A breaker exists to protect the wire behind it, not the appliance in front of it. So sizing is a two-step job: find the load’s current, then pick the smallest standard breaker that is above it — while making sure the wire can carry that breaker’s rating. Get either half wrong and you have either nuisance trips or an unprotected cable.
Step 1: find the full-load current
Read it off the nameplate, or work it from the power: amps = watts ÷ volts (divide again by power factor for motors and other inductive loads). A 3,000 W heater on a 240 V circuit draws 3,000 ÷ 240 = 12.5 A. On a 120 V circuit the same 3,000 W would draw 25 A — volts matter.
Step 2: add 25% for continuous loads
A continuous load runs at or near full current for three hours or more — EV chargers, water heaters, storage heaters, big freezers. Breakers heat up under a sustained load, so the rule is to load them to no more than 80%, which is the same as sizing the breaker at 125% of the running current. A 16 A continuous load becomes a 16 × 1.25 = 20 A design current.
Step 3: round up to a standard size
North American (NEC) and international (IEC) ladders differ, but the logic is identical:
| NEC (amps) | IEC (amps) | Typical use |
|---|---|---|
| 15 | 16 | Lighting, general receptacles |
| 20 | 20 | Kitchen, small appliances |
| 30 | 32 | Dryer, water heater |
| 40 / 50 | 40 / 50 | Range, EV charger, sub-panel |
Breaker Size Calculator
Enter the load in amps or in kW with voltage and power factor — it applies the 125% continuous factor and returns the next standard breaker size.
The rule people miss: the wire must match
The wire’s ampacity must be at least the breaker rating, allowing for installation method, grouping and temperature. This is why 14 AWG (2.5 mm²) pairs with a 15 A breaker and 12 AWG (4 mm²) with a 20 A. Never upsize a breaker beyond what the existing wire can carry to stop nuisance trips — upsize the wire instead, or you remove the cable’s protection.
Why does a breaker trip when a motor starts?
Motor inrush can hit 6–8× the running current for a fraction of a second, tripping a fast breaker even though the running load is fine. Motor circuits use an inverse-time or HACR breaker — a C or D curve in IEC terms — of the same rating that rides through the surge, rather than a bigger breaker that would leave the wire under-protected.
This gets the rating, not the whole design
Breaker choice also involves the interrupting rating (AIC), coordination with upstream devices, and ground-fault or arc-fault protection where the rules require it (GFCI / AFCI). Use this to get the size right, then confirm the full design against the wiring rules that apply to your installation.