Low Voltage Fault Tables 

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The following tables provide quick order of magnitude fault levels for a a range of typical low voltage situations.

Transformer LV Terminal Fault Levels (230/400 V)

Short circuit current, Isc (kA) at the LV terminals of a 3-phase HV/LV transformer.

Transformer rated power (kVA) 50 100 160 250 315 400 500 630 800 1000 1250 1600 2000 2500
Transformer rated (LV) current (A) 69 137 220 344 433 550 687 866 1100 1375 1718 2199 2749 3437
oil-immersed Psc = 250MVA 1.17 33.1 3.40 5.41 8.38 10.5 13.2 16.4 17.4 20.4 21.5 26.4 40.4 49.1
  Psc = 500 MVA 1.17 3.42 5.45 8.49 10.7 13.5 16.8 21.0 17.9 22.2 27.5 34.8 43.0 52.9
cast-resin Psc = 250 MVA 1.14 2.28 3.63 5.63 7.07 8.93 11.1 13.9 17.4 21.5 26.4 33.1 40.4 49.1
  Psc = 500 MVA 1.14 2.28 3.65 5.68 7.14 9.04 11.3 14.1 17.9 22.2 27.5 34.8 43.0 52.9

Psc - Short circuit current at HV windings

Short Circuit Current of Feeders

Short circuit current, Isc at receiving end of a feeder in terms of the short circuit at it's sending end.

Copper 230/400 V

Example of using the table - 10 mm2 cable, 6.5 m in length, upstream fault 35 kA, gives downstream fault of 11 kA

csa (mm²) length of circuit (m)
1.5                           0.8 1 1.3 1.6 3
2.5                         1 1.3 1.6 2.1 2.6 5
4                       0.8 1.7 2.1 2.5 3.5 4 8.5
6                       1.3 2.5 3 4 5 6.5 13
10                   0.8 1.1 2.1 4 5.5 6.5 8.5 11 21
16               0.9 1 1.4 1.7 3.5 7 8.5 10 14 17 34
25             1 1.3 1.6 2.1 2.6 5 10 13 16 21 26 50
35             1.5 1.9 2.2 3 2.5 7.5 15 19 22 30 37 75
50           1.1 2.1 2.7 3 4 5.5 11 21 27 32 40 55 110
70           1.5 3 3.5 4.5 6 7.5 15 30 37 44 60 75 150
95       0.9 1 2 4 5 6 8 10 20 40 50 60 80 100 200
120   0.9 1 1.1 1.3 2.5 5 6.5 7.5 10 13 25 50 65 75 100 130 250
150 0.8 1 1.1 1.2 1.4 2.7 5.5 7 8 11 14 27 55 70 80 110 140 270
185 1 1.1 1.3 1.5 1.6 3 6.5 8 9.5 13 16 32 65 80 95 130 160 320
240 1.2 1.4 1.6 1.8 2 4 8 10 12 16 20 40 80 100 120 160 200 400
300 1.5 1.7 1.9 2.2 2.4 5 9.5 12 15 19 24 49 95 120 150 190 240  
2 x 120 1.5 1.8 2 2.3 2.5 5.1 10 13 15 20 25 50 100 130 150 200 250  
2 x 150 1.7 1.9 2.2 2.5 2.8 5.5 11 14 17 22 28 55 110 140 170 220 280  
2 x 185 2 2.3 2.6 2.9 3.5 6.5 13 16 20 26 33 65 130 160 200 260 300  
3 x 120 2.3 2.7 3 3.5 4 7.5 15 19 23 30 38 75 150 190 230 300 380  
3 x 150 2.5 2.9 3.5 3.5 4 8 16 21 25 33 41 80 160 210 250 330 410  
3 x 185 2.9 3.5 4 4.5 5 9.5 20 24 29 39 49 95 190 240 290 300    
Isc (kA) upstream Isc (kA) downstream
100 94 94 93 92 91 83 71 67 63 56 50 33 20 17 14 11 9 5
90 85 85 84 83 83 76 66 62 58 52 47 32 20 16 14 11 9 4.5
80 76 76 75 75 74 69 61 57 54 49 44 31 19 16 14 11 9 4.5
70 67 67 66 66 64 61 55 52 49 45 41 29 18 16 14 11 5 4.5
60 58 58 57 57 57 54 48 46 44 41 38 27 18 15 13 10 8.5 4.5
50 49 48 48 48 48 46 42 40 39 36 33 25 17 14 13 10 8.5 4.5
40 39 39 39 39 39 37 35 33 32 30 29 22 15 13 12 9.5 8 4.5
35 34 34 34 34 34 33 31 30 29 27 26 21 15 13 11 9 8 4.5
30 30 29 29 29 29 28 27 26 25 24 23 19 14 12 11 9 7.5 4.5
25 25 25 25 24 24 24 23 22 22 21 20 17 13 11 10 8.5 7 4
20 20 20 20 20 20 19 19 18 18 17 17 14 11 10 9 7.5 6.5 4
15 15 15 15 15 15 15 14 14 14 13 13 13 9.5 8.5 8 7 6 4
10 10 10 10 10 10 10 9.5 9.5 9.5 9.5 9 8.5 7 6.5 6.5 5.5 5 3.5
7 7 7 7 7 7 7 7 7 6.5 6.5 6.5 6 5.5 5 5 4.5 4 2.9
5 5 5 5 5 5 5 5 5 5 5 5 4.5 4 4 4 3.5 3.5 2.5
4 4 4 4 4 4 4 4 4 4 4 4 3.5 3.5 3.5 3 3 2.9 2.2
3 3 3 3 3 3 3 3 3 2.9 2.9 2.9 2.8 2.7 2.6 2.5 2.4 2.3 1.9
2 2 2 2 2 2 2 2 2 2 2 2 1.9 1.9 1.8 1.8 1.7 1.7 1.4
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.9 0.9 0.9 0.8

