Breaker size calculator
Enter the load and whether it runs continuously (3 hours or more). The calculator applies 125% to the continuous part, rounds up to the next standard breaker, and shows the smallest wire for that breaker.
For general (non-motor) loads only. Motors, air conditioners and heat pumps follow Articles 430 and 440; use the unit's nameplate "MOCP" or "max fuse/breaker". The wire size shown assumes copper, 30 °C, no bundling and a short run. Check wire size for your conditions.
The 125% rule
NEC 210.20(A) (branch circuits) and 215.3 (feeders) require the overcurrent device to be rated at least the non-continuous load plus 125% of the continuous load, unless the whole assembly is listed for 100% operation. A continuous load is one expected to run at maximum current for 3 hours or more.
Put the other way round, a standard breaker should carry no more than 80% of its rating continuously: 12 A on a 15 A breaker, 16 A on 20 A, 24 A on 30 A, 32 A on 40 A, 40 A on 50 A.
Worked examples
- Level 2 EV charger, 32 A: 32 × 1.25 = 40 A → 40 A breaker, 8 AWG copper at 60 °C.
- 4,500 W water heater, 240 V: 18.75 A × 1.25 = 23.4 A → 25 A breaker. NEC 422.13 treats storage water heaters of 120 gallons or less as continuous loads.
- Wall oven, 8,000 VA at 240 V, treated as non-continuous: 33.3 A → 35 A breaker.
Standard breaker sizes, NEC 240.6(A)
15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000 and 6000 A. Fuses also come in 1, 3, 6 and 10 A.
Never replace a breaker with a larger one to stop nuisance tripping. The breaker protects the wire. A bigger breaker on the same wire lets it overheat inside the wall. Find out why it trips.
Questions people ask
How do I size a breaker for a continuous load?
Multiply the continuous load current by 125%, add any non-continuous load at 100%, and choose the next standard size from NEC 240.6(A). A 32 A EV charger needs 32 × 1.25 = 40 A, so a 40 A breaker.
What are the standard breaker sizes?
NEC 240.6(A) lists 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500 and 600 A, with larger ratings above that.
Does this apply to motors and air conditioners?
No. Motors (Article 430) and air-conditioning equipment (Article 440) have their own rules, and many allow a breaker larger than the wire ampacity. Follow the nameplate "maximum overcurrent protection" on HVAC units.