.
Consider the same building, still with 22,000 amps of available fault current. Whilethe breaker at the service entrance is rated at 22,000 amps, additional downstream
breakers could be rated at only 10,000 amps.
FIGURE 19. FULL RATING METHOD (AT LEFT) VS. SERIES-RATED METHOD (AT RIGHT)
In the typical single family home, available fault current is normally 10,000 amps orless.
Ratings
(continued)
MAIN BREAKER
22,000 AMPSMAIN BREAKER
22,000 AMPS
BRANCH BREAKERS
22,000 AMPS
BRANCH BREAKERS
10,000 AMPS
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LOADCENTERS
There are three different types of loadcenters made today. These are:
Main Breaker
Main Lug Only
Convertible
MAIN BREAKER MAIN LUG CONVERTIBLE
FIGURE 20. MAIN CONFIGURATIONS
The incoming supply cables of a main breaker loadcenter are connected to the
main breaker, which in turn feeds power to the loadcenter and its branch
circuits. Power from the mains is fed through the main breaker to the bus bars,
which are an extension of the utility power cables.
With the main circuit breaker in the off position, no power will flow to the bus barsfor the branch circuit breakers. This serves as a quick and simple way to disconnectpower to the entire building.
FIGURE 21. SIMPLIFIED MAIN BREAKER TYPE LOADCENTER
With the main breaker in the on position, power is allowed to flow from the
bus bars to the branch circuits. Branch circuits run throughout the building to
supply power for lighting and appliances. Because of these features, the mainbreaker is normally used as a service entrance panel.
LOADCENTER
TYPES
Main Breaker
Loadcenter
MAINS
MAIN BREAKER
BRANCH CIRCUIT
BREAKERS
BRANCH CIRCUITS
(TO LIGHTS, ETC.)
BUS BARS
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LOADCENTERS
The main lug only (MLO) loadcenterdoes not contain a main circuit breaker to
protect the loadcenter itself. Instead, it is protected by a breaker back in an
upstream panel. The supply cables from the upstream panel are connected to
the main lugs and bus bars. This type of loadcenter is used primarily used in a
distribution panel.
The MLO is often called an add-on, secondary, or downstream panel. It is mostoften used when the main breaker loadcenters circuit slots are full, or to providepower at a remote point.
The convertible loadcenteris supplied without lugs or breakers. Once the
customer decides to use main lugs or main breakers, the appropriate kit is
installed by an electrician.
In this home, a main breaker
loadcenter supplies power to amain lug only loadcenter in thehomes workshop.
Main lug only loadcenters canalso be fed from meteringequipment when used inapartment installations.
MAIN BREAKERLOADCENTER
MAIN LUG ONLYLOADCENTER
USING A MAIN LUG ONLY LOADCENTER
Main Lug Only
Loadcenter
Convertible
Loadcenter
IN THE WORKPLACE
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LOADCENTERS
When assisting a customer in selecting a loadcenter for an application, it is best tostart by interviewing the customer to determine the applications requirements.
1. Ask whether this application will involve an indoor or outdoor installation. Thiswill determine the enclosure type for the loadcenter.
NEMA Type 1General
Purpose
This enclosure type is for general purpose, indoor use. It issuitable for most applications where unusual serviceconditions do not exist. It provides protection fromaccidental contact with enclosed equipment.
NEMA Type 3R
Raintight
This enclosure type is intended for outdoor use. It providesprotection against falling rain and sleet, and damage fromexternal ice formation. It has a gasketed cover.
If it is an indoor installation, ask whether it will be flush-mounted or surfacemounted.
FIGURE 22. OUTDOOR, FLUSH AND SURFACE MOUNTING TYPES
2. Ask whether the loadcenter is to serve as a service entrance panel or adistribution panel.
3. Ask whether the application involves a single-phase, three-wire system or athree-phase, four-wire system.
4. Ask what type of main, either a Main Lugs Only or a Main Circuit Breaker.
5. Ask for the main ampere rating.
6. Ask what type and rating main breaker is required, if applicable.
7. Ask for the number of breakers required, and for their ratings.
8. Ask how many spaces are required. The customer may want to leave extraspaces in the loadcenter for future needs.
Once this information is obtained, go to the product catalog. These questionsshould cover everything needed to recommend products that fit the customersapplication needs.
