Convert between power (watts) and current (amps) for single- and three-phase circuits, with the formula and full working shown.
Convert between power (watts) and current (amps) for single- and three-phase circuits at any voltage and power factor.
Power, voltage and current are linked by the formula P = V × I × PF. Rearranged, current is I = P / (V × PF) for a single-phase circuit, or I = P / (√3 × V × PF) for three-phase. For resistive loads such as heaters and kettles the power factor is 1.0, so at the UK standard 230V a 3kW load draws about 13A — right at the limit of a standard 13A plug.
Knowing the current lets you choose the right protective device and cable size. Once you have the figure, use the cable size calculator to complete the design to BS 7671.
Max Zs values, diversity factors, cable ratings, voltage drop — one printable page. Plus occasional emails with calculator updates and useful tips.