Note: for a 3-phase system having 230 V between phases, divide above lengths by 1.732

Aluminium 230/400 V

csa (mm²) length of circuit (m)
2.5                           0.8 1 1.3 1.6 3
4                         1 1.3 1.6 2.1 2.6 5
6                       0.8 1.6 2 2.4 3 8 8
10                       1.3 2.6 3.5 4 5.5 6.5 13
16                   0.8 1.1 2.1 4 5.5 6.5 8.5 11 21
25               0.8 1 1.3 1.7 3.5 6.5 8.5 10 13 17 33
35             0.9 1.2 1.4 1.8 2.3 4.5 9 12 14 18 23 46
50             1.3 1.7 2 2.6 3.5 6.5 13 17 20 26 33 65
70           0.9 1.8 2.3 2.8 3.5 4.5 9 18 23 28 37 46 90
95           1.3 2.5 3 4 5 6.5 13 25 32 38 50 65 130
120         0.8 1.7 3 4 4.5 6.5 8 17 32 40 47 65 80 160
150         0.9 1.7 3.5 4.5 5 7 8.5 17 34 43 50 70 85 170
185       0.9 1 2 4 5 6 8 10 20 40 50 60 80 100 240
240   0.9 1 1.1 1.3 2.5 5 6.5 7.5 10 13 25 50 65 75 100 130 250
300 0.9 1 1.2 1.4 1.5 3 6 7.5 9 12 15 30 60 75 90 120 150 300
2 x 120 0.9 1.1 1.3 1.4 1.6 3 6.5 8 9.5 13 16 32 65 80 95 130 160 320
2 x 150 1 1.2 1.4 1.5 1.7 3.5 7 9 10 14 17 35 70 85 100 140 140  
2 x 185 1.2 1.4 1.6 1.8 2 4.1 8 10 12 16 20 41 80 100 120 160 200  
2 x 240 1.5 1.8 2 2.3 2.5 5 10 13 15 20 25 50 100 130 150 200 250  
3 x 120 1.4 1.7 1.9 2.1 2.4 4.5 9.5 12 14 19 24 48 95 120 140 190 240  
3 x 150 1.5 1.8 2.1 2.3 2.6 5 10 13 15 21 26 50 100 130 150 210 260  
3 x 185 1.8 2.1 2.4 2.7 3 6 12 15 18 24 30 60 120 150 180 240 300  
3 x 240 2.3 2.7 3 3.5 4 7.5 15 19 23 30 38 75 150 190 230 300 380  
Isc (kA) upstream Isc (kA) downstream
100 94 94 93 92 91 83 71 67 63 56 50 33 20 17 14 11 9 5
90 85 85 84 83 83 76 66 62 58 52 47 32 20 16 14 11 9 4.5
80 76 76 75 75 74 69 61 57 54 49 44 31 19 16 14 11 9 4.5
70 67 67 66 66 64 61 55 52 49 45 41 29 18 16 14 11 5 4.5
60 58 58 57 57 57 54 48 46 44 41 38 27 18 15 13 10 8.5 4.5
50 49 48 48 48 48 46 42 40 39 36 33 25 17 14 13 10 8.5 4.5
40 39 39 39 39 39 37 35 33 32 30 29 22 15 13 12 9.5 8 4.5
35 34 34 34 34 34 33 31 30 29 27 26 21 15 13 11 9 8 4.5
30 30 29 29 29 29 28 27 26 25 24 23 19 14 12 11 9 7.5 4.5
25 25 25 25 24 24 24 23 22 22 21 20 17 13 11 10 8.5 7 4
20 20 20 20 20 20 19 19 18 18 17 17 14 11 10 9 7.5 6.5 4
15 15 15 15 15 15 15 14 14 14 13 13 13 9.5 8.5 8 7 6 4
10 10 10 10 10 10 10 9.5 9.5 9.5 9.5 9 8.5 7 6.5 6.5 5.5 5 3.5
7 7 7 7 7 7 7 7 7 6.5 6.5 6.5 6 5.5 5 5 4.5 4 2.9
5 5 5 5 5 5 5 5 5 5 5 5 4.5 4 4 4 3.5 3.5 2.5
4 4 4 4 4 4 4 4 4 4 4 4 3.5 3.5 3.5 3 3 2.9 2.2
3 3 3 3 3 3 3 3 3 2.9 2.9 2.9 2.8 2.7 2.6 2.5 2.4 2.3 1.9
2 2 2 2 2 2 2 2 2 2 2 2 1.9 1.9 1.8 1.8 1.7 1.7 1.4
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.9 0.9 0.9 0.8

Note: for a 3-phase system having 230 V between phases, divide above lengths by 1.732



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus

  1. Sinnadurai's avatar Sinnadurai says:
    7/11/2012 11:59 AM

    Give a method of calculating/evaluating fault current contributed by motors(induction/synchrounous)

    • Steven's avatar Steven says:
      7/11/2012 12:44 PM

      IEC 60909 gives some guidelines on how to take into account motor contribution. You can also refer to the following (which gives a guideline on if it is necessary to consider motors):

      http://myelectrical.com/notes/entryid/192/fault-calculations-introduction

      At some stage in the future, I’ll put a post together on this (noted in the my list of future posts).


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