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Transformer Fault Current Calculator

Prospective three-phase fault current at the LV terminals, IEC 60909 simplified

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Transformer

kVA

Rated power from the nameplate

V

Nominal line-to-line, e.g. 400 V

%

uk or Z% from the nameplate; typically 4 to 6%

From the test report; blank treats the transformer as pure reactance

Supply

MVA

At the HV terminals, from the network operator; blank for an infinite source

IEC 60909-0 Table 1; use cmax for the maximum fault current

Fault Current

Enter the transformer's rating and impedance to see the fault current.

How It's Calculated

With an infinite (stiff) source, the transformer's impedance alone limits the fault current:

where is the rated power, the LV nominal voltage, the short-circuit impedance as a fraction and the IEC 60909-0 voltage factor. With a finite source the network impedance, referred to the LV side, is added in series (IEC 60909-0, clause 6.2, for a network with no other data):

split into and by the transformer's

Assumptions: a bolted three-phase fault at the LV terminals; no cable, busbar or arc impedance; no contribution from running motors; nominal tap; and no impedance correction factor , which IEC 60909-0 applies to network transformers and which raises the result by a few percent. For a design of record, run a full IEC 60909 study.

Worked Example

A 1000 kVA, 400 V transformer with 5% impedance and X/R of 5, fed from a 250 MVA source, :

  • Full load current =
  • Source: ()
  • Transformer: ()
  • Total , so 26.74 kA
  • , so

With an infinite source the same transformer gives 28.87 kA, so the 250 MVA network takes about 7% off the fault current.

Fault Current by Transformer Size

Prospective fault current at 400 V with an infinite source and c = 1.0, for typical impedances. Use the nameplate impedance of the actual transformer.

Rating (kVA)Impedance (%)Full load current (A)Fault current (kA)
10041443.6
20042897.2
315445511.4
500472218.0
630490922.7
80051,15523.1
100051,44328.9
125051,80436.1
150062,16536.1
200062,88748.1
250063,60860.1

Frequently Asked Questions

Fault Current Questions Answered

How do I calculate the fault current of a transformer?

Divide the full load current by the per-unit impedance: Ik = S / (√3 × V × Z%). A 1000 kVA, 400 V transformer with 5% impedance has a full load current of 1443 A and a prospective fault current of about 28.9 kA at its LV terminals with an infinite source.

Why does the source fault level matter?

The upstream network has impedance too, and it adds to the transformer's. A weak supply (a low HV fault level) lowers the LV fault current; ignoring it gives the higher, conservative value for rating switchgear.

What is the voltage factor c?

IEC 60909-0 multiplies the nominal voltage by a factor c to cover the highest voltage the system may run at. For low-voltage systems cmax is 1.05 where the tolerance is +6% and 1.10 where it is +10%. Use cmax when checking equipment ratings and cmin (0.95) for protection sensitivity.

What is the difference between the RMS and the peak fault current?

The initial symmetrical current I″k is the RMS value switchgear breaking capacity is checked against. In the first half-cycle a DC offset lifts the instantaneous current to the peak ip = κ × √2 × I″k, which sets the making capacity and the busbar forces. κ ranges from 1.02 for a resistive circuit to 2.0 for a purely reactive one.

Where do I find the transformer impedance?

The impedance voltage (uk or Z%) is on the nameplate and in the test report. Distribution transformers are typically 4% up to 630 kVA and 5 to 6% above. Tolerance on the measured value is ±7.5% to ±10%, so the minimum impedance gives the maximum fault current.

Does this give the fault current at the end of a cable?

No. This is the fault at the transformer terminals. Cable impedance reduces the fault current at downstream boards; use the cable size calculator's fault loop check for final subcircuits.

Reviewed by Suheib Sherif, electrical engineer at CableHero. The calculator is a design aid: the designer remains responsible for checking the installation against the standards.

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