Wattage to Amperage Calculator
Need to know how many amps a device draws? Divide watts by volts. A 1,500-watt space heater on a 120V circuit draws 12.5 amps. This calculator handles DC, single-phase AC, and three-phase AC, and it suggests the right breaker size based on the NEC 80% continuous load rule.
The formulas
- DC: I = P / V
- Single-phase AC: I = P / (V × PF)
- Three-phase AC: I = P / (√3 × V × PF)
Where I is current in amps, P is power in watts, V is voltage, and PF is the power factor.
Why power factor matters for AC
AC motors and other inductive loads do not use all the current they draw to do useful work. The power factor (typically 0.8 to 0.95) accounts for this. A motor rated at 1,000W with a power factor of 0.85 on 120V actually draws about 9.8 amps, not the 8.3 amps you would expect from simple division.
The 80% breaker rule
The National Electrical Code says that for continuous loads (running 3+ hours), a circuit breaker should only be loaded to 80% of its rating. So a 20A breaker should carry no more than 16A continuously. This calculator factors that in when suggesting a breaker size.
Quick references
- 1,500W on 120V = 12.5A (needs 20A breaker)
- 3,600W on 240V = 15A (needs 20A breaker)
- 5,000W on 240V = 20.8A (needs 30A breaker)