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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 (V) |
|---|---|---|
| Lighting | 3 | 6.9 |
| Other (power, heating, etc.) | 5 | 11.5 |
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.
Related Pages
See the Cable Ampacity Table for current carrying capacities, and the UK Cable Sizes reference for physical cable dimensions.
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Frequently Asked Questions
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.
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.
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.
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.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.