Circuit Breaker Size Calculator

Work out the load current first: amps = watts / (volts × power factor). A 3 kW load at 230 V with a power factor of 1.0 draws 13 A. Multiply by a safety factor (1.25 for continuous loads) to get the design current, then choose the next standard MCB rating above it, here 20 A. The breaker must also match the cable's capacity, so final selection is a job for a qualified electrician working to BS 7671.

UK miniature circuit breakers follow standard ratings of 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100 and 125 A. Typical assignments: 6 A for lighting circuits, 16 or 20 A for radials, 32 A for ring finals and many EV charge points, 40 to 50 A for showers and larger cookers. The calculator rounds your design current up to the next rating in this series.

Loads that run for hours at a stretch, such as immersion heaters, EV charge points, catering equipment and space heating, are treated as continuous. Sizing the protective device at 125 percent of the load current avoids nuisance heating of the breaker and gives headroom for tolerance in the load itself. For short-duration loads a factor of 1.0 or 1.1 is generally enough. Confirm the approach with a qualified electrician to BS 7671.

Yes, for anything inductive. Power factor is the ratio of real power to apparent power, and current scales with apparent power. A 2 kW motor at a power factor of 0.8 draws the same current as a 2.5 kW resistive heater. Use 1.0 for heating elements, 0.8 to 0.9 for motors, compressors and older fluorescent fittings, and check the nameplate where one exists.

Treat it as a single-phase tool. For a balanced three-phase load the line current is kW × 1000 / (1.732 × 400 × power factor), which is roughly a third of the single-phase equivalent at 230 V. Use the three-phase power calculator for those circuits, and have a qualified electrician confirm the protective device and cable to BS 7671, especially where motors introduce starting surges.

The cable. A circuit breaker's primary job under BS 7671 is to disconnect before the conductors overheat, so its rating must not exceed the cable's current-carrying capacity after derating for installation method, grouping and ambient temperature. Appliance protection is handled by the device's own fusing or controls. If the sums say you need a bigger breaker than the cable allows, the cable gets upsized, not the other way round.