Prospective Fault Current Calculator — BS 7671

Prospective fault current is the maximum current that would flow during a short circuit or earth fault. BS 7671:2018+A4:2026 (18th Edition) Regulation 434.5.1 requires that the prospective fault current be determined at every point of the installation. Each protective device should have a breaking capacity at least equal to the prospective fault current at its point of installation. A device with a lower breaking capacity may still comply where an upstream device limits the let-through energy (back-up protection) or where the assembly carries a conditional short-circuit rating — but otherwise it may fail to safely interrupt the fault and could cause fire or damage.

PSCC (Prospective Short Circuit Current) is the fault current that flows in a line-to-neutral short circuit. PEFC (Prospective Earth Fault Current) is the fault current that flows in a line-to-earth fault via the circuit protective conductor. PSCC is typically higher because the neutral conductor usually has lower impedance than the CPC. Both must be checked against the protective device's breaking capacity.

Maximum prospective fault current is calculated using Ohm's law with the voltage factor Cmax: Ipf = Cmax × Uo / Zs, where Uo is the nominal line-to-neutral voltage (230V single phase), Cmax = 1.10 and Zs is the loop impedance. The 1.10 factor accounts for the supply being permitted to sit up to +10% high (230V × 1.10 = 253V), so at the origin PSCC = 253 / Ze. At any point in the circuit, the conductor resistances (R1, Rn or R2) are added to Ze. Because Cmax assumes the worst-case high voltage, this calculator reads roughly 10% higher than a hand-held PFC tester, which measures at the actual supply voltage.

Standard domestic MCBs typically have a 6kA breaking capacity, which is adequate for most domestic installations where PSCC is usually below 6kA. For installations close to substations or with very low Ze values, the PSCC may exceed 6kA and MCBs with higher breaking capacity (10kA or 16kA) or HRC fuses (BS 88 with 80kA) may be required. Use our Circuit Breaker Sizing Calculator to select the right MCB rating.

TN-C-S (PME) systems typically have Ze values between 0.2 and 0.35 ohms. TN-S (lead sheath) systems typically range from 0.35 to 0.8 ohms. TT systems can have much higher values. The DNO (Distribution Network Operator) can provide the maximum expected Ze for your supply. Lower Ze means higher prospective fault current at the origin.