Voltage Drop Calculator — UK Wiring Regulations

BS 7671 18th Edition limits voltage drop to 3% for lighting circuits (6.9V on a 230V supply) and 5% for all other circuits (11.5V on a 230V supply). These limits apply from the origin of the installation (typically the meter) to the furthest point of the circuit.

There are two main ways: increase the cable size (a larger cross-sectional area has lower resistance) or reduce the cable length. For very long runs, such as to an outbuilding, you may need to go up one or two cable sizes beyond what the current capacity requires. SWA cable may also help as it can handle longer runs. Use our Cable Sizing Calculator to find the right cable for your current and installation method.

For typical domestic cable runs under 10-15 metres, voltage drop is rarely an issue. It becomes critical for longer runs such as garden buildings, EV chargers at the end of a long drive, or commercial installations. Always check both current capacity and voltage drop. Use our Cable Length Calculator to determine the actual cable run distance, then confirm the cable's current-carrying capacity holds up in its installed conditions with our Cable Derating Calculator (temperature, grouping and thermal-insulation factors).

mV/A/m stands for millivolts per amp per metre. It is the voltage drop rate for a given cable size listed in the BS 7671 appendix 4 tables — Table 4D5 for flat twin and earth (6242Y) and Table 4D4B for multicore steel wire armoured (SWA). The single-phase formula is: Voltage drop (V) = (mV/A/m x current x length) / 1000. For a balanced three-phase circuit the line-to-line drop is 0.866 (the square root of 3, divided by 2) times that single-phase value.

The UK nominal supply voltage is 230V +10% / -6% as harmonised with European standards. In practice, most UK supplies still deliver around 240V at the meter. BS 7671 calculations use 230V as the reference. A measured supply of 240V gives slightly more headroom on voltage drop limits.