© 2001 Cooper Bussmann, Inc.

Suggestion on How to Use

• Industry Trainers are encouraged to use this material in their sessions

• Download both the PowerPoint file (.ppt) and script file (.pdf)

• Print the script file (.pdf) and read the script as you view the PowerPoint presentation in the “Slide Show” view. In this way you see the slides in large format and have animation (if there is any)

• Must have PowerPoint and Adobe Reader application software on your system.

Types of Overcurrent&

Calculations

© 2001 Cooper Bussmann, Inc.

NEC® Article 100 – Definitions

Overcurrent Any current in excess of the rated current of

equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault.

© 2001 Cooper Bussmann, Inc.

NEC® Article 100 – Definitions

Overload Operation of equipment in excess of normal, full-load rating,

or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating. A fault, such as a short circuit or ground fault, is not an overload.

© 2001 Cooper Bussmann, Inc.

Normal LoadSo

urce

R R

RR

R

R

Load

R

IL= VS / RT

VS

IL

IL

© 2001 Cooper Bussmann, Inc.

OverloadSo

urce

Within the normal path

R R

RR

R

R

LoadR

VS

IOL= VS / RT

© 2001 Cooper Bussmann, Inc.

Short CircuitSo

urce

Outside the normal path

R R

RR

R

R

ISC= VS / RT

VSLoadR

Out of the circuit

© 2001 Cooper Bussmann, Inc.

Types of Faults

• Bolted Faults• Arcing Faults

© 2001 Cooper Bussmann, Inc.

BoltedShort Circuit

A B

Arcing Fault

A B

Current Thru Air

© 2001 Cooper Bussmann, Inc.

Bolted Faults –Various Types

ABC

GroundN

480Y/277 V3 / 4WSolid Grd

© 2001 Cooper Bussmann, Inc.

Bolted Faults – Three Phase

ABC

GroundN

480Y/277 V3 / 4WSolid Grd

Typically considered the “worst case” or highest magnitude

© 2001 Cooper Bussmann, Inc.

Bolted Faults – Line to Line

ABC

GroundN

480Y/277 V3 / 4WSolid Grd

87 % of the three phase bolted fault

© 2001 Cooper Bussmann, Inc.

Bolted Faults – Line to Ground

ABC

GroundN

480Y/277 V3 / 4WSolid Grd

Typically much lower than 3 fault, but can be > 3 fault near Xfmr terminals

© 2001 Cooper Bussmann, Inc.

A B

Arcing Faults – Many Variables Effect Current & Whether Sustainable

System VoltageGap spacingAvailable 3 Short Circuit AmpsAmount of Copper VaporizedDegree of ContainmentConfiguration of Equipment

Typically does not sustain on 208Y/120V

© 2001 Cooper Bussmann, Inc.

Arcing Faults – Progression

ABC

GroundN

480Y/277 V3 / 4WSolid Grd

© 2001 Cooper Bussmann, Inc.

Arcing Faults – Three Phase

ABC

GroundN

480Y/277 V3 / 4WSolid Grd

Can vary widely possibly up to 89% of 3 bolted fault

© 2001 Cooper Bussmann, Inc.

Arcing Faults – Line to Line

ABC

GroundN

480Y/277 V3 / 4WSolid Grd

Can vary widely possibly up to 74% of 3 bolted fault

© 2001 Cooper Bussmann, Inc.

Arcing Faults – Line to Ground

ABC

GroundN

480Y/277 V3 / 4WSolid Grd

Can vary widely

© 2001 Cooper Bussmann, Inc.

Arcing Faults – Sustainability

ABC

GroundN

480Y/277 V3 / 4WSolid Grd

Rule of thumb:Arcing faults will typically not sustain

at less than 38% of 3 bolted fault

© 2001 Cooper Bussmann, Inc.

How do you know what the short-circuit current is throughout a system?

andWhat are some typical values

© 2001 Cooper Bussmann, Inc.

MM

MSB

Short Circuit Currents Vary Depending on Many

Factors

Transformer Size & % ZVoltageConductor Size & Length

© 2001 Cooper Bussmann, Inc.

MM

MSB

Short Circuit Currents Vary Depending on Many

Factors

60,000 A

27,000 A40,000 A

60,000 A

18,000 A

9,000 A

© 2001 Cooper Bussmann, Inc.

500 KVA

480/277V

1

5 % Z

500 KVA

480/277V

1A

2 % Z

1500 KVA

480/277V

2

5% Z

1500 KVA

480/277V

2A

2 % Z

Short Circuit Current Examples #1

1500 KVA

208/120V

3

5% Z

1500 KVA

208/120V

3A

2 % Z

© 2001 Cooper Bussmann, Inc.

How to Calculate Transformer Secondary (assuming infinite primary)

Isca = (Xfmr FLA) x 100 / %Z

(increase result by 10% due to UL

tolerance for transformer impedances)

© 2001 Cooper Bussmann, Inc.

500 KVA

480/277V

1

5 % Z

500 KVA

480/277V

1A

2 % Z

13,222 A

33,055 A

1500 KVA

480/277V

2

5% Z

1500 KVA

480/277V

2A

2 % Z

39,666 A

99,165 A

Short Circuit Current Example #1 Answers

1500 KVA

208/120V

3

5% Z

1500 KVA

208/120V

3A

2 % Z

91,608 A

229,020 A

© 2001 Cooper Bussmann, Inc.

208/120V

4

4A

6

6A

480/277V

5

5A

480/277V

40,000 A. 40,000 A.40,000 A.

50 ft # 1 50 ft 250 kcm

Short Circuit Current Example #2

© 2001 Cooper Bussmann, Inc.

Calculating Short Circuit Currents

Utilize Point-to-Point MethodSteps 4, 5 & 6 for 3 Faults

© 2001 Cooper Bussmann, Inc.

Short Circuit Current Example #2 Answers

208/120V

4

4A

40,000 A.

50 ft # 1

12,367 A.

480/277V

5

5A

40,000 A.

20,322 A.

6

6A

480/277V

40,000 A.

50 ft 250 kcm

28,818 A.

© 2001 Cooper Bussmann, Inc.

Summary / Questions / To come• Overcurrent – overloads, bolted faults, & arcing

faults• Fault currents can be determined through out

distribution system• Short circuit currents needed to assess I.R.,

Component Pro, Coordination, Arc Flash Hazards & OCPD Selection