Volt drop is the voltage lost as current flows through the resistance of the line conductor. BS 7671 limits it to 3% of supply voltage for lighting circuits and 5% for power circuits, measured from the installation origin to the furthest point.
For single-phase circuits, current travels out via the line conductor and returns via the neutral, so the effective resistance is doubled over the cable's length: Vd = 2 × r1 × Ib × L ÷ 1000. For three-phase balanced circuits, the neutral currents cancel and only the line conductor resistance counts: Vd = r1 × Ib × L ÷ 1000 (r1 in mΩ/m, Ib in amps, L in metres, result in volts). A result within 1V of the limit is flagged as a caution — the cable passes, but the margin is too thin for comfort and the next size up is worth specifying.
Copper resistance rises with temperature, increasing volt drop proportionally. The ambient temperature factor scales r1 before it's used in the formula — colder installations (e.g. an unheated outbuilding in winter) reduce resistance and volt drop slightly, while hot environments (e.g. a loft in summer or plant room) increase it. The tool applies this correction automatically based on the selected ambient temperature.
Line conductor resistance (r1) values are for copper conductors at 20°C, covering Twin & Earth from 1.0mm² to 16mm² and Singles/SWA from 1.5mm² to 185mm². The neutral (r2) is not used in this calculation — for single-phase, the doubling in the formula already accounts for the return path, since standard T&E and SWA neutral conductors share the same cross-sectional area as the line conductor.
Excessive volt drop means equipment at the end of a long cable run receives less voltage than it needs — motors run hot and lose torque, lighting dims, and electronic equipment can misbehave or fail to start. Keeping volt drop within BS 7671's limits ensures connected equipment operates as designed across the full length of the installation.
Resistance values (Table B1) and the temperature correction factor (Table B2) used in this calculator are sourced from BS 7671:2018+A2:2022 (18th Edition Amendment 2), the IET Wiring Regulations — the current UK standard for electrical installations. This tool provides an estimate for guidance only; always verify against the full regulations and confirm with a qualified electrician before installation.