Short Circuit Calculation Basics for LV Systems
The short-circuit level at a switchboard decides breaker ratings, busbar bracing and cable withstand. Here is the quick estimate from a transformer, a worked example, and the effects that a real study adds on top.
The quick estimate
Ifl = kVA ÷ (√3 × V), and Isc ≈ Ifl ÷ Z, with Z the transformer impedance in per-unit. This assumes an infinitely strong upstream source, so it is an upper bound for the transformer's own contribution.
Worked example
Assumptions (illustrative): 1,600 kVA transformer, 415 V secondary, 6% impedance (check the actual nameplate).
- Ifl = 1,600 ÷ (1.732 × 0.415) = 2,226 A
- Isc ≈ 2,226 ÷ 0.06 = 37.1 kA
So the board must be rated for at least this fault level, with the next standard breaking capacity chosen above it. The short-circuit calculator runs this estimate for your values.
What a real study adds
- Upstream impedance: a finite source lowers the result.
- Cable impedance: reduces the fault level downstream of the board.
- Motor contribution: raises it for the first cycles.
- Standard factors: IEC 60909 applies voltage factors and separate maximum and minimum cases.
Maximum fault current sizes the equipment ratings; minimum fault current checks that protection operates quickly enough at the far end of a circuit.
Where the number goes
- Breaker breaking capacity and busbar short-circuit withstand
- Cable thermal withstand during the fault clearing time
- Protection coordination and arc-flash studies
Common mistakes
- Using the infinite-source estimate as the final answer.
- Using one impedance value for every transformer without reading the nameplate.
- Forgetting motor contribution.
- Checking only the maximum case.
Related: protection relays and cable sizing basics.
Written by the Vision Matrix Institute editorial team. Worked examples use stated, illustrative assumptions; check them against your project data, the applicable standards and manufacturer datasheets before use.
Frequently Asked Questions
How do you calculate short-circuit current at a transformer secondary?
For a quick estimate with an infinite upstream source, I_sc = I_full-load ÷ Z, where I_full-load = kVA ÷ (√3 × V) and Z is the transformer impedance as a per-unit value (percent impedance ÷ 100).
Why is the result only an estimate?
It ignores the upstream network impedance, cable impedance, motor contribution and the standard's voltage factors. A real study follows IEC 60909 or the project's chosen method and uses the actual data.
What is the short-circuit level used for?
To check that breakers, busbars and cables can break and withstand the prospective fault current, and as an input to protection coordination and arc-flash analysis.
Do motors add to the fault current?
Yes. Running motors act as generators briefly during a fault and add to the initial fault current, which matters at LV boards with large motor loads such as chillers.
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