Voltage-drop estimate
A resistivity estimate for a two-core copper run. It ignores reactance, temperature rise and installation method.
Method
R = ρ × 2L / A, with ρ = 1.72×10⁻⁸ Ω·m and area converted from mm². Drop = IR. Percent = drop / supply × 100.
Worked case: 16 A, 25 m one way, 2.5 mm², 230 V → about 5.5 V, about 2.4%.
Not a substitute for BS 7671 appendix tables, manufacturer data or a designer’s check.
Logic: how this figure is built.
Last checked 2 October 2026. Worked figures are scenarios. See methodology and the disclaimer.