Conductor Short Circuit Protection (10-6-99)
 

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Introduction:

This paper analyzes the protection of wire from fault currents.  It gives the specifier the necessary information regarding the short-circuit current rating of wire.  Proper protection of wire will improve reliability and reduce the possibility of injury.  Wire can be destroyed if the overcurrent devices do not limit the short-circuit current to within the short-circuit current rating of the wire.  Merely matching the ampere rating of the wire with the ampere rating of a protective device will not assure component protection of the wire under short-circuit conditions. 

In the past several years, there have been numerous reports in newspapers, magazines and insurance company files about destroyed electrical systems.  Recognizing this as a serious problem to safety of life and property, much more emphasis has been placed on COMPLIANCE with THE NATIONAL ELECTRICAL CODE®

The National Electrical Code covers COMPONENT PROTECTION in several sections. The first section to note is Section 110-10.

Component Protection and the National Electrical Code:

Per NEC® Section 110-10. Circuit Impedance and Other Characteristics:

"The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated as permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors, or between any circuit conductor and the grounding conductor or enclosing metal raceway.  Listed products applied in accordance with their listing shall be considered to meet the requirements of this section." 

This requires that overcurrent protective devices, such as fuses and circuit breakers be selected in such a manner that the short-circuit current ratings of the system components will not be exceeded should a short-circuit occur.

The “short-circuit current rating” is the maximum short-circuit current that a component can safely short-circuit current. Failure to provide adequate protection may result in component destruction under short-circuit conditions. 

The first step to verifying conductor short-circuit protection is to calculate the short-circuit current fault levels throughout the electrical system. The next step is to check the short-circuit current rating of wire and cable, based upon the calculated short-circuit fault current available and the overcurrent protective device selected.

Note: The let-through of the protective device must be equal to or less than the short-circuit current rating of the component being protected.

CAUTION: Choosing overcurrent protective devices strictly on the basis of voltage, current, and interrupting rating alone will not assure component protection from short-circuit currents.  High interrupting capacity electromechanical overcurrent protective devices, especially those that are not current-limiting, may not be capable of protecting wire, cable or other components within high short-circuit ranges.  The interrupting rating of a protective device pertains only to that device and has absolutely no bearing on its ability to protect connected down-stream components.  Quite often, an improperly protected component is completely destroyed under short-circuit conditions while the protective device is opening the faulted circuit without damage to itself.

CONDUCTOR SIZE:

Short-Circuit Currents for Insulated Cables

The recent increase in KVA capacity of power distribution systems has resulted in possible short-circuit currents of extremely high magnitude. Fault induced, high conductor temperatures may seriously damage conductor insulation.

As a guide in preventing such serious damage, maximum allowable short-circuit temperatures, which begin to damage the insulation, have been established for various insulation such as, Thermoplastic at 150°C.

The Insulated Cable Engineers Association (ICEA) protection chart, to the right, shows the currents, which, after flowing for the times indicated, will produce these maximum temperatures for each conductor size. The system short-circuit capacity, the conductor cross-sectional area and the overcurrent protective device opening time should be such that these maximum allowable short-circuit currents are not exceeded. 

Using the formula shown on the ICEA protection chart will allow the engineer to calculate short-circuit current ratings of cable not shown on these pages. It may be advantageous to calculate short-circuit current ratings below one cycle, when the opening time of the current-limiting device is known.

Protecting Equipment Grounding Conductors:

Safety issues arise when the analysis of equipment grounding conductors is discussed. Table 250-122 of the NEC offers minimum sizing for equipment grounding conductors. 

The problem of protecting equipment grounding conductors was recognized more than 30 years ago when Eustace Soares, wrote his famous grounding book “Grounding Electrical Distribution Systems for Safety”. In his book he states that the “validity” rating corresponds to the amount of energy required to cause the copper to become loose under a lug after the conductor has had a chance to cool back down. This validity rating is based upon raising the copper temperature from 75°C to 250°C. 

In addition to this and the ICEA charts, a third method promoted by Onderdonk allows the calculation of the energy necessary to cause the conductor to melt (if the copper conductor reaches a temperature 1,083°C). Table 1 offers a summary of these values associated with various size copper conductors. 

