What is the maximum permitted voltage drop?

BS 7671 Regulation 525.1 requires that the voltage drop between the origin of the installation and any load point does not exceed 3% for lighting circuits (6.9V on a 230V supply) and 5% for all other circuits (11.5V on a 230V supply). The DNO supply tolerance is 230V +10%/-6%, so the actual supply voltage may vary between 216V and 253V.

How do I calculate voltage drop?

Voltage drop = (mV/A/m value × design current in amps × cable length in metres) ÷ 1000. For example, 2.5 mm² T+E cable has a mV/A/m value of 18. A 20A circuit with a 15m cable run gives: (18 × 20 × 15) ÷ 1000 = 5.4V, which is within the 5% (11.5V) limit for power circuits.

Do I use single-phase or three-phase mV/A/m values?

For single-phase circuits, use the two-conductor (line + neutral) mV/A/m values. For three-phase circuits, use the three- or four-conductor values. The tables below show two-conductor values for domestic single-phase installations, which is the most common scenario in UK houses.

Does temperature affect voltage drop?

Yes. The tabulated mV/A/m values in BS 7671 are given at the maximum conductor operating temperature (70°C for PVC, 90°C for XLPE). If the actual conductor temperature is lower (which it usually is unless the cable is fully loaded), the voltage drop will be slightly less. A simplified correction can be applied using the formula in BS 7671 Appendix 4, but for most domestic work the tabulated values provide a conservative result.

Voltage Drop Coefficients — mV/A/m Reference

Voltage drop values (mV/A/m) for common UK cable types per BS 7671:2018 Table 4D1B-4D5B. Use to verify cable runs comply with maximum voltage drop limits.

Twin-and-Earth (T+E) — PVC 70°C Copper

Reference: BS 7671:2018 Table 4D5B. Two loaded conductors (single-phase).

Cable Size (mm²)	mV/A/m (at 70°C)	Max Run at 20A (m)*	Max Run at 32A (m)*
1.0	44	—	—
1.5	29	19.8	—
2.5	18	31.9	19.9
4.0	11	52.3	32.7
6.0	7.3	78.8	49.2
10.0	4.4	130.7	81.7
16.0	2.8	205.4	128.3

* Maximum cable run assuming 5% voltage drop limit (11.5V) on a 230V single-phase supply. Lighting circuits limited to 3% (6.9V).

SWA Cable — XLPE 90°C Copper

Reference: BS 7671:2018 Table 4D4B. Two loaded conductors (single-phase).

Cable Size (mm²)	mV/A/m (at 90°C)	Max Run at 40A (m)*
2.5	19	15.1
4.0	12	24.0
6.0	7.9	36.4
10.0	4.7	61.2
16.0	2.9	99.1
25.0	1.85	155.4

Voltage Drop Limits Summary

Circuit Type	Max Drop (%)	Max Drop at 230V
Lighting	3%	6.9V
Other (power, heating, etc.)	5%	11.5V

Note: Refer to BS 7671:2018 + Amendment 2:2022 for definitive values. The mV/A/m values above are for standard conditions. Reactive components of voltage drop become significant for cable sizes above 25 mm² — use the tabulated r and x values separately for larger cables.

See the Cable Ampacity Table for current carrying capacities, and the UK Cable Sizes reference for physical cable dimensions.