Prospective Fault Current Calculator — BS 7671

This calculator determines the maximum prospective fault current at the origin and at any point in a circuit using Ohm's law applied to the fault loop impedance, with the voltage factor C_max = 1.10 applied per BS 7671:2018+A4:2026 Appendix 3 / IEC 60909.

Prospective fault current formula

At the origin: PSCC = C_max × Uo / Ze, where Uo is the nominal line-to-neutral voltage (230V single phase), C_max = 1.10 (so 230V × 1.10 = 253V) and Ze is the external earth fault loop impedance.

At any point: PEFC = C_max × Uo / Zs, where Zs = Ze + R1 + R2 (the total line-earth loop impedance including circuit conductor resistances).

The C_max = 1.10 factor is a conservative design upper bound: ESQCR permits the 230V supply to sit up to +10% high (253V), so the calculated fault current is the worst case. It reads roughly 10% higher than a hand-held PFC tester, which measures at the actual supply voltage with no voltage factor applied.

Breaking capacity

BS 7671:2018+A4:2026 Regulation 434.5.1 requires the rated breaking capacity of each protective device to be not less than the prospective fault current at its point of installation. A device with a lower breaking capacity is still permitted where adequate upstream (back-up) protection limits the let-through energy, or where the assembly carries a conditional short-circuit rating. The calculator compares the unconditional PSCC at the origin against the device breaking capacity.

• MCBs to BS EN 60898: commonly 6kA, sometimes 10kA (suitable for most domestic supplies)
• BS 88-3 HRC fuses: up to 80kA breaking capacity
• BS 1361 Type I consumer-unit cartridge fuse: 16.5kA at 240V
• BS 1361 / BS 88-3 Type II house-service cut-out: 33kA at 415V (ASTA-enhanced to 80kA)
• Domestic consumer unit (BS EN 61439-3): commonly 16kA conditional rating behind a 100A BS 88-3 service cut-out