It becomes obvious that the word “Minimum” in the heading of table 250-122 means just that - the values in the table are a minimum - they may have to be increased due to the available short-circuit current and the current-limiting, or non-current-limiting ability of the overcurrent protective device. 

Good engineering practice requires the calculation of the available short-circuit currents (3-phase and phase-to-ground values) wherever equipment grounding conductors are used. Overcurrent protective device (fuse or circuit breaker) manufacturers’ literature must be consulted. Let-through energies for these devices should be compared with the short-circuit current rating of the equipment grounding conductors. Wherever let-through energies exceed the “minimum” equipment grounding conductor short-circuit current ratings, the equipment grounding conductor size must be increased until the short-circuit current ratings are not exceeded.

Table 1:

Comparison of Equipment Grounding Conductor Ratings (Based on RMS Symmetrical Amperes)

 
   

5 Second Rating (Amps)

I2t Rating (Amperes Squared Seconds)

Cond

Cond

ICEA

Soares

Onderdonk

ICEA

Soares

Onderdonk

Size

Area

Insulation

Annealing

Melting

Insulation

Annealing

Melting

 

Circ.

Deg. C

Deg. C

Deg. C

Deg. C

Deg. C

Deg. C

 

Mils

150

250

1083

150

250

1083

14

4110

97.31

139.84

251.35

47346

97774

315881

12

6530

154.61

222.18

399.34

119517

246812

797382

10

10380

245.76

353.17

634.79

301994

623640

2014813

8

16510

390.90

561.74

1009.68

764007

1577732

5097229

6

26240

621.27

892.79

1604.72

1929882

3985352

12875605

4

41740

988.25

1420.16

2552.63

4883238

10084258

32579535

3

52620

1245.85

1790.34

3218.00

7760768

16026576

51777572

2

66360

1571.17

2257.83

4058.27

12342860

25488942

82347942

1

83690

1981.48

2847.46

5118.10

19631350

40540229

130974616

1/0

105600

2500.23

3592.93

6458.01

31255818

64545637

208529659

2/0

133100

3151.33

4528.59

8139.79

49654561

102540438

331280679

3/0

167800

3972.91

5709.22

10261.88

78919977

162975743

526530956

4/0

211600

5009.94

7199.46

12940.49

125497294

259161440

837281168

250

250000

5919.11

8505.98

15288.86

175179408

361758775

1168745667

300

300000

7102.93

10207.18

18346.63

252258347

520932636

1682993761

350

350000

8286.76

11908.38

21404.40

343351639

709047199

2290741508

400

400000

9470.58

13609.57

24462.17

448459284

926102464

2991988908

500

500000

11838.22

17011.97

30577.71

700717631

1447035100

4674982669

600

600000

14205.87

20414.36

36693.26

1009033389

2083730545

6731975044

700

700000

16573.51

23816.75

42808.80

1373406557

2836188797

9162966032

750

750000

17757.33

25517.95

45866.57

1576614670

3255828976

10518711006

800

800000

18941.16

27219.15

48924.34

1793837136

3704409857

11967955634

900

900000

21308.80

30621.54

55039.88

2270325125

4688393726

15146943849

1000

1000000

23676.45

34023.93

61155.43

2802870525

5788140402

18699930677

Table 1 illustrates the conductor rating for 5 seconds as well as the maximum I2t rating based upon equipment grounding conductor size and the conductor damage level selected (ICEA, Soares or Onderdonk).  However, depending upon the device selected and the resulting opening time, the amount of current the equipment grounding (or phase) conductor can handle will need to be adjusted.  This is illustrated in the following tables and is valid for both phase and equipment grounding conductors. 

Table 2 shows the maximum short-circuit current rating of the conductor based upon the opening time of the device.  This table is based upon the ICEA Insulation damage level to raise the conductor from 75 degrees C to 150 degrees C.  The opening time of the device can depend upon the device selected or the short time setting of the device.  For example a low voltage power circuit breaker can have a short time delay of up to 30 cycles.  Molded case circuit breakers can have similar short time delay settings up to the instantaneous setting or the instantaneous override.

