Work out the correct circuit breaker or fuse rating (MCB / breaker) for a load from its power, voltage and continuous-load factor.
The calculator converts the load to a design current, applies the continuous-load factor where relevant, and rounds up to the next standard breaker or fuse rating.
Divide the load in watts by the voltage to get the current in amps, multiply continuous loads by 1.25, then choose the next standard breaker rating above that figure. This calculator does all of that for you.
A load that runs for three hours or more is treated as continuous. To stop the breaker running hot, it should be sized to at least 125% of the load current – so a 16 A continuous load needs a 20 A breaker.
A 7 kW shower at 230 V draws about 30 A, so it typically needs a 32 A (UK) breaker on a suitably sized cable. Always confirm against the cable rating and manufacturer instructions.
They work together: the breaker must protect the cable, so the cable current rating must be equal to or greater than the breaker rating. Use this alongside the Cable Size calculator.
The method and standard ratings shown apply to both UK MCBs and US breakers, but final circuit design must follow BS 7671 in the UK or the NEC in the US and be verified by a qualified electrician.
