Breaker Size Calculator

Work out the correct circuit breaker or fuse rating (MCB / breaker) for a load from its power, voltage and continuous-load factor.

Part of:Electrical hub
kW
V

How to Use This Calculator

How to Use This Calculator

  1. Enter by – Choose whether you know the load in kilowatts or already know the current in amps.
  2. Power or current – Enter the figure for the appliance or circuit.
  3. Voltage – 230 V is standard in the UK; US circuits are usually 120 V or 240 V.
  4. Phase – Single phase for most domestic circuits; three phase for larger machines.
  5. Power factor – Use 1 for heaters and resistive loads, around 0.8 for motors.
  6. Load type – A continuous load (running for 3 hours or more) is sized at 125% of its current, in line with common practice and the US NEC.

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.

Common Questions

How do I work out what size breaker I need?

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.

What is the 125% continuous load rule?

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.

What size breaker for a 7 kW shower?

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.

Should the breaker or the cable be sized first?

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.

Is this valid for UK and US installations?

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.

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