Breaker Size Calculator

How to Convert Watts to Amps

Every breaker sizing problem starts with one formula: amps = watts ÷ volts. A 1,500-watt space heater on a standard 120V circuit draws 1,500 ÷ 120 = 12.5 amps. The same 1,500 watts on a 240V circuit would draw only 6.25 amps, which is why large appliances like dryers, ranges, and water heaters run on 240V — the higher voltage cuts the current in half, allowing smaller wire for the same power.

Once you know the amps, you might think you simply pick the next breaker size up. For short, intermittent loads that is roughly true. But for anything that runs for hours at a time, the National Electrical Code adds a safety margin that changes the answer — and that margin is where most DIY sizing mistakes happen.

The 80% Continuous Load Rule

The NEC defines a continuous load as one expected to run at maximum current for three hours or more — think space heaters, water heaters, EV chargers, and baseboard heat. For these loads, the breaker and circuit must be sized so the load is no more than 80% of the breaker's rating. Put another way, the circuit needs 125% of the load's current.

Our 12.5-amp space heater is a continuous load, so it needs 12.5 × 1.25 = 15.6 amps of capacity. That rules out a 15A breaker and lands on 20A. This is exactly why space heaters routinely trip 15A bedroom circuits: 12.5A is technically under 15A, but a breaker carrying over 80% of its rating for hours will heat up and trip — by design. The calculator above applies this 125% factor automatically when the continuous load box is checked.

Motor loads (compressors, well pumps, large saws) get similar treatment because motors draw far more current at startup than while running. The NEC has a dedicated set of rules for motor circuits (Article 430) that can actually allow larger breakers to ride through inrush current; for planning purposes this calculator adds a 25% margin to the running load and flags the result.

Standard Breaker Sizes and Wire Pairings

Breakers come in standard sizes, and each pairs with a minimum copper wire gauge. This pairing is non-negotiable: the breaker exists to protect the wire, so the wire must be able to carry at least as much current as the breaker allows through.

These pairings assume copper conductors at a 75°C termination rating, which is standard for modern residential breakers and lugs. Aluminum wire, long runs (see our voltage drop calculator), high ambient temperatures, or bundled cables all require upsizing the wire — another reason to have a licensed electrician review anything beyond a simple like-for-like replacement.

Common Appliance Circuits

For air conditioners, heat pumps, and other equipment with a nameplate, always follow the "maximum overcurrent protection" value printed on the unit — the manufacturer has already done the motor-load math for you. A 4,500W water heater is a good worked example of the continuous rule: 4,500 ÷ 240 = 18.75A, times 1.25 = 23.4A, which rounds up to a 30A breaker on 10 AWG wire. Size the tank itself with our water heater calculator.

Why Oversizing a Breaker Is Dangerous

Here is the single most important idea in residential wiring: the breaker protects the wire, not the appliance. A 15A breaker on 14 AWG wire trips before the wire overheats. If you swap that breaker for a 20A because it "keeps tripping," the 14 AWG wire can now carry 20 amps continuously — more than it is rated for — and it will heat up inside your walls with nothing left to stop it. That is how electrical fires start.

A breaker that trips repeatedly is telling you the circuit is overloaded or faulty, and the fix is always one of three things: move loads to another circuit, run a new circuit with appropriately sized wire, or find and repair the fault. It is never a bigger breaker on the same wire. When in doubt about what gauge is behind a breaker, check the wire itself where it enters the panel — 14 AWG and 12 AWG look similar, and outlets alone tell you nothing. Our wire size calculator covers gauge selection in more depth.

This calculator is an educational planning aid, not a substitute for your local electrical code or a licensed electrician.