Loads
Power & Current Calculator
Convert between kW, kVA and amps for single-phase and three-phase loads. Enter either the power or the current and the rest follows from the voltage and power factor.
Last reviewed
Current
-A
Real power
-
Apparent power
-
Reactive power
-
The formulae
For a single-phase load:
I = P ÷ (V × pf) · S(kVA) = V × I ÷ 1000
For a balanced three-phase load using the line voltage:
I = P ÷ (√3 × VL × pf) · S(kVA) = √3 × VL × I ÷ 1000
Real power (kW) is the apparent power (kVA) multiplied by the power factor. Reactive power (kVAr) is the apparent power multiplied by the sine of the phase angle. With a power factor of 1, kW and kVA are equal and reactive power is zero.
A note on voltage
For three-phase, enter the line-to-line voltage (400 V on a standard UK supply). The calculator uses the √3 relationship, so you do not need to convert to phase voltage yourself.
Common questions
How do I convert kW to amps on three phase?
Divide the power in watts by the square root of three times the line voltage times the power factor. On 400 V that is watts ÷ (1.732 × 400 × pf).
What power factor should I assume?
Resistive loads such as heaters are close to 1. Motors and many electronic loads are lower, often 0.8 to 0.95. Use the figure from the equipment data where you have it.
Is this for balanced loads?
The three-phase calculation assumes a balanced load across all three phases, which is the usual design assumption. Unbalanced loads need to be assessed per phase.
For guidance only. These conversions support load assessment and are not a full design. Confirm equipment ratings, diversity and the supply arrangement before sizing protection or cables.