How many amps does it draw?

How many amps is 1,500 watts?

At 120 volts, 1,500 W ÷ 120 V = 12.5 amps — a typical space heater. The same 1,500 W at 240 V draws only 6.25 amps.

What is the formula for watts to amps?

For DC and for AC loads like heaters and incandescent bulbs, amps = watts ÷ volts. For other AC loads, divide by the power factor too: amps = watts ÷ (volts × PF). On a three-phase circuit, amps = watts ÷ (√3 × volts × PF), with the line-to-line voltage: 10,000 W at 208 V three-phase with a power factor of 0.9 is 10,000 ÷ (1.732 × 208 × 0.9) = 30.8 A.

How many watts can a 15 or 20 amp circuit handle?

A 15 A circuit at 120 V carries 15 × 120 = 1,800 W, and a 20 A circuit 2,400 W. For a continuous load — one that runs three hours or more — the National Electrical Code keeps you at 80% of the breaker: 1,440 W on 15 A and 1,920 W on 20 A.

What size breaker do I need for my load?

For a continuous load, the breaker must be at least 125% of the current (NEC 210.20(A)), rounded up to a standard size from NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60 A and so on. A 12.5 A heater that runs for hours needs 12.5 × 1.25 = 15.6 A, so a 20 A breaker. The wire must be rated for the breaker, too — see the wire size calculator.

What is power factor?

Power factor is the share of the current that does real work. Heaters, toasters and incandescent lamps are close to 1, so watts = volts × amps. Motors, compressors and many power supplies are lower, so they draw more amps for the same watts. The nameplate or the manual lists it; if it gives volt-amps (VA) instead, amps = VA ÷ volts.

How many amps is a 5,000 watt load at 240 volts?

20.8 amps: 5,000 ÷ 240 = 20.8. A load of that size, such as an electric dryer, is typically on a 30 amp breaker; the smallest standard breaker for 20.8 amps that does not run continuously is 25 amps. The wire must match the breaker.

How do I convert watts to amps for a 12 volt DC device?

Divide the watts by the volts, with no power factor for DC. A 100 W device on 12 V draws 8.33 A, and 8.33 A × 12 V gives back 100 W. The same 100 W on 120 V draws 0.83 A, which is why low-voltage circuits need thicker wire.

The three formulas

For direct current and for resistive AC loads, power equals voltage times current: P = V × I, so amps = watts ÷ volts. For single-phase AC loads with a power factor PF, P = V × I × PF. For balanced three-phase loads, P = √3 × V × I × PF, where V is the line-to-line voltage. You can solve each for any of the three quantities; the calculator does that, and it shows the same load at 120 V, 240 V, 208 V three-phase and 480 V three-phase so that you can see the effect of the voltage. Power factor is capped at 1, and a missing value is treated as 1.

Worked example: the same 1,500 W at different voltages

A 1,500 W space heater (power factor 1) draws 1,500 ÷ 120 = 12.5 A at 120 V and 6.25 A at 240 V. If the load were a motor with a power factor of 0.8, the 120 V current becomes 1,500 ÷ (120 × 0.8) = 15.63 A. A three-phase 10,000 W load at 208 V and PF 0.9 draws 30.8 A on each line. Solving the other way, 1,500 W at 12.5 A means 120 V. For the 12.5 A heater running three hours or more, the breaker must be rated for 125% of the current, 15.6 A, so the smallest standard breaker is 20 A; if it runs for less than three hours, 15 A is the smallest.

Continuous and non-continuous loads

The National Electrical Code treats a load that runs for three hours or more as continuous and asks for a breaker rated at least 125% of the load (NEC 210.20(A)). Equivalent: the load should not exceed 80% of the breaker rating. The standard sizes listed in NEC 240.6(A) are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 amps and so on; the calculator picks the smallest one that meets the need. The breaker protects the wire, so the conductor must have an ampacity at least equal to the breaker. Motors, heaters with fans and other equipment have their own rules; check the nameplate and the code that applies in your area.

Common mistakes

Using the volt-ampere (VA) rating of a UPS or power supply as watts overstates or understates the load, because the two differ by the power factor. Applying the single-phase formula to a three-phase load is another: the √3 changes the answer by 73%. Forgetting that startup current of motors and compressors is several times the running current is a third; the figure here is the running current. Using the breaker rating as the load, or the reverse, mixes up what the circuit can deliver and what the device draws. A calculator result does not replace a licensed electrician for work on a panel.

Sources: the power relations P = VI, P = VI·PF and P = √3·VI·PF (OpenStax University Physics, Volume 2, chapters 9 and 15 on power and AC circuits); National Electrical Code (NFPA 70) 210.20(A) for continuous loads and 240.6(A) for standard overcurrent device ratings. Related: wire size calculator, voltage drop calculator and Ohm's law calculator.