Current is power divided by voltage, with two corrections for AC: the power factor, which is one for heaters and lights but lower for motors, and for three-phase circuits the square root of three.
How to use this calculator
- Choose DC, single-phase AC (a household circuit) or three-phase AC.
- Choose how the power is given and enter it: watts, kilowatts, a motor's horsepower or kVA.
- Enter the voltage. For a motor, set its power factor and efficiency under More options, from the nameplate if you have it.
Worked examples
A space heater
Circuit: Single-phase AC (household) · Power is given in: Watts · Power: 1,500 · Voltage: 120 V
- Current
- 12.5 amps
- Power drawn
- 1,500 watts
- Apparent power
- 1.50 kVA
- Wire and breaker must carry, if it runs for hours
- 15.6 amps
An electric dryer
Circuit: Single-phase AC (household) · Power is given in: Watts · Power: 5,000 · Voltage: 240 V
- Current
- 20.8 amps
- Power drawn
- 5,000 watts
- Apparent power
- 5.00 kVA
- Wire and breaker must carry, if it runs for hours
- 26.0 amps
A one-horsepower pump
Circuit: Single-phase AC (household) · Power is given in: Horsepower (a motor) · Power: 1 · Voltage: 240 V · Power factor: 0.9
- Current
- 4.18 amps
- Power drawn
- 903 watts
- Apparent power
- 1.00 kVA
- Wire and breaker must carry, if it runs for hours
- 5.2 amps
A three-phase load
Circuit: Three-phase AC · Power is given in: Kilowatts · Power: 15 · Voltage: 208 V · Power factor: 0.9
- Current
- 46.3 amps
- Power drawn
- 15,000 watts
- Apparent power
- 16.67 kVA
- Wire and breaker must carry, if it runs for hours
- 57.8 amps
A car inverter
Circuit: DC (batteries, solar, cars) · Power is given in: Watts · Power: 300 · Voltage: 12 V
- Current
- 25.0 amps
- Power drawn
- 300 watts
- Wire and breaker must carry, if it runs for hours
- 31.3 amps
The formulas
- DC: amps = watts ÷ volts.
- Single-phase AC: amps = watts ÷ (volts × power factor).
- Three-phase AC: amps = watts ÷ (volts × power factor × 1.73), the square root of three, with the line-to-line voltage.
- Horsepower: one horsepower is 745.7 watts; divide by the motor's efficiency for the power it draws.
Power factor and efficiency
Heaters, incandescent bulbs and other resistive loads have a power factor of 1, so watts and volt-amps are the same. Motors draw some current that does no work: a University of Kentucky review puts refrigerators around 0.8 to 0.99 and central air conditioning around 0.9 to 0.92, and a motor's power factor drops further when it runs below about 65 percent of its load.
A motor's horsepower is what it delivers at the shaft. Federal rules require a one-horsepower capacitor-start motor to be at least 82.6 percent efficient, so it draws more than its rating; the calculator divides by the efficiency.
From amps to wire and breaker
A load that runs at full current for 3 hours or more counts as continuous, and the wire and breaker for it must carry 125 percent of its current. The wire size calculator takes it from there.
Common questions
How do I convert watts to amps?
Divide the watts by the volts for a DC or resistive load. For AC loads with motors, divide by the power factor too. A heater of fifteen hundred watts on a hundred-twenty-volt circuit draws twelve and a half amps.
How many amps does a horsepower motor draw?
Multiply the horsepower by 745.7 watts, divide by the motor's efficiency and power factor, then by the voltage. The nameplate gives the full-load amps directly.
What is the formula for three-phase amps?
Amps = watts ÷ (volts × power factor × 1.73), using the line-to-line voltage.
How do I calculate amps needed for a house?
Add up the watts of everything that can run at once and divide by the service voltage. For sizing a service, an electrician uses the code's load calculation, which allows for not everything running together.