Short-circuit current calculator — IEC 60909
Estimate the available short-circuit current at a transformer secondary — the figure used to check a breaker’s interrupting rating and cable let-through.
Transformer secondary short-circuit current
Impedance Z
Secondary line voltage
Formulas: I = kVA × 1000 / (√3 × V) and I = I × 100 / Z — the impedance figure is the transformer nameplate Z%. A real installation adds cable impedance and motor contribution, so the breaker or contactor interrupting rating must be selected above the value shown here.
Typical transformer impedance
Quick reference: small dry-type transformers run about 4%, medium distribution transformers about 5–6%, and generator step-up transformers about 10–12%. Lower impedance means higher fault current — a breaker fed by a 4% unit must interrupt a lot more than one fed by an 8% unit.
Enter the transformer nameplate impedance when available — the presets are a starting point, not a substitute for the actual rating.
Fault-current figures drive the interrupting rating of every protective device downstream.AO Ctrl stocks the breakers, transformers and contactors sized for these numbers:
More calculator tools
Frequently asked questions
How is available short-circuit current calculated from transformer kVA?
The tool first finds the rated secondary current with I = kVA × 1000 / (√3 × V), then divides by the transformer nameplate impedance: Isc = I × 100 / Z. A 1000 kVA, 400 V transformer with 6% impedance gives roughly 24 kA of available fault current at its terminals. Any upstream impedance you add (as a % on the transformer base) adds in series with the transformer impedance: Isc_eff = I × 100 / (Z + extra).
What is a typical transformer impedance percentage?
Small dry-type transformers run about 4% impedance, medium distribution transformers about 5-6%, and generator step-up transformers about 10-12%. Lower impedance means higher fault current — a breaker fed by a 4% unit must interrupt far more than one fed by an 8% unit. Use the nameplate Z% when available; the 4/5/6/8% presets are only a starting point.
Is the transformer short-circuit current the breaker interrupting rating I need?
No — the figure at the transformer terminals is the ceiling, but a real installation adds cable impedance and motor contribution, which the tool does not model. Interrupting ratings must be selected above the calculated value, and this estimate is a screening check rather than a substitute for a full IEC 60909 study.
AO Ctrl provides this tool for reference only.