Breaker Size Calculator

Start with the load, not the breaker. The calculator follows the same three steps an electrician uses, all from the National Electrical Code:

1. Find the load current in amps. Read it off the nameplate, or convert watts to amps (watts ÷ volts). A 1,500 W heater on 120 V draws 12.5 A.
2. Apply the 125% rule if the load is continuous. A load that runs three hours or more is sized at 125% per NEC 210.20(A), so 12.5 A becomes 15.6 A.
3. Round up to the next standard breaker, then match the wire. NEC 240.6(A) lists the standard sizes (15, 20, 25, 30, 35, 40, 45, 50 A and up), so 15.6 A rounds up to a 20 A breaker on 12 AWG copper.

The breaker protects the wire, so the conductor always comes from the breaker rating, never from the load directly. The next two sections show the exact wire pairings and why continuous loads change the answer.

Once you know the breaker, the minimum copper wire follows from NEC Table 310.16, and the column you read depends on the wiring and the terminations. NM-B cable (Romex) is held to the 60 °C column: Southwire's NM-B specification prints "ampacity limited to that for 60°C conductors" and lists the cable from 14 AWG to 2 AWG. THHN/THWN conductors in conduit read the 60 °C column too on a circuit of 100 A or less unless the breaker and lugs are marked 75 °C; above 100 A, or with that marking, they read the 75 °C column (NEC 110.14(C), as Schneider Electric's terminations bulletin prints it). Cited to NEC 334.80. The code text itself is not freely available to link; confirm the figure against the current edition or your authority having jurisdiction before acting. The 60 °C and 75 °C columns part at 45 A, so the table gives the NM-B size and both conduit readings:

*NM-B cable is not made in the size the 60 °C column calls for at these breakers (1 AWG and larger): large feeders and services use THHN or SER and should be sized by an electrician. Southwire's NM-B table lists no 3 AWG; the next size it lists is 2 AWG, rated 95 A at 60 °C, so the NM-B column shows 2 AWG at 80 A. For conduit wire this page reads the terminal rule by the circuit's rating (the 60 °C column at 100 A or less, the 75 °C column above); the wire distance and inverter cable calculators read it by the conductor's size. A long run can also force a thicker wire to hold voltage drop in check, which our guide to voltage drop explains. These figures assume copper, no more than three current-carrying conductors, and a 30 °C ambient.

A breaker cannot carry its full rating around the clock. NEC 210.20(A) requires it to be rated for 125% of a continuous load, which is the same as saying a standard breaker only carries 80% of its rating for anything that runs three hours or more. That is why a 1,500 W heater at 12.5 A needs a 20 A breaker, not a 15 A one: 12 A is all a 15 A breaker will hold continuously. Cited to NEC 210.20(A). The code text itself is not freely available to link; confirm the figure against the current edition or your authority having jurisdiction before acting.

Set the load type to continuous above and the calculator applies the 125% factor for you. Tallying every circuit for a whole panel or service is a separate job: our electrical load calculator handles dwelling demand by the NEC method.

This tool sizes general branch circuits and feeders: receptacles, lighting, heaters, ranges, and similar resistive or mixed loads. It does not size circuits for motors, air conditioners, heat pumps, or welders. Those loads draw a heavy starting surge and follow separate NEC rules: motors size from full-load amps by the percentages in NEC 430.52, and HVAC equipment uses the minimum circuit ampacity (MCA) and maximum overcurrent protection (MOCP) printed on the nameplate. Run a motor load through the general rule and you will undersize its breaker.

An appliance's own installation instructions set its circuit. GE's dryer owner's manual (Pub. No. 49-9996, models DBL333, DCL333, DVL223, NLL113 and NVL333), for example, prints under "Electrical Connection Information for Electric Dryers" that the dryer "must be connected to an individual branch circuit", with "208V or 240V installation 30 amps" and a "30A, 240V, 3 #10 AWG minimum copper conductor power cord". For its gas dryers the same manual prints a 120V supply protected by "a 15 or 20 amp circuit breaker or time-delay fuse". Wire such a circuit to those instructions.

Treat every result as a starting estimate, not a wiring permit. The figures assume copper conductors, no more than three current-carrying conductors in a run, and a 30 °C ambient; heat, conduit fill, and bundling all cut ampacity and can push you to a larger wire. Size and install real circuits with a licensed electrician, and confirm everything against the current NEC and your local Authority Having Jurisdiction (AHJ) before energizing.