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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.
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See the Cable Ampacity Table for current carrying capacities, and the UK Cable Sizes reference for physical cable dimensions.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.