Choose what you want to find, then enter the line-to-line voltage and either the current or the power. The calculator gives real, apparent and reactive power and the line current.
Result
Real power
kW
Apparent power
kVA
Reactive power
kVAR
Line current
A
Three-phase power formulas
In a balanced three-phase system the power is the line voltage times the line current times √3, multiplied by the power factor for real power:
Real power: P = √3 · V · I · PF
Apparent power: S = √3 · V · I
Reactive power: Q = √(S² − P²)
Current: I = P ÷ (√3 · V · PF)
V
line-to-line voltage in V
I
line current in A
PF
power factor, cos φ
√3
1.732
Worked example
A motor draws 50 A at 480 V with a power factor of 0.85. Apparent power is 1.732 × 480 × 50 = 41.6 kVA. Real power is 41.6 × 0.85 = 35.3 kW, and reactive power is √(41.6² − 35.3²) = 21.9 kVAR.
Full-load current per kW
Line voltage
PF 1.0
PF 0.85
208 V
2.78 A per kW
3.27 A per kW
400 V
1.44 A per kW
1.70 A per kW
415 V
1.39 A per kW
1.64 A per kW
480 V
1.20 A per kW
1.42 A per kW
600 V
0.96 A per kW
1.13 A per kW
Electrical input power. For a motor, divide the rated shaft power by its efficiency first, or use the full-load current on the nameplate.
Single-phase or three-phase?
For the same power, a three-phase supply needs less current per conductor than single-phase, so cables can be smaller. That is why motors above a few kW, large heaters and EV fast chargers run on three-phase. To size the cable for the current you found, use the cable size calculator.
Questions and answers
How do you calculate three-phase power?
Real power P = √3 × V × I × PF, where V is the line-to-line voltage, I the line current and PF the power factor. Divide by 1,000 for kW. Apparent power S = √3 × V × I, reactive power Q = √(S² − P²).
How do I convert kW to amps for three-phase?
I = P × 1,000 ÷ (√3 × V × PF). For 30 kW at 480 V and power factor 0.85 that is 30,000 ÷ (1.732 × 480 × 0.85) = 42.5 A.
What is the difference between kW and kVA?
kVA is the apparent power the supply has to deliver; kW is the part that does useful work. Their ratio is the power factor. Generators, transformers and UPS units are rated in kVA, motors and heaters in kW.
Should I use line-to-line or line-to-neutral voltage?
Use line-to-line voltage (208, 400, 480 V) with the √3 formula. If you only know the phase voltage (120, 230, 277 V), total power is 3 × V(phase) × I × PF, which gives the same result.