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.
How to read the voltage-drop estimate
The estimate is current times the go-and-return resistance of a copper conductor at 20°C. It is labelled as an estimate. It is not the voltage drop in BS 7671, which uses tabulated values at the operating temperature and the installation method.
Length is the route length. Go and return means the outward and the return conductor are both in the resistance. A one-way length used with a go-and-return factor already included double-counts the run.
Temperature, grouping, power factor and a reactive term are not applied. A long run and a short run do not share a percentage just because the current is the same.
Use it to see the order of a copper run before an electrical design. The design uses the regulation method.
- Related: Ohm's law
- electrical limits
Last checked 3 October 2026. Worked figures are scenarios. See methodology and the disclaimer.