BS 7671 requirements for voltage drop
Under BS 7671 (IET Wiring Regulations 18th Edition Appendix 4), the maximum permitted voltage drop between the origin of an installation and any point of utilization is 3% for lighting circuits (6.9V at 230V) and 5% for other uses (11.5V at 230V).
UK calculations use tabulated millivolts per ampere per metre (mV/A/m) values for standard British copper cables: Vd = (mV/A/m × Ib × L) / 1000, where Ib is design current and L is length in metres.
If the calculated voltage drop exceeds 3% on lighting or 5% on power circuits, a larger cable size (such as upgrading from 2.5 mm² to 4 mm² or 6 mm²) must be selected.
Worked example: Outbuilding power submain
Supplying a 32A garden office submain 35 metres away from the main consumer unit at 230V using 6 mm² SWA cable (mV/A/m = 7.3):
Voltage drop Vd = (7.3 × 32 A × 35 m) ÷ 1,000 = 8.18 Volts.
Percentage drop = (8.18 V ÷ 230 V) × 100% = 3.55%, successfully passing the 5% BS 7671 power limit.
Inputs for the UK BS 7671 voltage drop calculator
BS 7671 cable size
Select standard UK conductor cross-section (1.5, 2.5, 4, 6, 10, 16, 25, 35, or 50 mm²).
Run length in metres
One-way route distance in metres from consumer unit to load.
CALCULATOR QUESTIONS
UK BS 7671 voltage drop calculator questions
What are the BS 7671 voltage drop limits in the UK?
3% (6.9V at 230V) for lighting circuits and 5% (11.5V at 230V) for all other power, socket, and heating circuits.
What is mV/A/m in UK cable tables?
It represents the millivolts dropped per Ampere of current per metre of cable length, incorporating both conductor resistance and reactance.