Amps to Watts Calculator

Multiply amps by volts. On a DC circuit that is the whole calculation: 10 A at 12 V is 120 W. On an AC circuit you multiply by the power factor as well, because amps times volts gives apparent power in volt-amperes, and watts is only the part of it doing work. On a three-phase circuit you also multiply by the square root of three, about 1.732, using the line-to-line voltage. The tool applies whichever formula matches the system you select and shows the arithmetic it used.

At 120 volts, 2,400 watts for a resistive load with a power factor of 1. At 240 volts, 4,800 watts. But if the 20 amps is a breaker rating rather than a load, 2,400 watts is not what the circuit can run all evening: a load that continues for three hours or more is limited to 80 percent of the rating, which is 16 amps or 1,920 watts at 120 volts. And on a 20 amp circuit that serves several outlets, NEC 210.23(A)(1) limits any single plug-in appliance to 16 amps as well.

1,800 watts at 120 volts is the nominal figure — 15 amps times 120 volts. A load that runs for three hours or more is limited to 80 percent, which is 12 amps or 1,440 watts. A single cord-and-plug appliance on a multi-outlet 15 amp circuit is also limited to 12 amps under NEC 210.23(A)(1), which is why a 1,500 watt space heater, drawing 12.5 amps, is already over the line on a 15 amp circuit before anything else is plugged in.

Because on an AC circuit with a motor or compressor, current and voltage fall out of step, and part of the current flowing does no useful work. Amps times volts measures everything flowing, which is apparent power in volt-amperes. Watts counts only the working part. So 10 amps at 120 volts is 1,200 VA regardless, but at a power factor of 0.85 it is only 1,020 watts. The conductor and the breaker carry the full 10 amps either way, which is why a circuit is limited in amps rather than in watts.

NEC 210.19(A)(1) and 210.20(A) require the conductor and the overcurrent device serving a continuous load — one where the maximum current is expected to continue for three hours or more — to be rated at not less than 125 percent of that load. Read from the breaker's side, that is the same rule: a continuous load may use 80 percent of the rating, because 80 percent is the inverse of 125. The exception is an assembly listed for operation at 100 percent of its rating, which is uncommon in residential panels.

Two things, on a branch circuit that serves more than one outlet. Under 210.23(A)(1), the rating of any one cord-and-plug-connected appliance not fastened in place may not exceed 80 percent of the branch-circuit rating — 12 amps on a 15 amp circuit, 16 amps on a 20 amp circuit. Under 210.23(A)(2), the total of utilization equipment fastened in place, other than luminaires, may not exceed 50 percent of the rating where lighting or plug-in equipment is also supplied — 7.5 amps and 10 amps respectively. That second limit is why a permanently installed appliance usually ends up on a circuit of its own.

No. It reproduces the conversion arithmetic and the continuous-load and permissible-load provisions of the NEC as a preparation aid. A circuit's capacity is its breaker rating only if the conductor behind it was sized for that breaker, which this tool cannot see. Jurisdictions adopt different NEC editions and amend them, and equipment listings govern. Confirm against the edition your building department has adopted.