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REEFLINEELECTRICAL SAFETY
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Short-Circuit Current Calculator

A rough estimate of fault current at a transformer's secondary terminals — not a substitute for a proper fault level study.

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Read before using this one: this figure only covers the transformer's own impedance at its secondary terminals. It ignores the upstream source fault level, cable impedance, and everything downstream — all of which reduce the real fault current at your switchboard. Never use this number alone to select or verify a breaker's interrupting (kA) rating. That requires a full fault level study by a licensed designer.

Transformer secondary fault current

Isc = Full-load current ÷ (impedance % ÷ 100)

Check the transformer's nameplate — typically 4–6% for distribution transformers.

Approx. three-phase fault current

0 kA

Transformer-terminal only — the real figure at your switchboard will be lower once source and cable impedance are included. Get a proper fault level study before relying on this for protective device selection.

Export this result as a PDF

A clean, branded PDF of your inputs and result — handy for records or sending on.

Please note: General guidance only — verify against the current standard before relying on this result. Not a substitute for a licensed electrical design.

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