Circuit Breaker Size Calculator
Not sure what size breaker you need? Enter your load in watts and the circuit voltage, and this free calculator works out the load current, applies the NEC 125% continuous-load rule when needed, and recommends the next standard breaker size - plus a suggested wire gauge.
Enter your numbers
Total watts on the circuit (check the appliance nameplate).
A continuous load runs at full power for 3+ hours (heaters, EV chargers, lighting). Select Yes to apply the NEC 125% rule.
Your results
Results update as you change the inputs.
How to use this calculator
Enter the total load in watts - the number on the appliance nameplate, or the sum of everything sharing the circuit. Pick 120V for standard outlets and lighting circuits, or 240V for large appliances like water heaters, dryers, and EV chargers. Then say whether the load is continuous: anything expected to run at full power for 3 hours or more, such as space heaters, grow lights, or EV charging.
Results update instantly as you change any input, or press Calculate to recompute. The recommended breaker size, the required current, and a suggested wire gauge all appear on the right (or below on phones).
How it works
First the calculator finds the load current with I = P ÷ V (amps = watts ÷ volts). A 1500W heater on 120V draws 1500 ÷ 120 = 12.5 amps.
Then it applies the NEC continuous-load rule honestly: circuits carrying full load for 3 hours or more must be sized at 125% of the load current (equivalently, a breaker may only carry 80% of its rating continuously). So 12.5A × 1.25 = 15.625A required. Non-continuous loads skip this step and use the raw load current.
Finally, it rounds up to the next standard breaker size that meets or exceeds the required current, from 15, 20, 25, 30, 35, 40, 50, and 60 amps. For 15.625A required, a 15A breaker is too small, so the recommendation is 20A. The wire gauge hint follows the breaker: 14 AWG copper for 15A, 12 AWG for 20A, 10 AWG for 25–30A.
Safety disclaimer: this calculator is guidance only. Breakers protect the wire, not the appliance - never install a breaker larger than your wiring is rated for. Electrical work must follow your local electrical code, and installation should be done or checked by a licensed electrician. If a breaker trips repeatedly, find the cause rather than upsizing it.
Frequently asked questions
What size breaker do I need for a 1500 watt space heater?
A 1500W heater on a 120V circuit draws 12.5 amps. Heaters run for hours at a time, so they count as continuous loads: 12.5 x 1.25 = 15.625 amps required. The next standard breaker size is 20 amps, typically wired with 12 AWG copper. Never plug two heaters into the same circuit.
What is the 125% continuous load rule?
The National Electrical Code (NEC) says a circuit that runs at full load for 3 hours or more must be sized at 125% of the load current. Put another way, a breaker should only carry 80% of its rating continuously: a 20A breaker is rated for 16A of continuous load. This calculator applies the 125% rule automatically when you select a continuous load.
Can I just install a bigger breaker to stop it tripping?
No. The breaker protects the wire, not just the appliance. A 20A breaker on 14 AWG wire rated for 15A will let the wire overheat before it trips, which is a fire hazard. If a breaker trips, find out why - the circuit is overloaded, shorted, or the breaker itself is failing - and have an electrician fix it.
Does the same math work for 240V circuits?
Yes. Amps = watts divided by volts works for any voltage: a 4500W water heater on 240V draws 18.75A, and as a continuous load needs 23.4A, so a 25A breaker. Just select 240V in the calculator and the math is identical.
What wire gauge goes with each breaker size?
As general guidance: 15A breakers pair with 14 AWG copper wire, 20A with 12 AWG, and 30A with 10 AWG. Wire sizing also depends on wire length, insulation type, and how many conductors share a conduit, so treat this as a starting point and confirm with a licensed electrician or your local code.