Table 2:

Maximum Short-Circuit Current Rating In Amperes (Per ICEA Insulation Damage 150 Deg C)

   
   

Maximum Short-Circuit Current Rating In RMS Symmetrical Amperes

 

Cond

Cond

1/2*

1

2

3

6

12

18

24

30

 

Size

Area

Cycles

Cycle

Cycle

Cycle

Cycle

Cycle

Cycle

Cycle

Cycle

 
 

Circ. Mils

0.0083

0.0167

0.0333

0.0500

0.1

0.2

0.3

0.4

0.5

 
 

 

Seconds

Seconds

Seconds

Seconds

Seconds

Seconds

Seconds

Seconds

Seconds

 

14

4110

2384

1685

1192

973

688

487

397

344

308

 

12

6530

3787

2678

1894

1546

1093

773

631

547

489

 

10

10380

6020

4257

3010

2458

1738

1229

1003

869

777

 

8

16510

9575

6771

4788

3909

2764

1954

1596

1382

1236

 

6

26240

15218

10761

7609

6213

4393

3106

2536

2197

1965

 

4

41740

24207

17117

12104

9883

6988

4941

4035

3494

3125

 

3

52620

30517

21579

15259

12459

8810

6229

5086

4405

3940

 

2

66360

38486

27213

19243

15712

11110

7856

6414

5555

4968

 

1

83690

48536

34320

24268

19815

14011

9907

8089

7006

6266

 

1/0

105600

61243

43305

30621

25002

17679

12501

10207

8840

7906

 

2/0

133100

77192

54583

38596

31513

22283

15757

12865

11142

9965

 

3/0

167800

97316

68813

48658

39729

28093

19865

16219

14046

12563

 

4/0

211600

122718

86775

61359

50099

35426

25050

20453

17713

15843

 

250

250000

144988

102522

72494

59191

41854

29596

24165

20927

18718

 

300

300000

173986

123026

86993

71029

50225

35515

28998

25113

22461

 

350

350000

202983

143531

101492

82868

58596

41434

33831

29298

26205

 

400

400000

231981

164035

115990

94706

66967

47353

38663

33484

29949

 

500

500000

289976

205044

144988

118382

83709

59191

48329

41854

37436

 

600

600000

347971

246053

173986

142059

100451

71029

57995

50225

44923

 

700

700000

405966

287062

202983

165735

117192

82868

67661

58596

52410

 

750

750000

434964

307566

217482

177573

125563

88787

72494

62782

56154

 

800

800000

463962

328070

231981

189412

133934

94706

77327

66967

59897

 

900

900000

521957

369079

260978

213088

150676

106544

86993

75338

67384

 

1000

1000000

579952

410088

289976

236764

167418

118382

96659

83709

74871

 

* When comparing these values for 1/2 cycle with the available RMS symmetrical fault current, multiply

the available RMS symmetrical by 1.3 to account for asymmetry during the first half cycle.

   

Table 3 illustrates the maximum short-circuit current rating of copper conductor based upon the Soares damage level.  This is the damage level associated with raising the conductor temperature from 75 degrees C to 250 degrees C.  This would be the temperature that the conductor becomes loose under the lug (also known as the annealing point of copper).

Table 3:

Maximum Short-Circuit Current Rating In Amperes (per Soares Annealing 250 Deg C)

     
   

Maximum Short-Circuit Current Rating In RMS Symmetrical Amperes

 

Cond

Cond

1/2*

1

2

3

6

12

18

24

30

 

Size

Area

Cycles

Cycle

Cycle

Cycle

Cycle

Cycle

Cycle

Cycle

Cycle

 
 

Circ. Mils

0.0083

0.0167

0.0333

0.0500

0.1

0.2

0.3

0.4

0.5

 
 

 

Seconds

Seconds

Seconds

Seconds

Seconds

Seconds

Seconds

Seconds

Seconds

 

14

4110

3425

2422

1713

1398

989

699

571

494

442

 

12

6530

5442

3848

2721

2222

1571

1111

907

786

703

 

10

10380

8651

6117

4325

3532

2497

1766

1442

1249

1117

 

8

16510

13760

9730

6880

5617

3972

2809

2293

1986

1776

 

6

26240

21869

15464

10934

8928

6313

4464

3645

3156

2823

 

4

41740

34787

24598

17393

14202

10042

7101

5798

5021

4491

 

