Operating line current and apparent power
Current is an operating estimate for the entered real input, voltage, and power factor. Three-phase output is line current, not the sum of three line currents. It does not include start-up inrush or select a breaker or conductor rating.
For the resistance of a separately specified wire length and material, the Wire Resistance Calculator provides a conductor calculation without selecting a circuit rating.
The formula
The power input is electrical real power, so no motor efficiency correction is applied. The three-phase equation uses total real power and line-to-line voltage for a balanced supply. Single-phase voltage is measured across the load.
Worked example
A balanced three-phase load using 10 kW at 400 V line-to-line and 0.8 power factor draws 10,000/(√3 × 400 × 0.8) = 18.042 A per line. Apparent power is 12.5 kVA. The same real power at 250 V single phase draws 50 A.
How to use this calculator
- Enter supply type, real electrical input power using the units shown.
- Set rms voltage, power factor.
- Calculate and review operating line current alongside the supporting quantities.
Use electrical input power
Mechanical output ratings on motors require efficiency conversion before they become electrical input power. Enter measured or specified electrical real kW here. If the nameplate supplies current directly, its rating can be more useful than combining assumptions. Power factor is separate from energy efficiency and should describe the same operating point as the power input.
Balanced three-phase scope
The √3 relation assumes a balanced system. Unequal phase loading, harmonics, or unusual supply waveforms may need phase-specific measurements. Use line-to-line RMS voltage for the three-phase mode; entering line-to-neutral voltage changes the result incorrectly. Circuit protection, wiring capacity, starting current, and local installation requirements require separate electrical design checks.
Assumptions & limitations
What this calculation assumes
- Three-phase loads are balanced; real power and power factor describe the same operating point.
What to keep in mind
- Excludes motor efficiency conversion, phase imbalance, harmonics, inrush, and wiring design.
Common questions
Which voltage do I enter for three phase?
Use line-to-line RMS voltage. The displayed current is the current in each supply line.
Can I use a motor output kW rating directly?
No. Convert mechanical output to electrical input using its efficiency, or use a specified electrical input rating.
Is apparent kVA the same as real kW?
They match only at unity power factor. Apparent power equals real power divided by the entered power factor.
Sources & further reading
Reviewed on