Circuit breaker size calculator
Choose the right breaker for a circuit from its power and check that it protects the wire.
How it is calculated
Under the NEC, a breaker feeding a continuous load (3 hours or more) must be rated at least 125 % of that current, rounded up to the next standard size: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A…
Small conductors are limited by NEC 240.4(D): 14 AWG to 15 A, 12 AWG to 20 A and 10 AWG to 30 A.
IEC: Ib ≤ In ≤ Iz
Under IEC rules the breaker rating (In) must be at least the load current (Ib) and no more than the wire ampacity (Iz). Standard MCB ratings are 6, 10, 13, 16, 20, 25, 32, 40, 50, 63 A. Curve B trips at 3–5 × In, C at 5–10 × In and D at 10–20 × In.
Frequently asked questions
What size breaker for a 3500 W load at 240 V?
3500 / 240 = 14.6 A. As a continuous load, 14.6 × 1.25 = 18.2 A, so a 20 A breaker on 12 AWG copper.
What breaker size for a 7.2 kW EV charger?
At 240 V it draws 30 A continuously, so it needs a 40 A breaker (30 × 1.25 = 37.5 A) and 8 AWG copper.
Can I install a bigger breaker if it keeps tripping?
Only if the wire is rated for it. A breaker larger than the wire ampacity leaves the wire unprotected and is a fire risk.