Prospective Fault Current
Circuit Breakers

Determination of Prospective Fault Current
The following information is provided to assist with the calculation of Prospective Fault Current (assuming the voltage 415/240Vac).

Obtain the data:

(a) Transformer sc(kA) rating using the formula

Short circuit (kA) =
or the data shown in the following table.

kVA

% X

FLC (A)

SC (kA)

100

4.75

139

2.93

200

4.75

278

5.86

250

4.75

348

7.32

400

4.75

556

11.72

500

4.75

696

14.64

800

4.75

1113

23.43

1000

4.75

1391

29.29

1250

5

1739

34.78

1600

5.5

2226

40.47

2000

5.5

2782

50.59

(b) Cable sizes and lengths from transformer to the relevant point of installation.

Read off the added circuit resistance value (milliohms) from the table below for copper conductors.

Notes:

(a) This applies for 3-phase symmetrical fault for a short circuit across all three phases.

(b) For single phase line-neutral faults, take the cable resistance and double (x2) the resistance to obtain the line-neutral value.

Knowing the resistance read off the prospective Fault Current from the graph.

Nominal
Area
mm²

Conductor
strands/
dia.

Resistance in milli ohms of single-core
cable of stated lengths (metres)

5

10

25

50

75

100

150

200

300

400

500

750

1000

1

1/1 - 13

86

177

442

885

-

-

-

-

-

When resistance values have been omitted for small conductors the fault level will be less than 0.25kA.

1.5

1/1 - 38

60

119

297

595

892

1190

-

-

-

2.5

1/1 - 78

36

71

159

357

515

714

1071

-

-

4

7/0 - 85

23

45

113

226

339

452

678

904

-

6

7/1 - 04

15

30

76

151

227

302

453

604

906

1028

-

-

-

10

7/1 - 35

9

18

45

90

135

180

270

360

540

720

900

-

-

16

7/1 - 70

6

11

28

57

85

113

170

226

339

452

565

847

1130

25

7/2 - 14

4

7

18

36

53

71

106

142

212

285

356

534

712

35

19/1 - 53

3

5

14

26

32

51

78

103

154

206

267

390

514

50

19/1 - 78

2

4

9

19

29

38

57

76

114

152

190

294

379

70

19/2 - 14

1

3

8

13

21

26

39

52

79

105

131

206

262

95

19/2 - 52

-

2

5

9

12

19

28

38

59

76

94

122

189

120

37/2 - 03

-

2

4

8

11

15

25

30

45

60

75

113

150

150

37/2 - 25

-

1

3

6

9

12

18

24

37

49

61

91

122

185

37/2 - 52

-

-

2

5

7

10

15

19

29

39

49

73

97

240

61/2 - 25

-

-

2

4

6

7

11

15

22

30

37

56

74

300

61/2 - 52

-

-

1

3

4

6

9

12

18

24

30

44

59

400

61/2 - 85

-

-

1

2

3

5

7

9

14

18

23

34

46

500

61/3 - 20

-

-

-

2

3

4

6

8

1

15

19

28

38

630

127/2 - 52

-

-

-

1

2

3

4

6

9

11

14

22

28

800

127/2 - 85

-

-

-

1

2

2

3

4

7

9

11

17

22

1000

127/3 - 20

-

-

-

-

1

2

3

4

5

7

9

13

18

Example:

To calculate the prospective fault current at the end of 50m of 70mm² cable from a 1000kVA transformer.

Fault Current for 1000kVA Transformer = 29.29 kA.

Read off the cable resistance for the copper conductors.

Resistance for 50m of 70mm² copper conductors = 13 milliohms

Knowing the resistance, read off short circuit current from graph using the 1000kVA curve.

From graph Short Circuit current = 13kA.

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