Three-Phase Power Calculator
Calculate real power (kW), apparent power (kVA) and reactive power (kVAR) of a balanced three-phase load from voltage, current and power factor, or work out the line current from a kW or kVA rating.
For motors use the rated shaft output and enter the efficiency
1 for resistive loads, 0.8-0.9 typical for motors
How to use the Three-Phase Power Calculator
- Choose what to calculate: power from current, or current from kW or kVA.
- Enter the voltage and say whether it is line-to-line or phase voltage.
- Enter the current, kW or kVA, plus efficiency for motors.
- Set the power factor and the star or delta connection.
- Read kW, kVA, kVAR and amps and check the voltage table.
Three-phase power formulas
| Quantity | Formula (balanced load) |
|---|---|
| Real power | P (kW) = √3 × VL × IL × PF ÷ 1000 |
| Apparent power | S (kVA) = √3 × VL × IL ÷ 1000 |
| Reactive power | Q (kVAR) = √(S² − P²) = S × sin φ |
| Current from kW | IL = P × 1000 ÷ (√3 × VL × PF × η) |
| Current from kVA | IL = S × 1000 ÷ (√3 × VL) |
| Star (wye) | VL = √3 × Vph, IL = Iph |
| Delta | VL = Vph, IL = √3 × Iph |
Amps per kVA at common voltages
| Line voltage | Amps per kVA | 100 kVA | Where used |
|---|---|---|---|
| 208 V | 2.776 A | 277.6 A | North American commercial (120/208 V) |
| 230 V | 2.510 A | 251.0 A | Older and some regional systems |
| 400 V | 1.443 A | 144.3 A | Europe, UK, Asia, Australia (230/400 V) |
| 415 V | 1.391 A | 139.1 A | Older UK/Australian rating |
| 480 V | 1.203 A | 120.3 A | North American industrial (277/480 V) |
| 600 V | 0.962 A | 96.2 A | Canadian industrial |
These formulas assume a balanced, sinusoidal load. For sizing cables and breakers, follow your local electrical code and the equipment nameplate, and consult a qualified electrician.
Frequently asked questions
What is the formula for three-phase power?
For a balanced load, real power P = √3 × V_L × I_L × PF, where V_L is the line-to-line voltage, I_L the line current and PF the power factor. Apparent power is S = √3 × V_L × I_L, and reactive power Q = √(S² − P²).
How do I calculate three-phase current from kW?
I = P × 1000 ÷ (√3 × V_L × PF). A 15 kW load at 400 V and PF 0.85 draws 15,000 ÷ (1.732 × 400 × 0.85) ≈ 25.5 A. For a motor rated by shaft output, divide by the efficiency too.
How do I convert kVA to amps in three-phase?
I = kVA × 1000 ÷ (√3 × V_L). 50 kVA at 208 V is about 138.8 A; at 400 V it is about 72.2 A. The power factor is not needed for this conversion.
What is the difference between line and phase voltage?
Line voltage is measured between two phase conductors; phase voltage is across one winding or load. In a star (wye) system V_line = √3 × V_phase (400 V line, 230 V phase). In delta, line and phase voltage are the same.
What is the difference between kW, kVA and kVAR?
kW is real power that does useful work, kVAR is reactive power that magnetizes motors and transformers, and kVA is the total apparent power the supply must deliver. They form a right triangle: kVA² = kW² + kVAR².
Is the formula the same for star and delta?
Yes. P = √3 × V_L × I_L × PF works for both when you use line values. Only the conversion between line and phase values differs: in star the currents are equal, in delta the voltages are equal.