Short Circuit Current Calculator

Work out short circuit current instantly with clear inputs, formula shown and shareable results.

About Short Circuit Current Calculator

Fault current at a transformer's terminals is full load current divided by the per-unit impedance, so a 4.5% transformer gives roughly 22 times full load current. Cable impedance between the transformer and the fault reduces it, which is why the highest prospective fault current is always closest to the source.

Formula

Prospective fault current

Isc at terminals = FLC / (Z% / 100); adding cable impedance reduces it downstream

Worked example

With the default values below, Short Circuit Current Calculator returns Prospective fault current of 10.31. Change any input above and every figure here is recalculated in your browser.

Inputs used

Transformer rating: 500 kVA
Secondary voltage: 400 V
Transformer impedance: 4.5 %
Cable impedance to the fault: 8 mΩ

Results

Prospective fault current: 10.31 kA
Fault current at the terminals: 16.04 kA
Transformer full load current: 721.69 A
Minimum device breaking capacity: 16 kA

Step by step

1Values used
2Prospective fault current
3Prospective fault current= 10.31 kA
4Fault current at the terminals= 16.04 kA
5Transformer full load current= 721.69 A
6Minimum device breaking capacity= 16 kA

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why does the switchgear need to know this?

Every protective device has a breaking capacity. If the prospective fault current exceeds it, the device can fail explosively instead of clearing the fault.

Where is fault current highest?

At the transformer secondary terminals. It falls with distance as cable impedance accumulates, which is why remote sub-boards can use lower-rated devices.