Circuit Breaker Size Calculator

Circuit breakers protect wiring from overheating by cutting power when current exceeds a safe level. Choosing the wrong size, in either direction, causes problems that range from nuisance trips to house fires.

The NEC 80% Rule:

The National Electrical Code (NEC) requires that continuous loads (running for 3+ hours) use no more than 80% of the breaker’s rated capacity.

Minimum Breaker Size = Total Load (amps) ÷ 0.80

If your circuit draws 16 amps continuously: 16 ÷ 0.80 = 20A breaker minimum

Standard breaker sizes (US), from NEC 240.6(A): 15A → 20A → 25A → 30A → 35A → 40A → 45A → 50A → 60A → 70A → 80A → 90A → 100A → 110A → 125A → 150A → 175A → 200A

Always round up to the next standard size after the calculation. The odd ones, 25, 35, 45, 90 and 110, are real code sizes but not what a hardware store stocks, so treat them as a signal to check the next common size up rather than as a shopping list.

Sizing a new circuit or checking one that exists?

This page sizes a new circuit: you know the load, you want the breaker and the wire that go with it. If the breaker is already in the panel and the question is whether something else will fit on it, that is the circuit breaker load capacity calculator, which runs the same 80% rule from the other end.

Wire gauge must match the breaker, and the column depends on the cable:

That 50A row is the one people get wrong. Plenty of sources say 8 AWG is fine for 50 amps, and it is, if the cable is THHN in conduit rated to 75°C. NM-B cable, which is the plastic-sheathed Romex almost every American house is wired in, is capped at the 60°C column by NEC 334.80 no matter what the conductors themselves are rated. There 8 AWG is only good for 40 amps, so a 50A breaker on 8 AWG Romex is a violation and a genuine fire risk. Pick the cable type in the calculator and it uses the matching column.

Never install a breaker larger than the wire can handle. It is the most dangerous mistake in the whole of home electrics, because the failure is silent: the circuit works perfectly, the wire runs hot inside a wall you cannot see, and nothing trips until something is already burning.

Why 14, 12 and 10 AWG are held back. Those three carry more current on the ampacity table than their breaker is ever allowed to be. 14 AWG copper is good for 20 amps at 75°C, yet it is never protected above 15. That is NEC 240.4(D), the small-conductor rule, and it exists because those are the sizes that get nicked by drywall screws, over-tightened in backstab connectors, and buried in insulation. The margin is deliberate. Do not spend it.

Aluminum is a different table. If your feeder is aluminum, everything above moves up roughly two sizes: a 100A aluminum feeder wants 1/0, not 3, and a 200A one wants 4/0.

Common circuits and typical breaker sizes:

Important: Always have a licensed electrician verify calculations and perform wiring work in your jurisdiction. Electrical codes vary by location.