Low Voltage Fault Tables 

By on

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).


Comments are closed for this post:
  • have a question or need help, please use our Questions Section
  • spotted an error or have additional info that you think should be in this post, feel free to Contact Us



Write your best report

Years ago I was told that you should always try to write the best report you can.  Many years later I still think on this as one of the better pieces of...

Microsoft OneNote

A couple of months ago I came Microsoft's OneNote and downloaded the 60 day free trail. Since then I have been using it regularly and now have a full license...

Robotics - Home Innovations

We have a sister note to this (Robots - Interesting Video), in which I have posted some videos of interesting robots developed by commercial corporations...

Periodic Electrical Installation Inspection – What to Inspect?

This is the second post in a series of two on periodic electrical inspections. In the first post, I discussed how often inspections should be carried out...

UPS Battery Sizing

Various techniques exist to enable the correct selection of batteries for UPS applications.  The procedure described below is one of the more common. ...

Arc Flash Calculations

Working in the vicinity of electrical equipment poses an hazard. In addition to electric shock hazard, fault currents passing through air causes Arc Flash...

How a Digital Substation Works

Traditionally substations have used circuit breakers, current transformers (CT), voltage transformers (VT) and protection relays all wired together using...

Capacitor Theory

Capacitors are widely used in electrical engineering for functions such as energy storage, power factor correction, voltage compensation and many others...

Lead Acid Batteries

Lead acid batteries are cost effect and reliable, making them suitable for many applications.This note examines topics of interest associated with the...

Cable Sizing Software

When sizing cables nearly, everyone uses some form of software. This ranges from homespun spreadsheets to complex network analyses software. Each has its...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note