Estimate prospective short-circuit current at origin and end of a cable run using I ≈ V / Z.
Choose a source model and cable impedance method.
Does not account for motor contribution, arc impedance, asymmetric peak, or protective device let-through.
For design guidance only. Always verify with a qualified electrical engineer and switchgear ratings.
About this short circuit fault current calculator
This free short circuit fault current calculator estimates the prospective short-circuit current (PSCC) at the origin of a supply and at the end of a cable run, using the simplified I ≈ V / Z method. You choose a source model, a transformer (kVA and %Z), a known PSCC at origin in kA, or a known external source impedance Ze, then describe the cable so the tool can add its impedance. It is built for UK building-services engineers, electrical designers and contractors who need a quick fault-level figure to check switchgear and protective-device ratings.
How the fault current is calculated
The calculator works out a per-phase source impedance Zs from your chosen source model, then adds the cable’s loop impedance. Cable resistance comes either from a preset R20 value by conductor size (copper or aluminium) or from custom R and X in Ω/km, with R corrected for conductor temperature (20, 70 or 90°C); reactance X is temperature-independent. The loop impedance is Zcable = √(R² + X²), doubling the conductor resistance for single-phase L-N and L-PE loops to account for the return conductor. The total is combined with the source as Ztotal = √(R² + (X + Zs)²). For a three-phase supply the fault current is I = VLL / (√3·Z); for single-phase it is I = V / Z. The method follows the simplified approach of BS 7671 and IEC 60909, with R20 values from the BS 7671 On-Site Guide and temperature coefficients per IEC 60228. It does not account for motor contribution, arc impedance, asymmetric peak or protective-device let-through.
Frequently asked questions
Which source model should I use?
Use Transformer (kVA + %Z) when you know the supply transformer rating and impedance, Known PSCC at origin when the DNO has quoted a prospective fault current (kA), or Known Ze when you have measured the external earth-fault loop impedance directly. Each is converted to a per-phase source impedance before the cable is added.
Why does temperature change the result?
Conductor resistance rises with temperature, so the tool corrects R20 to your selected 20, 70 or 90°C using copper or aluminium coefficients. The cold/test value (20°C) gives the highest fault current, while operating temperatures lower it. Reactance X stays the same regardless of temperature.
Can I rely on this for final design?
No. It is a simplified estimate for design guidance only and ignores motor contribution, arc impedance, asymmetric peak and device let-through. Always verify fault levels with a qualified electrical engineer and against the actual switchgear and protective-device ratings.