types of overcurrents & calculations - nec

© 2001 Cooper Bussmann, Inc.

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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

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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