Watts tell you how much power a load uses. Amps tell you how much current it draws, and that is the figure that sizes a breaker and a cable. The conversion depends on the voltage, and on AC it also depends on the power factor, so the same wattage draws very different currents on different supplies.
Formulas
AC single-phase: I = P / (V × PF)
AC three-phase: I = P / (√3 × VLL × PF)
I is current in amps, P is real power in watts, V is voltage and PF is the power factor from 0 to 1. For three-phase, VLL is the line-to-line voltage and I is the current in each line, assuming a balanced load. If your power is in kilowatts, use the kW to amps calculator, which is the same formula with the power entered in kW.
Worked examples
A 1,500 W heater on a 120 V single-phase supply, power factor 1:
I = 1,500 / 120 = 12.5 A. On 240 V the same heater draws 6.25 A.
A 3,000 W single-phase motor load on 240 V at a power factor of 0.9:
I = 3,000 / (240 × 0.9) = 13.89 A.
A 10,000 W three-phase load on 400 V at 0.8 power factor:
I = 10,000 / (1.732 × 400 × 0.8) = 18.04 A.
Current for common loads
At a power factor of 1, in amps:
| Power | 120 V | 208 V | 230 V | 240 V | 277 V |
|---|---|---|---|---|---|
| 1,000 W | 8.33 | 4.81 | 4.35 | 4.17 | 3.61 |
| 1,500 W | 12.50 | 7.21 | 6.52 | 6.25 | 5.42 |
| 2,000 W | 16.67 | 9.62 | 8.70 | 8.33 | 7.22 |
On a 12 V DC system, 100 W draws 8.33 A. At 24 V it draws 4.17 A and at 48 V 2.08 A, which is why higher-voltage DC systems need thinner cable for the same power.
From amps to a breaker
Once you have the current, the breaker and the cable follow. Under the NEC, a load that runs for three hours or more is a continuous load and the circuit must be rated for 125 % of it. The 1,500 W heater above draws 12.5 A, and 12.5 × 1.25 = 15.6 A, so a 15 A breaker is too small and the next standard size is 20 A. The breaker size calculator does this step for you.
Common mistakes
- Using the line-to-neutral voltage on three-phase. With the √3 formula, use the line-to-line voltage, such as 400 V or 208 V. Using 230 V understates the power, or overstates the current, by a factor of 1.732.
- Ignoring power factor. A motor load at 0.8 draws 25 % more current than the same watts at unity. Take the power factor from the nameplate or from a measurement.
- Confusing watts and volt-amps. A label that gives VA, as on many UPS units, is apparent power. The real power in watts is lower by the power factor.
- Sizing from the running current alone. Motors and compressors draw several times their running current for a moment at start, so a breaker or a generator sized to the running figure may trip.
Questions
How many amps is 1,000 watts?
At 120 V it is 8.33 A. At 230 V it is 4.35 A, and on a 12 V DC supply it is 83.3 A. The answer always depends on the voltage.
How many watts can a 15 A, 120 V circuit carry?
15 × 120 = 1,800 W at most. For a continuous load the practical limit is 80 % of that, 1,440 W, because the breaker must be 125 % of a continuous load.
Do watts to amps and kW to amps give the same answer?
Yes, if you enter the same power. 1,000 W is 1 kW, so both calculators agree.
Why does the power factor not appear for DC?
DC has no phase shift between voltage and current, so the power is simply volts times amps.
Related calculators
Found a wrong value or formula? Report an error. Confirmed fixes are listed on the corrections log.