Watts to Amps Calculator

Divide watts by volts. On a DC circuit that is the whole calculation: a 1,200 W load at 12 V draws 100 A. On an AC circuit you divide by the power factor as well, because watts measures only the part of the power doing work. On a three-phase AC circuit you divide by the square root of three (about 1.732) in addition to the voltage and the power factor. The tool applies whichever of the three formulas matches the system you select, and shows the arithmetic it used.

Because a conductor carries current, not watts. On an AC circuit with an inductive load — a motor, a compressor, an air handler — current and voltage fall out of step, so some of the current flowing does no useful work. Watts counts only the useful part. The quantity the conductor and the breaker actually see is apparent power, measured in volt-amperes, and it equals watts divided by power factor. A 1,000 W motor at 0.85 power factor draws the same current as a 1,176 VA resistive load, which is why a wattage-only conversion under-reports a motor circuit by roughly 18 percent.

A continuous load is one where the maximum current is expected to continue for three hours or more. NEC 210.19(A)(1) requires the branch-circuit conductor to have an ampacity of not less than 125 percent of that load, and 210.20(A) requires the overcurrent device to be rated not less than 125 percent of it. The factor exists because overcurrent devices are thermal devices calibrated in open air, and a breaker sitting in a loaded panel for hours behaves differently from one tested on a bench. EV charging, PV output, electric heat and a water heater recovering from cold are the common residential cases.

Because standard overcurrent devices only come in the ratings listed in NEC 240.6(A) — 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 and upward — so the design current is rounded up to the smallest standard rating that is not less than it. The tool also reports the same relationship from the other direction: a device may carry 80 percent of its rating continuously, which is the arithmetic inverse of the 125 percent factor. If the load you entered is above that figure, the circuit is undersized even though the breaker will hold.

No, and it deliberately does not try. Conductor ampacity depends on the conductor material, the insulation temperature rating, the rating of the terminals at both ends, how many current-carrying conductors share the raceway and the ambient temperature — none of which this tool asks for. Reporting a wire gauge from a wattage and a voltage alone would be a figure that looks authoritative and is not. The overcurrent device shown here is the starting point for that calculation, not a replacement for it.

Use it to sanity-check a motor, not to size its circuit. Branch-circuit conductors and short-circuit protection for a motor are taken from the tables in NEC Article 430, using the full-load current values in those tables rather than the motor's nameplate wattage, and the permitted overcurrent device can be far larger than the running current because it has to let the motor start. A conversion from nameplate watts will give you an approximate running current and will not give you a compliant motor branch circuit.

No. It reproduces the conversion arithmetic and the continuous-load and standard-rating provisions of the NEC as a preparation aid. Jurisdictions adopt different NEC editions and amend them, the equipment's own listing and instructions govern, and the 125 percent factor does not apply where the assembly containing the overcurrent device is listed for operation at 100 percent of its rating. Confirm against the edition your building department has adopted.