What people use it for
- Sizing a circuit Pick a breaker and a cable that match the load.
- Motors The power factor is what makes a motor pull more current than the label says.
- Inverters and generators Check that the supply covers the load with room to spare.
The formulas
Single phase: amps equal kilowatts times 1,000 divided by volts divided by power factor. Three phase: amps equal kilowatts times 1,000 divided by the square root of three divided by volts divided by power factor. The power factor only matters for motors, transformers and other inductive loads; resistive loads like heaters sit at 1.00.
The result is the running current. Motors draw several times that for a moment on start up, so a breaker is normally sized above the running figure and the circuit is sized for the starting current.
| Load | Supply | Current |
|---|---|---|
| 1 kW | 120 V single phase | 8.3 A |
| 7.5 kW | 240 V single phase | 34.7 A |
| 7.5 kW | 400 V three phase, pf 0.9 | 12.0 A |
| 22 kW | 400 V three phase, pf 0.9 | 35.3 A |