Watts to Amps Calculator
Convert watts to amps or amps to watts for DC, single-phase and three-phase AC circuits, with the power factor.
System
Find
kW
A
V
—
The formulas
| Circuit | Amps from watts | Watts from amps |
|---|---|---|
| DC | I = P / V | P = V × I |
| AC single-phase | I = P / (V × PF) | P = V × I × PF |
| AC three-phase | I = P / (√3 × V × PF) | P = √3 × V × I × PF |
Examples
- A 1,800 W hair dryer on 120 V: 1,800 / 120 = 15 A – the whole capacity of a 15 A circuit.
- A 11 kW three-phase motor on 480 V with PF 0.85: 11,000 / (1.732 × 480 × 0.85) = 15.6 A.
- A 12 V, 5 A DC device: 12 × 5 = 60 W.
Power factor
In AC circuits with motors or electronics, current and voltage are not perfectly in step. The power factor (cos φ) is the share of the current that does useful work. Heaters are 1; motors 0.8–0.9. Apparent power, in kVA, is what the wiring and breakers must carry.
Which voltage for three-phase?
Use the line-to-line voltage: 208 V or 480 V in US commercial buildings, 400 V in Europe. When you switch the system, the voltage changes to the usual value as long as you have not typed your own.
Frequently asked questions
How many amps is 1,500 watts at 120 volts?
12.5 A.
How many watts can a 20 A, 120 V circuit carry?
2,400 W at most, and 1,920 W for a continuous load (80%).