The watts to amps formulas
DC: I = P ÷ V
AC single phase: I = P ÷ (V × PF)
AC three phase: I = P ÷ (1.732 × V × PF)
I is the current in amps, P is power in watts, V is the voltage and PF is the power factor. In three-phase systems V is the line-to-line voltage, such as 208 V or 480 V.
Step by step
- Find the watts on the nameplate. If it shows VA instead, multiply by the power factor, or use the VA to watts calculator.
- Find the voltage the device runs on.
- Choose the formula for DC, single phase or three phase.
- Divide. Round up, because the breaker and the wire must handle the full current.
Worked examples
DC: a 100 W 12 V pump
100 ÷ 12 = 8.33 A. Add a margin, because motors draw more when they start.
Single phase: a 2,400 W heater on 240 V
2,400 ÷ 240 = 10 A. On 120 V the same heater would draw 20 A, which is why high-power loads use 240 V.
Single phase with power factor: a 1,200 W motor at PF 0.8 on 120 V
1,200 ÷ (120 × 0.8) = 12.5 A, not 10 A. Ignoring the power factor would undersize the breaker and wire.
Three phase: a 10,000 W load on 208 V at PF 0.9
10,000 ÷ (1.732 × 208 × 0.9) = 30.8 A per phase.
Why amps matter more than watts
Wires, breakers and outlets are rated in amps, not watts. A 15 A circuit on 120 V is 1,800 W, but a continuous load should stay at or below 80% of that, which is 1,440 W. A 20 A circuit gives 2,400 W, or 1,920 W continuous. Once you know the amps, the wire size calculator gives the wire and the breaker to wire size chart gives the matching breaker.
Common mistakes
- Using the wrong voltage. The same watts at 240 V draw half the amps of 120 V.
- Ignoring power factor on motors, compressors and some LED drivers.
- Forgetting startup surge. Motors can draw several times the running current for a moment. Size breakers and generators with that in mind.
- Mixing up VA and watts. A 1,000 VA UPS at 0.6 PF delivers 600 W.
For a ready-made table of common values, see the watts to amps chart.
Frequently asked questions
What is the formula to convert watts to amps?
Amps = watts ÷ volts for DC. For AC single phase divide by volts times power factor, and for three phase by 1.732 times volts times power factor.
How many amps is 1,500 watts?
12.5 amps at 120 V and 6.25 amps at 240 V.
Do I need power factor to convert watts to amps?
Only for AC loads with a power factor below 1, such as motors. Heaters, incandescent lights and DC loads have a power factor of 1.
How do I convert watts to amps at 12 volts?
Divide the watts by 12. 60 W is 5 A and 240 W is 20 A.