3

52620

43854

31010

21927

17903

12660

8952

7309

6330

5662

 

2

66360

55305

39107

27653

22578

15965

11289

9218

7983

7140

 

1

83690

69748

49320

34874

28475

20135

14237

11625

10067

9004

 

1/0

105600

88008

62231

44004

35929

25406

17965

14668

12703

11362

 

2/0

133100

110927

78437

55464

45286

32022

22643

18488

16011

14321

 

3/0

167800

139847

98887

69923

57092

40370

28546

23308

20185

18054

 

4/0

211600

176350

124698

88175

71995

50908

35997

29392

25454

22767

 

250

250000

208353

147328

104177

85060

60146

42530

34726

30073

26898

 

300

300000

250024

176794

125012

102072

72176

51036

41671

36088

32278

 

350

350000

291694

206259

145847

119084

84205

59542

48616

42102

37658

 

400

400000

333365

235725

166683

136096

96234

68048

55561

48117

43037

 

500

500000

416706

294656

208353

170120

120293

85060

69451

60146

53797

 

600

600000

500048

353587

250024

204144

144351

102072

83341

72176

64556

 

700

700000

583389

412518

291694

238168

168410

119084

97231

84205

75315

 

750

750000

625060

441984

312530

255180

180439

127590

104177

90220

80695

 

800

800000

666730

471449

333365

272191

192468

136096

111122

96234

86075

 

900

900000

750071

530381

375036

306215

216527

153108

125012

108263

96834

 

1000

1000000

833413

589312

416706

340239

240586

170120

138902

120293

107593

 

* When comparing these values for 1/2 cycle with the available RMS symmetrical fault current, multiply

the available RMS symmetrical by 1.3 to account for asymmetry during the first half cycle

   

Table 4 illustrates the maximum short-circuit current rating of copper conductor based upon the Onderdonk damage level.  This is the damage level associated with raising the conductor temperature from 75 degrees C to 1083 degrees C.  This would be the temperature at which the copper conductor melts.  This value should obviously never be reached if good design practice is exercised.

Table 4:

Maximum Short-Circuit Current Rating In Amperes (per Onderdonk Melting 1083 Deg C)

   
   

Maximum Short-Circuit Current Rating In RMS Symmetrical Amperes

 

Cond

Cond

1/2*

1

2

3

6

12

18

24

30

 

Size

Area

Cycles

Cycle

Cycle

Cycle

Cycle

Cycle

Cycle

Cycle

Cycle

 
 

Circ.

0.0083

0.0167

0.0333

0.0500

0.1

0.2

0.3

0.4

0.5

 
 

Mils

Seconds

Seconds

Seconds

Seconds

Seconds

Seconds

Seconds

Seconds

Seconds

 

14

4110

6157

4353

3078

2513

1777

1257

1026

889

795

 

12

6530

9782

6917

4891

3993

2824

1997

1630

1412

1263

 

10

10380

15549

10995

7775

6348

4489

3174

2592

2244

2007

 

8

16510

24732

17488

12366

10097

7139

5048

4122

3570

3193

 

6

26240

39307

27795

19654

16047

11347

8024

6551

5674

5075

 

4

41740

62526

44213

31263

25526

18050

12763

10421

9025

8072

 

3

52620

78825

55737

39412

32180

22755

16090

13137

11377

10176

 

2

66360

99407

70291

49704

40583

28696

20291

16568

14348

12833

 

1

83690

125367

88648

62684

51181

36190

25590

20895

18095

16185

 

1/0

105600

158188

111856

79094

64580

45665

32290

26365

22833

20422

 

2/0

133100

199383

140985

99692

81398

57557

40699

33231

28778

25740

 

3/0

167800

251364

177741

125682

102619

72562

51309

41894

36281

32451

 

4/0

211600

316976

224136

158488

129405

91503

64702

52829

45752

40921

 

250

250000

374499

264811

187249

152889

108109

76444

62416

54054

48348

 

300

300000

449399

317773

224699

183466

129730

91733

74900

64865

58017

 

350

350000

524299

370735

262149

214044

151352

107022

87383

75676

67687

 

400

400000

599198

423697

299599

244622

172974

122311

99866

86487

77356

 

500

500000

748998

529622

374499

305777

216217

152889

124833

108109

96695

 