SELECTING A
LOADCENTER
Required
Application
Information
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LOADCENTERS
To assist you in visualizing the product selection process, lets consider a simple,commonplace customer application.
Suppose a customer comes to you and wants to add two branch circuit breakers toan existing loadcenter for a newly remodeled kitchen. How would you go aboutsizing the breaker?
First, interview the customer to find out what loads will be on the branch circuits.Lets say that one will be dedicated to a dishwasher, and the other will be dedicatedto a garbage disposal.
Next, obtain the amperage requirements of the loads from the customer. Supposethe dishwasher runs on 12.5 amps and the garbage disposal runs on 9.8 amps.
Based on these numbers alone, you might think that it would be a good idea torecommend a 15 amp breaker for the dishwasher circuit, and a 10 amp breaker forthe garbage disposal circuit. But this would result in a lot of nuisance tripping.
So, we need to do a little simple math. Figure in a 25% safety margin by multiplyingthe amp ratings by 1.25.
Dishwasher 12.5 amps x 1.25 = 15.625 amps
Garbage Disposal 9.8 amps x 1.25 = 12.25 amps
This will be sufficient to eliminate nuisance tripping, but the breakers will still trip inthe case of a true overload condition.
Based on these calculations, you would recommend a 20 amp breaker for thedishwasher circuit, and a 15 amp breaker for the garbage disposal circuit.
Sample
Application
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LOADCENTERS
Answer the following questions without referring to the material just presented.
1. List the three main loadcenter types made today.
____________________________________________________________
____________________________________________________________
____________________________________________________________
2. In your own words, explain why a double pole breaker draws twice the voltageof a single pole breaker installed in the same panel.
____________________________________________________________
____________________________________________________________
____________________________________________________________
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3. Regardless of ampere ratings, a lighting and appliance type of loadcenter islimited to _____ circuits in one enclosure.
4. List four application questions to ask when working with a customer.
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____________________________________________________________
____________________________________________________________
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REVIEW 2
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Bonding The permanent joining of metallic parts to form anelectrically conductive path that will ensure electricalcontinuity to ground.
Branch Circuit A circuit that supplies power to a load in a building and isterminated at the loadcenter.
Branch Circuit
Breaker
Also Miniature Circuit Breaker. A reusable overcurrentprotection device, used to protect a branch circuit. Aftertripping to break the circuit, it can be reset to protect thebranch circuit again.
Bus Bars A component of a loadcenter that serves as an extension ofthe main service conductors. Simplifies the connection ofbranch circuit breakers to the main service conductors.
Circuit Breaker A reusable overcurrent protection device. After tripping tobreak the circuit, it can be reset to protect the circuit again.
Convertible
Loadcenter
A loadcenter supplied without main lugs or a main breaker.Once in the field, it is up to the electrician to install theappropriate device.
Distribution Panel A loadcenter used at a point beyond the buildings serviceentrance. It is not supplied with a main breaker. This type ofpanel can be useful when adding additional electricalservice to an existing building. It has an isolated neutral.
Distribution
Transformer
A device that converts utility voltage into a voltage andcurrent supply suitable for use in most residentialapplications.
Double Pole Term used to describe a breaker that draws power fromboth poles of a loadcenter.
Equipment
Grounding Bus
A bus connected directly to the enclosure. Used forgrounding all feeder and branch circuit equipment.
Feed-Through
Lugs
A set of lugs in a service entrance panel where feedercables for a distribution panel are terminated.
Flush-Mounted A style of mounting a loadcenter in which the loadcenter isrecessed into a hole in the wall.
Full Rating
Method
A method of selecting circuit protection devices for use in aloadcenter. All devices must have ratings equal to orgreater than the available fault current.
