Short-Circuit Current Calculator
Estimate symmetric single-phase short-circuit current at low-voltage points from transformer and cable impedance.
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Short-circuit current (Icc)
At any point in the circuit, the symmetrical short-circuit current comes from Icc = V / Z, with Z standing for the equivalent impedance of the transformer, cables and busbars together. Take a 500 kVA transformer at 440 V with Z = 5%: that works out to Icc ≈ 13.1 kA at its secondary. Whatever protective device sits downstream needs a breaking capacity of Icu ≥ Icc at the spot where it's installed. If it doesn't, the breaker can weld shut during a fault and never open the circuit.
Applications and standards
This calculation sizes circuit breakers and fuses, feeds the design of industrial MCC panels and grid-tie PV systems, and lets you check that cables and busbars can ride out the dynamic and thermal stress of a fault. On the Brazilian side the standards are NBR 14039 for medium voltage and NBR 5410 for low voltage. The international reference is IEC 60909, which covers short-circuit calculation in three-phase AC systems.
FAQ
Why include cable impedance? Cables add to Z, which pulls Icc down as you move downstream. Leave them out and you'll overestimate the fault current at the panels farthest from the source.
What is the difference between Icu and Ics? Icu is the ultimate breaking capacity, the most the device can interrupt at all. Ics is the service capacity, usually somewhere from 25% to 100% of Icu, and after clearing a fault at that level the breaker still works normally.
Is this enough for a project? No, treat it as a first estimate. The final design has to follow IEC 60909, with the whole network modeled and every source contribution accounted for, from the utility to motors and generators.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.