Breaker Size Calculator

Find the load’s full-load current — from its nameplate or from kW ÷ (volts × power factor) — then pick the next standard breaker rating above it. For loads that run for hours at a time (heaters, EV chargers, water heaters), size the breaker at 125% of the running current first, so it doesn’t sit near its limit all day. So a 16 A continuous load becomes a 20 A design current and takes a 20 A breaker.

North American NEC breakers come in 15, 20, 30, 40, 50, 60, 70, 90, 100 A and up (per NEC 240.6); the rest of the world uses the IEC ladder — 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100 and 125 A. The calculator rounds up within a standard breaker series; the sizing logic is the same either way.

Yes — this is the rule people miss. The breaker protects the wire, so the wire’s ampacity must be at least the breaker rating, accounting for installation method, grouping and temperature. This is why 14 AWG (2.5 mm²) goes on a 15 A breaker and 12 AWG (4 mm²) on a 20 A. Never upsize a breaker beyond what the existing wire can carry; upsize the wire instead.

A load that runs at or near full current for three hours or more — water heaters, EV chargers, storage heaters, big freezers. Breakers heat up when loaded continuously, so the convention is to load them to no more than 80% — which is the same as sizing the breaker at 125% of the load.

Motor starting current can hit 6–8× the running current for a fraction of a second, which trips a fast breaker even though the running load is fine. Motor circuits normally use an inverse-time or HACR-rated breaker (in IEC terms, a C or D curve) of the same rating that rides through the inrush, rather than a bigger breaker.

No. Breaker choice also involves the interrupting rating (AIC), coordination with upstream devices, ground-fault protection (GFCI/AFCI where required) and your local wiring rules. Use this to get the rating right, then confirm the full design against the regulations that apply to the installation.