Calculators

Voltage Drop Calculator

Calculate voltage drop along a copper or aluminium cable from its size (mm² or AWG), length and current, and check it against the 3% and 5% limits.

Metres for mm² cable, feet for AWG.
A
V

Enter your values and press Calculate.

How to use the Voltage Drop calculator

BS 7671 (the IET Wiring Regulations) suggests a maximum drop of 3% for lighting and 5% for other circuits on a public supply. This is an estimate that ignores reactance — cable sizing must be done by a qualified electrician using the full BS 7671 method, including current-carrying capacity.

Formula

Voltage drop = 2 × L × ρ ÷ A × I for single phase (√3 instead of 2 for three phase), where L is the one-way length, A the cross-section and ρ the conductor resistivity at a typical 70 °C operating temperature (copper 0.0206, aluminium 0.0338 Ω·mm²/m).

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