Circuit protection
Short-Circuit Current Calculator (Prospective Isc)
Estimate the prospective short-circuit current at a transformer secondary from the transformer rating and impedance, and find the minimum breaker breaking capacity (Icu) you need. This is the maximum at the transformer terminals — real values downstream are lower once cable impedance is included.
How it is calculated
The transformer rated secondary current and the prospective fault current at its terminals are:
where S is the transformer rating (VA), V the secondary line voltage, and uk the impedance voltage in percent. This simplified method assumes an infinite upstream source and ignores cable impedance, so it gives the maximum credible fault current at the transformer secondary. A rigorous value follows IEC 60909, adding source, cable, and motor contributions.
The selected breaker’s rated breaking capacity — Icu for an MCCB, Icn for an MCB — must be at least this Isc at the operational voltage. The tool rounds up to the next common capacity tier (10, 15, 25, 36, 50, 70, 85, 100 kA).
Frequently asked questions
Is this the fault current everywhere in the installation?
No — it is the maximum at the transformer secondary. Cable impedance reduces the fault current downstream, so devices further from the transformer can use a lower breaking capacity if a study confirms it.
Why does a lower impedance give a higher fault current?
The impedance voltage limits how much current flows into a short circuit. A 4 % transformer lets more fault current through than a 6 % one of the same rating, so it demands a higher breaker breaking capacity.
What about motor contribution?
Large running motors briefly feed additional current into a fault. For a rigorous result, add motor contribution per IEC 60909; this simplified tool does not include it.
Match the breaking capacity
Select an MCCB whose Icu / Ics at your operational voltage covers this fault level.
Estimate for guidance only. A rigorous short-circuit study to IEC 60909, including source, cable, and motor contributions, and verification by a qualified person, governs the final device selection.