Fault Current Calculator
Calculate the symmetrical three-phase fault current (short-circuit current) at a point in an electrical system from system impedance or transformer % impedance.
Results
What is it?
Fault current (short-circuit current) is the maximum current that flows when a bolted three-phase fault occurs. For a transformer: Isc = I_rated / (%Z/100). The result is used to select circuit breakers (whose interrupting rating must exceed Isc) and design busbar systems.
How to use
For a transformer-fed system, enter the transformer kVA, its nameplate percentage impedance (%Z), and the secondary voltage. For utility/source impedance method, enter the Thevenin equivalent impedance at the fault point in milliohms.
Example scenario
A 500 kVA, 415 V transformer with %Z = 4.5%. Rated current = 500×1000/(√3×415) = 695.6 A. Isc = 695.6/0.045 = 15,457 A = 15.5 kA. Specify 25 kA breakers for margin.
Pro tip
Always size circuit breakers and busbars for the AVAILABLE fault current, not the load current. Add cable and busbar impedances to reduce the calculated Isc at downstream points. IEC 60909 peak asymmetrical fault current ≈ 2.5 × Irms for LV systems, which must also be considered for mechanical switchgear ratings.