# 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 3.4 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 17.9 22.2 27.5 34.8 43 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 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

More interesting Notes:

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

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#### View 2 Comments (old system)

7/11/2012 11:59 AM

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

• 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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