Three-Phase Power Calculator guide
Three-Phase Power Calculator: kW, kVA and Amps
Use our three-phase power calculator to convert between amps, kW, kVA and kVAr on UK 400V supplies, with the power factor and efficiency explained.
By the AllCalculator editorial teamUpdated 15 October 20262 min read
Three-phase supplies power most commercial buildings, workshops and larger equipment such as motors, heat pumps and EV chargers. Our Three-Phase Power Calculator converts between current and power so you can size supplies, cables and protective devices.
How to use the calculator (Three-Phase Power Calculator)
- Choose power from current or current from power.
- Enter the line voltage, 400V for UK three-phase supplies.
- Enter the current or the power in kW.
- Enter the power factor and, for a motor, its efficiency.
The formulas
- Real power: P = √3 × V × I × power factor
- Apparent power: S = √3 × V × I
- Reactive power: Q = √(S² − P²)
V is the line-to-line voltage and I is the line current.
Real, apparent and reactive power
- kW (real power) does the useful work.
- kVA (apparent power) is what the supply has to carry.
- kVAr (reactive power) builds the magnetic fields in motors and transformers.
The power factor is kW ÷ kVA. A low power factor means more current for the same useful power.
A worked example
With the calculator's default settings, a load drawing 20A at 400V with a power factor of 0.85:
- Real power: √3 × 400 × 20 × 0.85 ≈ 11.78kW
- Apparent power: √3 × 400 × 20 ≈ 13.86kVA
Going the other way, an 11kW load at 0.85 power factor draws about 18.7A.
For cable runs, check the Voltage Drop Calculator.
Frequently asked questions
What is UK three-phase voltage?
400V between lines and 230V between each line and neutral.
How many amps is 22kW three phase?
At 400V and a power factor of 1, about 31.8A per phase. That's why 22kW EV chargers need a 32A three-phase supply.
Why include efficiency?
For a motor, the power rating is usually the output. The calculator divides by efficiency to find the input power and current.
Use the Three-Phase Power Calculator to check loads before planning a supply.
Sources and further reading
- IET: BS 7671 Wiring Regulationselectrical.theiet.org
- Energy Networks Association: Connecting to the networkenergynetworks.org
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.