600

600000

898798

635546

449399

366933

259460

183466

149800

129730

116034

 

700

700000

1048597

741470

524299

428088

302704

214044

174766

151352

135373

 

750

750000

1123497

794432

561748

458666

324326

229333

187249

162163

145043

 

800

800000

1198397

847394

599198

489243

345947

244622

199733

172974

154712

 

900

900000

1348196

953319

674098

550399

389191

275199

224699

194595

174051

 

1000

1000000

1497996

1059243

748998

611554

432434

305777

249666

216217

193390

 

* When comparing these values for 1/2 cycle with the available RMS symmetrical fault current, multiply

 

the available RMS symmetrical by 1.3 to account for asymmetry during the first half cycle

     
   


Current Limiting Devices:

As indicated in the footnote for the above tables, a device, which opens in one-half cycle or less, may or may not be classified as "current-limiting".  The device can be labeled current limiting only if it meets the requirements of UL/CSA (489/22.2 No. 5 for molded case circuit breakers or 248 for fuses).

The requirement for molded case circuit breakers is that the device must limit the asymmetrical fault current to a value below the equivalent symmetrical fault current.  If the circuit breaker is not current-limiting, but clears within approximately one-half cycle, the available symmetrical fault current must be multiplied by a factor of 1.3 to account for asymmetry.  Even if the molded case circuit breaker is current-limiting, the degree of current limitation is typically less than a rejection type fusible device.

The requirements for current-limiting fuses are based on the fuse type (class) and upon the maximum I2t let-through as shown in Table 5.  If the device meets the performance criteria, it meets the standard for that class of fuse and can be listed/certified per UL/CSA.

The values shown in the table to the right can be used to compare the fuse maximum I2t let-through for the fuse to the I2t short-circuit rating of the conductor.  As long as the I2t let-through of the fuse is less than the I2t short-circuit rating of the conductor, the conductor is protected under short-circuit conditions. 

An example examining both the phase and grounding conductor is given on the following page.

Table 5:

Max Let-Through per UL 248 (Based on Table A)

 
     

Fuse

I2t Max Let-through Value

 

Class

50

100

200

 
 

kA

kA

kA

 

 Class J

       

30

7,000

7,000

7,000

 

60

30,000

30,000

30,000

 

100

60,000

80,000

80,000

 

200

200,000

300,000

300,000

 

400

1,000,000

1,100,100

1,100,100

 

600

2,500,000

2,500,000

2,500,000

 

 Class RK1

     

30

10,000

10,000

11,000

 

60

40,000

40,000

50,000

 

100

100,000

100,000

100,000

 

200

400,000

400,000

400,000

 

400

1,200,000

1,200,000

1,600,000

 

600

3,000,000

3,000,000

4,000,000

 

 Class RK5

     

30

50,000

50,000

50,000

 

60

200,000

200,000

200,000

 

100

500,000

500,000

500,000

 

200

1,600,000

1,600,000

2,000,000

 

400

5,200,000

5,000,000

6,000,000

 

600

10,000,000

10,000,000

12,000,000

 

 Class T

       

30      300V

3,500

3,500

3,500

 

600V

7,000

7,000

7,000

 

60      300V

15,000

15,000

15,000

 

600V

30,000

30,000

30,000

 

100    300V

40,000

40,000

40,000

 

600V

60,000

80,000

80,000

 

200    300V

150,000

150,000

150,000

 

600V

200,000

300,000

300,000

 

400    300V

550,000

550,000

550,000

 

600V

1,000,000

1,100,000

1,100,000

 

600    300V

1,000,000

1,000,000

1,000,000

 

600V

2,500,000

2,500,000

2,500,000

 

800    300V

1,500,000

1,500,000

1,500,000

 

600V

4,000,000

4,000,000

4,000,000

 

1200   300V

3,500,000

3,500,000

4,000,000

 

600V

7,500,000

7,500,000

7,500,000

 

 Class L

       

800

10,000,000

10,000,000

10,000,000

 

1200

12,000,000

12,000,000

15,000,000

 

1600

22,000,000

22,000,000

30,000,000

 

2000

35,000,000

35,000,000

40,000,000

 

2500

 

75,000,000

75,000,000

 

3000

 

100,000,000

100,000,000

 

4000

 

150,000,000

150,000,000

 

5000

 

350,000,000

350,000,000

 

6000

 

350,000,000

500,000,000

 

Short-Circuit Protection of Wire and Cable:

Fusible Systems


The circuit shown originates at a distribution panel where 40,000 amperes RMS symmetrical are available. The # 10 THW copper conductor is protected by a Bussmann LOW-PEAK® fuse sized per NEC 240-3 (30A maximum for # 10 conductor). 


