Circuits
Watts, Amps and Volts Calculator
Convert watts to amps or back, single or three phase, and get the breaker size including the continuous-load multiplier.
Use 1.0 for resistive loads — heaters, incandescent lamps. Motors run 0.8-0.9.
Continuous loads are sized at 125% per NEC 210.19(A)(1). Enter 1 for yes, 0 for no.
Reference data and planning arithmetic. Does not apply ambient or bundling derating, motor rules, or local amendments. Not a substitute for the NEC edition your jurisdiction has adopted, an inspection, or a licensed electrician.
The formulas
Single phase: P = V × I × PF
Three phase: P = √3 × V × I × PF
Rearranged for current, which is what you usually want: amps = watts ÷ (volts × power factor) for single phase, or watts ÷ (√3 × volts × power factor) for three phase.
Power factor, and why the wire does not care about watts
Power factor is the ratio of real power (watts, what does work) to apparent power (volt-amps, what flows). For resistive loads — heaters, incandescent lamps, kettles — it is 1.0 and the two are identical. Motors typically run 0.8 to 0.9. Older fluorescent ballasts and some switch-mode supplies can be lower.
The practical consequence: conductors and breakers are sized on amps, and amps follow VA, not watts. A 1,000 W motor at 0.8 power factor draws the same current as a 1,250 W heater. Size the circuit on the current, always.
| Load | Watts | Volts | Amps | Continuous? Breaker |
|---|---|---|---|---|
| LED shop light | 40 W | 120 V | 0.3 A | yes, 15 A |
| Refrigerator (running) | 150 W | 120 V | 1.3 A | no, 15 A |
| Microwave | 1,100 W | 120 V | 9.2 A | no, 15 A |
| Toaster / kettle | 1,500 W | 120 V | 12.5 A | no, 15 A |
| Space heater | 1,500 W | 120 V | 12.5 A | yes, 20 A |
| Window air conditioner | 1,200 W | 120 V | 11.1 A | no, 15 A |
| Table saw (running) | 1,800 W | 120 V | 17.6 A | no, 20 A |
| Electric dryer | 5,000 W | 240 V | 20.8 A | no, 25 A |
| Electric range | 8,000 W | 240 V | 33.3 A | no, 40 A |
| EV charger, level 2 | 7,680 W | 240 V | 32.0 A | yes, 40 A |
The 125% continuous-load rule
A continuous load is one expected to run for three hours or more — shop lighting, a commercial freezer, an EV charger. NEC 210.19(A)(1) and 215.2(A)(1) require the branch circuit to be sized at 125% of the continuous load, so a 16 A continuous load needs a 20 A circuit rather than a 20 A load's worth of headroom.
The reason is thermal: breakers and terminations are tested for a limited duration at their rating, and sustained operation at 100% runs them hotter than the test conditions allow.
Where it bites in practice is EV charging. A 40 A continuous charger needs 40 × 1.25 = 50 A of circuit, which is 8 AWG copper and a 50 A breaker — not the 8 AWG-on-40 A that a straight ampacity lookup suggests.
Common questions
How many amps is 1,500 watts at 120 volts?
12.5 A at power factor 1.0. If that load runs three hours or more it is continuous, so the circuit is sized at 15.6 A and needs a 20 A breaker.
How do I convert watts to amps for three phase?
Divide the watts by √3 times the voltage times the power factor. For 1,500 W at 208 V three phase with unity power factor, that is 4.16 A.
What is power factor and does it matter?
The ratio of real power to apparent power. It is 1.0 for resistive loads and 0.8-0.9 for motors. It matters because conductors carry current, and current follows volt-amps rather than watts — so a low power factor means more current for the same useful output.
What is a continuous load?
One expected to run for three hours or more. The branch circuit must be sized at 125% of it per NEC 210.19(A)(1). A 40 A EV charger therefore needs a 50 A circuit.