Tools · Calculator

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.

Get a Free Quote Call 0404 961 061

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)

Transformer rating (kVA)

Secondary voltage (V)

Transformer impedance (%)

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.

Get a quote

Need the real numbers signed off?

This tool is a starting point. Get a licensed design or on-site assessment for anything going to certification.

Instant online quote for test & tag
Booked in as soon as we can fit you in
Certificate + asset register within 24h of completion

Or call 0404 961 061

Other calculators

Keep going

See all 8 →

Need it done, not just calculated?

Switchboard Upgrades

Replace old ceramic-fuse boards with a modern, compliant switchboard built for your load.

Get a quote