Voltage Drop Calculator

The cable's round-trip resistance is R = ρ × 2L / A, where ρ is the conductor resistivity (1.72 × 10⁻⁸ Ω·m for copper, 2.82 × 10⁻⁸ Ω·m for aluminium), L is the one-way run in metres and A is the cross-sectional area. The drop is then Vdrop = I × R. The length doubles because current flows out along one conductor and back along the other.

The wiring regulations' convention for an installation supplied directly from the public network is 3% for lighting circuits and 5% for other uses, measured from the origin of the installation to the load. On a 230 V supply that is about 6.9 V for lighting and 11.5 V for power. These are the standard design checks an electrician applies when sizing a circuit; longer runs often need a larger conductor purely to stay inside them.

Excessive drop means equipment at the end of long runs receives less than its rated voltage: lighting dims, motors run hot and draw more current, and sensitive equipment can misbehave. The lost voltage is also dissipated as heat in the cable itself, P = Vdrop × I, on every operating hour. Car park lighting, outbuildings, distant pumps and EV charge points are the usual problem circuits because of their cable lengths.

Increase the conductor cross-section, shorten the route, or reduce the current. Doubling the cross-sectional area halves the resistance and therefore halves the drop for the same load. On large sites it can be cheaper to distribute at a higher voltage and transform down near the load than to pull very large cables over long distances. Rerun the calculator with the next standard cable size up until the drop sits inside your design limit.

Copper's resistivity is about 1.72 × 10⁻⁸ Ω·m against 2.82 × 10⁻⁸ Ω·m for aluminium, so an aluminium conductor of the same cross-section has roughly 64% more resistance and proportionally more drop. Aluminium is still common on larger distribution cables because it is lighter and cheaper per amp carried, but it must be sized up to match copper's performance. Switch the material selector to compare both for the same run.

It models a single-phase or DC circuit with current flowing out and back, which is why it uses twice the one-way length. For a balanced three-phase circuit the line-to-line drop is the square root of 3 times the one-way conductor drop, which works out at 0.866 times the single-phase result this tool shows. Three-phase distribution is one reason large sites move heavy loads onto 400 V circuits: drop and losses fall for the same power delivered.