To convert watts to amps, divide the watts by the volts: amps = watts ÷ volts. A 1,500 watt heater on 120 volts draws 12.5 amps; the same wattage at 240 volts draws 6.25 amps. Motors and three-phase circuits need two more numbers, the power factor and (for three-phase) √3, and the calculator handles both.
How the amp calculator works
HyperPhysics gives the base relationship: power equals voltage times current, P = V × I. In an AC circuit the average power is V × I × cos φ, where cos φ is the power factor. The U.S. Department of Energy's motor fact sheet writes three-phase power as kW = V × I × PF × √3 ÷ 1,000, with V the line-to-line voltage. Turned around for current:
Single-phase: I = W ÷ (V × PF); three-phase: I = W ÷ (1.732 × V × PF); DC: I = W ÷ V
For kVA, which already includes the power factor, the formulas drop PF: I = kVA × 1,000 ÷ V, or ÷ (1.732 × V) on three-phase.
Worked example: 1,500 W at 120 V
The defaults are a space heater, a resistive load with a power factor of 1:
- Current: 1,500 W ÷ (120 V × 1) = 12.5 A
- Breaker: the next standard size (240.6(A)) above 12.5 A is 15 A. If it runs for three hours or more it is a continuous load, sized at 125%: 12.5 × 1.25 = 15.6 A, so a 20 A circuit.
Watts to amps at 120 V and 240 V
| Watts | Amps at 120 V | Amps at 240 V |
|---|---|---|
| 500 W | 4.17 A | 2.08 A |
| 1,000 W | 8.33 A | 4.17 A |
| 1,500 W | 12.5 A | 6.25 A |
| 1,800 W | 15 A | 7.5 A |
| 2,400 W | 20 A | 10 A |
| 3,600 W | 30 A | 15 A |
| 4,800 W | 40 A | 20 A |
| 7,200 W | 60 A | 30 A |
| 10,000 W | 83.33 A | 41.67 A |
These assume a power factor of 1 (heaters, water heaters, incandescent lights, toasters). For anything with a motor or an electronic power supply the current is somewhat higher than the table.
Three-phase amps
A three-phase circuit carries the power on three wires, so each carries less current. For a 10 horsepower motor with 90% efficiency and a 0.85 power factor on 480 V:
- Output: 10 hp × 745.7 = 7,457 W. DOE converts horsepower to input kW by dividing by the efficiency: 7,457 ÷ 0.90 = 8,286 W
- Current: 8,286 W ÷ (1.732 × 480 V × 0.85) = 11.72 A
The same motor on 208 V three-phase draws 27.1 A. Pick "Horsepower" in the power menu and set the efficiency to the nameplate figure. Motor branch circuits follow their own NEC article, so treat this as an estimate for planning, not as the size of the motor circuit.
kVA to amps
Transformers and generators are rated in kVA. A 75 kVA transformer on 208 V three-phase supplies 75,000 ÷ (1.732 × 208) = 208.2 A; at 480 V it is 90.2 A. A 25 kVA single-phase unit at 240 V gives 104.2 A. Switch to "Amps to kVA" to go the other way, for example to see what a 200 A panel at 240 V represents: 48 kVA.
Power factor and efficiency
Power factor is the share of the current that does real work. DOE describes it as the fraction of power actually delivered compared with what the same voltage and current would deliver without a phase shift, and notes it falls as a motor runs lightly loaded. Leave it at 1 for resistive loads. Efficiency matters only when the watts or horsepower you enter is a motor's shaft output: the motor draws more than it delivers.
DC amps: 12 V and 24 V
On DC there is no power factor: amps = watts ÷ volts. A 120 W inverter load on a 12 V battery draws 10 A before inverter losses; enter the inverter's efficiency to include them. Long 12 V runs lose a lot of voltage, so size the cable with the voltage drop calculator.
Next steps
Take the amps to the breaker size calculator and the wire size calculator. For a whole house, add up the loads with the electrical load calculator. If you know resistance instead of watts, use the Ohm's law calculator. All tools are on the electrical calculators page.