Circuit Breaker Load Capacity Calculator

The breaker is already in the panel. The question is what it can still carry, and whether the thing you want to plug in will fit alongside what is on there now.

That is a different question from sizing a new circuit, and it runs the code rule from the other end. If you are wiring something from scratch and need the breaker and the conductor, use the circuit breaker size calculator instead.

The 125% rule and the 80% cap are the same sentence

Those two numbers get quoted as if they were separate rules. They are not. Section 210.20(A) of the NEC (National Electrical Code) requires a breaker to be sized at 125% of a continuous load, which is the same as saying a continuous load may only use 80% of the breaker, because 1 divided by 1.25 is 0.8. The 125% form is the one you want when sizing. The 80% form is the one you want here.

A 20 A breaker is good for 20 A of anything that runs in short bursts, and only 16 A of anything that runs three hours or more. Lighting, heating, air conditioning and EV charging all count as continuous. A space heater on high is 12.5 A on its own, which is 78% of that 16 A ceiling before you plug in anything else on the circuit.

Adding it up is the part people skip

A branch circuit usually feeds several outlets, and the breaker sees the sum of whatever is running at once. The third box asks for the load already on that circuit precisely because a “will this 1500 W heater fit on a 20 A circuit” answer is worthless without it. Yes on an empty circuit, no on a kitchen counter that already has a toaster on it.

If you do not know what is already there, switch off the breaker and see what dies. That is genuinely how it is done.

120V vs 240V

Household outlets and most lighting are 120 V. Large appliances (dryer, range, central air conditioning, EV charger, water heater) are 240 V. For the same wattage a 240 V load draws half the amps, and the breaker is a double-pole that takes two spaces in the panel.

Never fix a shortage by fitting a bigger breaker

This is the classic mistake and it is worth being blunt about. Swapping a 15 A breaker for a 20 A one does not give the circuit more capacity. It removes the thing that was stopping 14 AWG wire from overheating inside a wall you cannot see. The circuit will work perfectly right up until it does not.

The wire that belongs with each breaker, for NM-B cable (the plastic-sheathed stuff sold as Romex) at the 60°C column NEC 334.80 requires: 15 A on 14 AWG, 20 A on 12 AWG, 30 A on 10 AWG, 40 A on 8 AWG, 50 A on 6 AWG. AWG is American Wire Gauge, and it runs backwards, so a smaller number is a thicker wire. The calculator prints the conductor that matches your breaker so you can check the wire in the box against it. If they disagree, the breaker is the thing that has to come down.

AFCI and GFCI requirements

NEC 2020 requires arc-fault circuit interrupters (AFCI) in most living areas and ground-fault protection (GFCI) in bathrooms, kitchens, garages, and outdoors. When replacing a breaker in one of those areas, replace it with the same protective type, not a plain one.