Prospective Fault Current Calculator
Estimate the prospective short-circuit current (PSCC, line–neutral) and prospective earth fault current (PEFC, line–earth) at the end of a copper cable run, then check it against the breaking capacity of the protective device.
Supply at the Origin
Get these from the generator supplier for the set and its earthing arrangement, at its terminals. Fault current from an alternator is low and falls away as the subtransient and transient reactances take over. It is often far below a mains supply.
Cable Run (Copper)
Fault Currents
| At origin | End of run | |
|---|---|---|
| PSCC line–neutral (max) | — | — |
| PSCC 3-phase ≈ 2 × L–N | — | — |
| PEFC line–earth (max) | — | — |
| PEFC line–earth (min, hot, Cmin) | — | — |
Next: use the minimum PEFC in the Adiabatic CPC Calculator to check the CPC size, or size the run with the Cable Gauge Calculator.
Breaking Capacity Check
A device must be able to break the fault current at the point where it is installed (Reg. 434.5.1). A device feeding this cable sits at the origin; the outgoing devices in a distro fed by this cable sit at the end.
Domestic-style consumer units tested to BS EN 61439-3 Annex ZB have a 16 kA conditional rating (Icc) when supplied through an upstream BS 88-3 (or BS 1361) fuse up to 100 A. That is why 6 kA MCBs are accepted in them on a 16 kA supply. It does not cover other upstream devices, or an MCB used on its own (e.g. in a portable distro). A lower breaking capacity is otherwise only acceptable where an upstream device with enough breaking capacity is coordinated to protect it (Reg. 434.5.1).
Typical Supply Values
| Supply | Max Ze (DNO declared) | Notes |
|---|---|---|
| TN-C-S (PME) | 0.35 Ω | PEFC at origin ≈ 230 ÷ 0.35 ≈ 657 A minimum. |
| TN-S | 0.8 Ω | Earth via cable sheath or separate conductor. |
| TT | 21 Ω | DNO source earth only. Add the installation electrode RA. PEFC is usually too low to operate overcurrent devices, so RCDs are used. |
| Max PFC | 16 kA | DNO declared maximum for single-phase supplies up to 100 A. Ask the DNO for three-phase and larger supplies. |
| Generator | — | From the generator supplier. Often a few hundred amps to a few kA, so check Zs / disconnection as well as breaking capacity. |
Notes & References
- Method: I = c × U0 ÷ (Zorigin + Rcable). PSCC uses the line + neutral resistance (neutral assumed the same size as the line); PEFC uses line + CPC. Resistances are added arithmetically to the source impedance, as in the IET On-Site Guide and Guidance Note 3. This is the normal approximation and is close enough for small cables. A measured value at the far end is always better.
- Maximum vs minimum: maximum fault currents (for breaking capacity, Reg. 434.5.1) use cold conductors at 20 °C. The voltage factor c can be raised to Cmax = 1.10 (PD CLC/TR 50480) to allow for the supply being up to 10 % high (ESQCR). The minimum PEFC (for disconnection and the adiabatic check) uses conductors at 70 °C (× 1.20) and Cmin = 0.95, as BS 7671 Appendix 3.
- Three-phase rule of thumb: a symmetrical three-phase fault is driven by U0 through one conductor's impedance, while a line–neutral fault goes through two (out on the line, back on the neutral). With equal-size neutral, the three-phase fault current is about twice the single-phase L–N value. Guidance Note 3 uses × 2 for this; it errs on the high (safe) side.
- Conductor resistance: copper, IEC 60228 maximum values at 20 °C (as the IET On-Site Guide), e.g. 2.5 mm² = 7.41 mΩ/m.
- Breaking capacities: MCB/RCBO Icn is the value marked on the device (BS EN 60898 / 61009-1). Fuse values are typical for the standard; always check the marking or the manufacturer's data. Record the higher of PSCC and PEFC as the prospective fault current on certificates.
This tool is guidance only. It does not replace BS 7671, the IET Guidance Notes, manufacturer's data, or the judgement of a competent person. Where the result is close to a device's rating, measure it.