Short-Circuit Current Calculator
Every breaker and panel has a maximum fault current it can safely interrupt. The biggest contributor is usually the supply transformer: its kVA rating and impedance percentage set the available short-circuit current at its terminals. This calculator gives that figure so you can check equipment ratings against it.
Formulas: Three-phase: Isc = kVA × 1000 ÷ (√3 × V × %Z ÷ 100) · Single-phase: Isc = kVA × 1000 ÷ (V × %Z ÷ 100)
Worked example: a 500 kVA, 400 V three-phase transformer at 5% impedance: Isc = 500,000 ÷ (1.732 × 400 × 0.05) ≈ 14,434 A (14.43 kA). Any breaker downstream must have an interrupting rating above that (allowing for cable impedance, which only reduces it). A single-phase 50 kVA, 230 V transformer at 4% gives 50,000 ÷ (230 × 0.04) ≈ 5,435 A.
This is the infinite-bus value at the transformer terminals — cable length and impedance between the transformer and the fault reduce the real figure, so this is the conservative number for equipment selection. Size the transformer first with the transformer kVA sizing calculator.
Frequently asked questions
Why does lower impedance mean higher fault current?
Impedance is what limits the current. A transformer at 4% impedance lets through roughly 25 times its full-load current under a dead short (1 ÷ 0.04); at 7% it is about 14 times. Stiffer transformers fault harder.
Is this the current at the far end of a long cable run?
No — this is at the transformer terminals, the worst case. Cable resistance and reactance reduce fault current with distance, which is why a proper fault study calculates it at each panel.
What do I do with this number?
Check that every breaker, fuse and panel downstream has a short-circuit (interrupting) rating above it. If any device is rated below it, it must be replaced or the system redesigned.