GLOSSARY
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Fuse A non-reusable overcurrent protection device. After trippingto break the circuit, it must be replaced to restore power tothe circuit.
Ground Fault Current leakage from an ungrounded conductor to thegrounding path in an electrical system
Grounding Path A solid conducting path for electricity to follow to ground.
Half-Size Branch
Circuit Breakers
A specialized reusable overcurrent protection devicedesigned to take up only half as much space in aloadcenter as a normal branch circuit breaker. Can only beinstalled in loadcenters equipped with notched stabs.
Interchangeable A type of branch circuit breaker that is standardized in sizeto fit in many different manufacturers loadcenters.
Loadcenter A device that delivers electricity from a supply source toloads in light commercial or residential applications.
Mains Also Main Service Conductors. The conductors that bringelectricity into a building from the power source.
Main Circuit
Breaker
Also Main Breaker. A reusable overcurrent protectiondevice designed to protect an entire loadcenter.
Main Lugs Component of a loadcenter not equipped with a mainbreaker. Termination point for a feeder cable from anotherloadcenter.
Main Lug Only
(MLO) Loadcenter
A loadcenter where power from the mains is fed directly to
the bus bars.
Main Service
Conductors
Also Mains. The conductors that bring electricity into abuilding from the power source.
Miniature Circuit
Breaker
Also Branch Circuit Breaker. A reusable overcurrentprotection device, used to protect a branch circuit. Aftertripping to break the circuit, it can be reset to protect thebranch circuit again.
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Non-
Interchangeable
A type of branch circuit breaker that is unique to a singlemanufacturers loadcenters. This type of breaker cannot beinstalled into another manufacturers loadcenter because itwill not physically fit.
Plug-In A style of miniature circuit breaker, so named for the
method of installation into the loadcenter. It is literallyplugged into the bus bar stabs.
Series-Rated
Method
A method of selecting circuit protection devices for use in aloadcenter. The main upstream circuit protection devicemust have an interrupting rating equal to or greater than theavailable fault current of the system. Downstream devicesconnected in series can be rated at lower values.
Service Entrance
Panel
The term used to describe a loadcenter used as a serviceentrance.
Single Pole Term used to describe a breaker that draws power fromonly one pole of a loadcenter.
Stab A protrusion on the bus bars of a loadcenter which acceptsa miniature circuit breaker.
Surface-Mounted A style of mounting a loadcenter in which the loadcenter isattached to and projects out from the wall.
Terminated The end connection of a circuit. For example, branchcircuits are terminated at the service entrance panel.
Voltmeter A device used to find the potential voltage between two
points.
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1.
distribution panel
service entrance panel
2.
residential
commercial / light industrial
3. neutral, insulated, isolated
4. Answer should basically say If all the loads in a panel are connected exactlyevenly between A-N and B-N, no current will flow in the neutral. The amountcurrent flowing in the neutral conductor at any one time is the differencebetween the current flowing through leg A and leg B.
5. Answer should basically say To supply power to lights, receptacles and loadslike dryers, washers, and air conditioners.
1.
Main Breaker
Main Lug Only
Convertible
2. Answer should basically say A double pole breaker is plugged onto twoadjacent bus stabs. The circuit is made across both ungrounded legs of thetransformer. A single pole breaker is plugged onto a single bus stab. The circuitis made from an ungrounded leg of the transformer to the transformersgrounded leg.
3. 42
4. Any four of the following:
Indoor or outdoor installation?
If indoor, flush-mount or surface-mount?
Service entrance panel or a distribution panel?
Single-phase, three-wire system or a three-phase, four-wire system?
What type of main, Main Lugs Only or Main Circuit Breaker?
Main ampere rating?
What type and rating main breaker is required? (if applicable)
How many branch breakers required, and the rating of each?
How many spaces are required?
REVIEW 1
ANSWERS
REVIEW 2
ANSWERS
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Cutler-HammerM ilw aukee, W isconsin U.S.A.