Table 1, shows the I2t short-circuit rating of # 10 THW copper to be 301,994 per the ICEA Insulation damage level.  Since the Bussmann LOW-PEAK® is a Class RK1 current limiting fuse, the maximum I2t let-through per UL/CSA 248 can be found per the table on the previous page.  The maximum I2t let-through for a Class RK1 fuse at a fault not greater than 50,000A is 10,000 A2sec.  Since the I2t let-through of the fuse, 10,000, is considerably less than the I2t short-circuit rating of the # 10 conductor, 301,994, the conductor is protected against short-circuits.  In addition, since the minimum equipment grounding conductor per NEC 250-122 is also # 10, the equipment grounding conductor is protected as well.

If the phase conductor was # 3, the maximum fuse size per NEC 240-3 would be 100A. Looking at Table 1, the I2t short-circuit rating of # 3 is shown to be 7,760,768 (per ICEA).  If a Bussmann LOW-PEAK® Class RK1 current limiting fuse was selected, the maximum I2t let-through per UL/CSA 248, at a fault not greater than 50,000A is 100,000 A2sec.  Since the I2t let-through of the fuse, 100,000, is less than the I2t short-circuit rating of the # 3 phase conductor, 7,760,768, the conductor is protected against short-circuits.  In order to meet NEC 110-10, we must also analyze the equipment grounding conductor as well.  The minimum equipment grounding conductor per NEC 250-122 is # 8.  The I2t short-circuit rating of # 8 is shown to be 764,007 (per ICEA).  Since the phase-ground fault current can equal (or more if near the transformer) the bolted three-phase fault current, we can use the previous I2t let-through of the 100A RK1 fuse.  Since the I2t let-through of the fuse, 100,000, is less than the I2t short-circuit rating of the # 8 equipment grounding conductor, 764,007, the equipment grounding conductor is protected against short-circuits as well.

If the phase conductor was 500 kcmil, the maximum fuse size per NEC 240-3 would be 400A. Looking at Table 1, the I2t short-circuit rating of 500 kcmil is shown to be 700,717,631 (per ICEA). If a Bussmann LOW-PEAK® Class RK1 current limiting fuse was selected, the maximum I2t let-through per UL/CSA 248, at a fault not greater than 50,000A is 1,200,000.  Since the I2t let-through of the fuse, 1,200,000, is considerably less than the I2t short-circuit rating of the 500 kcmil conductor, 700,717,631, the conductor is protected against short-circuits.  In order to meet NEC 110-10, we must also analyze the equipment grounding conductor as well.  The minimum equipment grounding conductor per NEC 250-122 is # 3.  The I2t short-circuit rating of # 3 is shown to be 7,760,768 (per ICEA).  Since the phase-ground fault current can equal (or more if near the transformer) the bolted three-phase fault current, we can use the previous I2t let-through of the 400A RK1 fuse.  Since the I2t let-through of the fuse, 1,200,000, is less than the I2t short-circuit rating of the # 3 equipment grounding conductor, 7,760,768, the equipment grounding conductor is protected against short-circuits as well.

Table 6, on the next page, compares the maximum I2t let-through for the fuse with the I2t short-circuit rating of the conductor.  If the I2t short-circuit rating of the conductor is greater than the I2t let-through of the fuse; the conductor will be protected.  The table shows the minimum size conductor able to be protected under short-circuit conditions by the fusible device. 


