Watts to Amps Calculator
Enter watts and voltage to get the current draw in amps. The calculator handles DC, single-phase AC, and both three-phase conventions, then applies the NEC 125% rule for continuous loads so you get a usable breaker size rather than a bare number.
The formula depends on the system
Watts, volts, and amps are related by P = V × I, so amps = watts ÷ volts. That is exact for DC — batteries, solar panels, automotive circuits.
AC adds power factor. Real power (watts) and apparent power (volt-amps) diverge whenever the load is not purely resistive, so the formula becomes amps = watts ÷ (volts × PF). A 1,000 W motor at 0.85 power factor pulls more current than a 1,000 W space heater, because the supply has to deliver 1,176 VA to do 1,000 W of work.
Three phase is where most errors happen, and it is almost always a voltage confusion rather than a math error. If you are using line-to-line voltage (208 V, 480 V — the usual case), divide by √3 × V × PF. If you are using line-to-neutral voltage (120 V, 277 V), divide by 3 × V × PF. Both give the same answer when the voltages correspond, since line-to-line = √3 × line-to-neutral.
Choosing a power factor
Set it to 1.0 for anything that just makes heat or light with a filament: space heaters, water heaters, toasters, incandescent bulbs. These are purely resistive and there is no phase shift to account for.
Use 0.8–0.9 for motors — refrigerator compressors, air conditioners, pumps, power tools. Small motors run lower than large ones, and any motor runs at a worse power factor when lightly loaded than at full load.
Modern electronics with active power factor correction — LED drivers, computer supplies, EV chargers — are usually 0.95 or better. Cheap unregulated supplies can be much worse. If the nameplate lists both watts and amps, divide to find the actual power factor rather than guessing.
Reading the breaker and wire outputs
The required ampacity applies the NEC 125% rule: a load running three hours or more continuously must be sized at 1.25× the calculated current. This is why a 16 A continuous load needs a 20 A breaker rather than a 20 A breaker "with a little room."
The breaker size is the next standard rating at or above that figure, from the list in NEC 240.6(A). Breakers only come in specific sizes.
The wire size is a starting point based on copper THHN at 75°C in typical conditions. It does not account for voltage drop on long runs, ambient temperature above 30°C, or more than three current-carrying conductors in a conduit — all of which force a larger conductor. Treat it as a sanity check, not a specification.
This calculator is for estimating and planning. Permanent wiring should be designed and installed by a licensed electrician to your local code.
Frequently asked questions
How many amps is 1000 watts?
At 120 V single phase with a power factor of 1, 1,000 watts draws 8.33 amps. At 240 V the same load draws 4.17 amps, and at 12 V DC it draws 83.3 amps. The answer always depends on voltage — watts alone cannot be converted to amps.
What is power factor and do I need to set it?
Power factor is the ratio of real power (watts) to apparent power (volt-amps), and it drops below 1 whenever current and voltage fall out of phase. Leave it at 1.0 for resistive loads like heaters and incandescent bulbs. Use 0.8–0.9 for motors. If your nameplate lists both watts and amps, you can calculate the actual figure instead of estimating.
Why does three phase use the square root of 3?
In a balanced three-phase system the line-to-line voltage is √3 (about 1.732) times the line-to-neutral voltage, because the three phases are 120 degrees apart. When you calculate current from line-to-line voltage, that factor has to appear in the denominator. Using line-to-neutral voltage instead, the formula uses a plain 3.
What size breaker do I need?
For a load running three hours or more, NEC requires sizing at 125% of the calculated current, then rounding up to the next standard breaker rating. A 16 A continuous load therefore needs a 20 A breaker. This calculator applies both steps when the continuous load toggle is on.
Is the suggested wire size safe to use?
Treat it as a starting point only. It assumes copper THHN at 75°C under ordinary conditions and does not adjust for voltage drop on long runs, high ambient temperature, or conduit fill. All of those push toward a larger conductor. Permanent wiring should be designed and installed by a licensed electrician to your local code.