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)