Every cable has resistance, so some voltage is lost along its length. On short runs it doesn't matter. On long runs to a garage, workshop, pump or EV charger, it can mean dim lights, motors that struggle and a run that fails inspection.
The formula
For single-phase: VD = 2 × ρ × L × I ÷ A, where ρ is about 0.0225 Ω·mm²/m for copper at working temperature, L is the one-way length in metres, I is amps and A is the cross-section in mm². For three-phase, use √3 instead of 2.
Typical limits
BS 7671 Appendix 4 gives typical limits from the origin of the installation on a public supply: 3% for lighting and 5% for other uses.
Voltage drop is only one check
Cable sizing also depends on current-carrying capacity, installation method, grouping, ambient temperature, protective device and fault protection. Voltage drop often decides the size on long runs, but it's not the only check. This work must be designed and installed by a qualified electrician to BS 7671, and notified under Part P where required.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the Building Regulations and Building Control for your project.