Appliance labels list watts, but breakers, wires and extension cords are rated in amps. Converting between them is simple for a heater on a household outlet and less obvious for a motor on a three-phase supply. This calculator handles DC, single-phase and three-phase power, accounts for power factor and motor efficiency, and gives the 125% figure that electricians use for continuous loads, along with the current at other common voltages.
How to use the watts to amps calculator
- Choose the current type: DC, AC single-phase or AC three-phase.
- Enter the power in W, kW, MW or hp.
- Enter the voltage. For three-phase, say whether it is line-to-line (208, 480 V) or line-to-neutral (120, 277 V).
- For AC, enter the power factor — 1 for resistive loads, roughly 0.8 to 0.9 for motors.
- For a motor rated by its output, enter its efficiency so the calculator uses the electrical input power.
Watts to amps formulas
With line-to-neutral voltage, the three-phase factor becomes 3 instead of √3. The √3 (about 1.732) appears because the voltage between two lines of a balanced three-phase system is √3 times the voltage of each phase.
Worked examples
A 1,500 W heater on 120 V (the default)
I = 1,500 W ÷ (120 V × 1) = 12.5 A. For a continuous load, the circuit must handle 125% × 12.5 = 15.63 A — more than a 15 A circuit allows, so a heater running for hours belongs on a 20 A circuit.
A three-phase motor. A 10 hp motor that is 90% efficient with a power factor of 0.85 on 480 V: output 10 × 745.7 = 7,457 W, input 7,457 ÷ 0.9 = 8,286 W, and I = 8,286 ÷ (1.732 × 480 × 0.85) = 11.72 A per line. The same load at 208 V would draw 27.06 A.
Typical current at 120 V and 240 V
| Load | Watts | Amps at 120 V | Amps at 240 V |
|---|---|---|---|
| LED bulb | 10 | 0.08 | 0.04 |
| Laptop charger | 90 | 0.75 | 0.38 |
| Microwave oven | 1,200 | 10 | 5 |
| Hair dryer, space heater | 1,500 | 12.5 | 6.25 |
| Electric dryer | 5,000 | — | 20.8 |
| Level 2 EV charger | 9,600 | — | 40 |
These are resistive or near-unity power factor loads; nameplate ratings of real devices vary.
Things the formula does not tell you
The result is the steady running current. Motors and compressors draw several times that for a moment when they start, and power supplies have a brief inrush when switched on, which is why breakers have time-delay curves. Nameplate wattage is usually a maximum, so real average consumption is often lower — the electricity cost calculator estimates what a device costs to run. For the reverse conversion, use the amps to watts calculator; for transformers and generators rated in kVA, the kVA calculator. Once you know the current, the wire gauge calculator shows what size conductor carries it.
Calculations are for estimation only. Circuit, breaker and wire sizes must follow your local electrical code; have a licensed electrician design and install circuits.
Frequently asked questions
How many amps is 1,500 watts?
At 120 V, 1,500 W ÷ 120 V = 12.5 A. The same heater on 240 V draws 6.25 A. A 1,500 W space heater therefore uses most of a 15 A circuit, and because heaters run for hours, it should not share that circuit with other large loads.
What is the formula for watts to amps?
DC: I = P ÷ V. Single-phase AC: I = P ÷ (V × PF). Three-phase AC: I = P ÷ (√3 × V × PF), using the line-to-line voltage. PF is the power factor, which is 1 for heaters and incandescent lamps.
Why do I need the power factor?
Motors, transformers and many electronic power supplies draw current that is not fully in step with the voltage. That extra current does no useful work but still flows in the wires and breaker. A load with a power factor of 0.8 draws 25% more current than its watts alone suggest.
How do I convert horsepower to amps?
Convert horsepower to watts (1 hp = 745.7 W), divide by the motor's efficiency to get the electrical input, then use the formula for your system. A 10 hp, 90% efficient motor on 480 V three-phase at PF 0.85 draws about 11.7 A. For sizing conductors, the NEC uses its full-load current tables instead of nameplate math.
What does the 125% figure mean?
The NEC treats a load that runs for three hours or more as continuous and requires the circuit to be rated for at least 125% of its current. Put another way, a continuous load should use no more than 80% of a breaker's rating.