Table 6:

                   

Conductor Short-Circuit Protection (ICEA Values)

Conductor Short-Circuit Protection (Soares Values)

Minimum Conductor Size, Based on UL 248

Minimum Conductor Size, Based on UL 248

Fuse    Class

Available Short-Circuit Current      (RMS Symmetrical Amperes)

Fuse   Class

Available Short-Circuit Current              (RMS Symmetrical Amperes)

    

50 kA

100 kA

200 kA

 

50 kA

100 kA

200 kA

 Class J

     

 Class J

     

30

14

14

14

30

14

14

14

60

14

14

14

60

14

14

14

100

12

12

12

100

14

14

14

200

10

10

10

200

12

10

10

400

6

6

6

400

8

8

8

600

4

4

4

600

6

6

6

 Class RK1

   

 Class RK1

   

30

14

14

14

30

14

14

14

60

14

12

12

60

14

14

14

100

12

12

12

100

12

12

12

200

8

8

8

200

10

10

10

400

6

6

6

400

8

8

6

600

4

4

4

600

6

6

4

 Class RK5

   

 Class RK5

   

30

12

12

12

30

14

14

14

60

10

10

10

60

12

12

12

100

8

8

8

100

10

10

10

200

6

6

4

200

6

6

6

400

3

3

3

400

4

4

4

600

2

2

2

600

4

4

3

 Class T

     

 Class T

     

30        300V

14

14

14

30       300V

14

14

14

600V

14

14

14

600V

14

14

14

60        300V

14

14

14

60       300V

14

14

14

600V

14

14

14

600V

14

14

14

100      300V

14

14

14

100     300V

14

14

14

600V

12

12

12

600V

14

14

14

200      300V

10

10

10

200     300V

12

12

12

600V

10

10

10

600V

12

10

10

400      300V

8

8

8

400     300V

10

10

10

600V

6

6

6

600V

8

8

8

600      300V

6

6

6

600     300V

8

8

8

600V

4

4

4

600V

6

6

6

800      300V

6

6

6

800     300V

8

8

8

600V

4

4

4

600V

4

4

4

1200    300V

4

4

4

1200   300V

6

6

4

600V

3

3

3

600V

4

4

4

 Class L

     

 Class L

     

800

2

2

2

800

4

4

4

1200

1

1

1

1200

3

3

3

1600

1/0

1/0

1/0

1600

2

2

1

2000

2/0

2/0

2/0

2000

1

1

1

2500

3/0

3/0

3/0

2500

2/0

2/0

2/0

3000

4/0

4/0

4/0

3000

2/0

2/0

2/0

4000

250

250

250

4000

3/0

3/0

3/0

5000

400

400

400

5000

250

250

250

6000

500

500

500

6000

250

250

300


Short-Circuit Protection of Wire and Cable:

Circuit Breaker Systems

In the previous example a 30A, Class RK1 fuse was protecting a # 10 conductor.  If the 30A fusible device were replaced with a 30A, molded case circuit breaker with a clearing of 1/2 cycle would the # 10 conductor be protected? 

Since the I2t let-through of the circuit breaker is not known, Tables 2 through 4 must be used.  If the 30A circuit breaker is current limiting, at a 40,000A fault, with a clearing time of 1/2 cycle, per UL 489, the let-through current could be as high as 40,000A RMS symmetrical and still be marked current limiting.  If the device was not current limiting, the let-through current could be as high as 40,000 X 1.3, or 52,000A.  The short-circuit current rating of # 10 conductor is 6,020A for 1/2 cycle (per ICEA).  Since the let-through current of either the current limiting or non-current limiting circuit breaker could be much greater than the short-circuit current rating of the conductor, protection can not be assured for either the phase or equipment grounding conductor.  Even if higher damage levels (Soares, Table 3 or Onderdonk, Table 4) where analyzed the let-through current could also exceed the higher short-circuit current damage levels.

If the same fault level was applied to # 3 conductor protected by a 100A circuit breaker, the phase conductor would not be protected to the ICEA level even if the circuit breaker device cleared within a half cycle.  The short-circuit current rating of # 3 conductor is 30,517A for 1/2 cycle (per ICEA).  Likewise the equipment grounding conductor sized as a No. 8, per NEC 250-122, may not be protected.  Even if the device is current limiting and clears within a half cycle, 40,000A could be let-through.  Since, at best, the # 8 conductor can withstand a short-circuit current of only 9,575A for 1/2 cycle, the equipment grounding conductor could be damaged on a phase-ground fault if the phase-ground fault was equal to the bolted three-phase fault level.

If the same fault level was applied to 500 kcmil conductor protected by a 400A circuit breaker, the phase conductor would be protected to ICEA level if the circuit breaker device cleared within 3 cycles.  The short-circuit current rating of 500 kcmil conductor is 118,382A for 3 cycles (per ICEA).  However, the equipment grounding conductor sized as a No. 3, per NEC 250-122 may not be protected.    Even if the device is current limiting and clears within a half cycle, 40,000A could be let-through.  Since, at best, the # 3 conductor can withstand a short-circuit current of only 30,517A for 1/2 cycle, the equipment grounding conductor could be damaged on a phase-ground fault if the phase-ground fault was equal to the bolted three-phase fault level.

In the above three examples, all with 40,000A available fault current, the circuit breaker could not assure short-circuit protection (per ICEA) of the phase or equipment grounding conductor in a 30A or 100A circuit.  In the 400A circuit, the circuit breaker could provide short-circuit protection (per ICEA) for the phase conductors, but could not assure short-circuit protection (per ICEA) for the equipment grounding conductor.  However, referring back to the previous example with the fusible systems, utilizing RK1 fuses, BOTH the phase conductor and equipment grounding conductor were protected in all cases. Typically, with current limiting fuses, the fuse will protect both the phase and equipment grounding conductor to the ICEA levels.

Table 7 & 8, on the next page, can be utilized to analyze conductor protection by current-limiting and non-current limiting circuit breakers.  Note in the tables, the size of the conductor has increased due to the limited degree of current-limitation or the increased opening time of the device (compared to a fusible system).  When using molded case circuit breakers, insulated case circuit breakers or low voltage power circuit breakers, conductors must be carefully analyzed for protection.  This is especially true for equipment grounding conductors since the NEC allows for reduced sizing of the equipment grounding conductor despite the fact that the available phase-ground short-circuit fault current can be equal to or perhaps greater than the available bolted three-phase short-circuit fault current. 


Table 7:

Conductor Short-Circuit Protection (ICEA Values 150 Degrees C)

 

Minimum Conductor Size

       

Circuit

Available Short-Circuit Current (RMS Symmetrical Amperes)

Breaker's

         

(Clearing

10

25

50

100

200

or STD***)

KA

kA

KA

kA

kA

Setting

         

1/2 cycle C.L.*

#6

#3

1/0

4/0

350

1/2 cycle N.C.L.**

#6

#2

2/0

250

500

 1 Cycle

#6

#2

2/0

250

500

 3 Cycle

#3

1/0

4/0

500

900

 6 Cycle

#2

3/0

300

600

-

 12 Cycle

1/0

4/0

500

900

-

 18 Cycle

1/0

300

600

-

-

 24 Cycle

2/0

300

600

-

-

 30 Cycle

3/0

350

700

-

-

*  C.L. = Current Limiting Circuit Breaker.  Values are based upon UL 489/CSA 22.2 No. 5

requirements to limit short-circuit currents to the symmetrical values.

** N.C.L. = Non-Current Limiting Circuit Breaker

*** STD = Short Time Delay

 
           

Table 8:

         

Conductor Short-Circuit Protection (Soares Values 250 Degrees C)

Minimum Conductor Size

     

Circuit

Available Short-Circuit Current (RMS Symmetrical Amperes)

Breaker's

         

(Clearing

10

25

50

100

200

or STD***)

KA

kA

kA

kA

kA

Setting

         

1/2 Cycle C.L.*

#8

#4

#2

2/0

250

1/2 Cycle N.C.L.**

#8

#4

#1

3/0

350

 1 Cycle

#6

#3

1/0

4/0

350

 3 Cycle

#4

#1

3/0

300

600

 6 Cycle

#4

1/0

4/0

500

900

 12 Cycle

#2

3/0

300

600

-

 18 Cycle

#1

4/0

400

750

-

 24 Cycle

#1

4/0

500

900

-

 30 Cycle

1/0

250

500

1000

-

* C.L. = Current Limiting Circuit Breaker.  Values are based upon UL 489/CSA 22.2 No. 5

Requirements to limit short-circuit currents to the symmetrical values.

** N.C.L. = Non-Current Limiting Circuit Breaker

*** STD = Short Time Delay

 

 

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