Voltage Drop Tables
mV/A/m values for calculating voltage drop in cable runs
BS 7671:2018+A2:2022 Tables 4D1B-4J2B
Lighting Circuits
3%
Maximum voltage drop
Other Circuits
5%
Maximum voltage drop
Formula
VD = mV/A/m × Ib × L / 1000
Result in volts
Copper Conductors (Thermoplastic 70°C)
Reference: Table 4D1B
| Size (mm²) | 2-core / Single-phase mV/A/m | 3/4-core / Three-phase mV/A/m |
|---|---|---|
| 1 | 44 | 38 |
| 1.5 | 29 | 25 |
| 2.5 | 18 | 15 |
| 4 | 11 | 9.5 |
| 6 | 7.3 | 6.4 |
| 10 | 4.4 | 3.8 |
| 16 | 2.8 | 2.4 |
| 25 | 1.75 | 1.5 |
| 35 | 1.25 | 1.1 |
| 50 | 0.93 | 0.8 |
| 70 | 0.63 | 0.55 |
| 95 | 0.46 | 0.4 |
| 120 | 0.36 | 0.31 |
| 150 | 0.29 | 0.25 |
| 185 | 0.235 | 0.2 |
| 240 | 0.18 | 0.155 |
| 300 | 0.145 | 0.125 |
Aluminium Conductors (Thermoplastic 70°C)
Reference: Table 4H1B
| Size (mm²) | 2-core / Single-phase mV/A/m | 3/4-core / Three-phase mV/A/m |
|---|---|---|
| 16 | 4.6 | 4 |
| 25 | 2.9 | 2.5 |
| 35 | 2.1 | 1.8 |
| 50 | 1.55 | 1.35 |
| 70 | 1.05 | 0.9 |
| 95 | 0.77 | 0.67 |
| 120 | 0.6 | 0.52 |
| 150 | 0.49 | 0.42 |
| 185 | 0.39 | 0.34 |
| 240 | 0.3 | 0.26 |
| 300 | 0.235 | 0.205 |
Example Calculation
Problem: 32A load on 25m of 6mm² T+E cable (single-phase)
mV/A/m value = 7.3 (from table)
VD = 7.3 × 32 × 25 / 1000
VD = 5.84V
VD% = 5.84 / 230 × 100 = 2.54%
✓ Within 5% limit for power circuit
Notes
- • Values are for thermoplastic (PVC) insulated cables at 70°C conductor operating temperature.
- • For thermosetting (XLPE) cables at 90°C, values will be slightly higher.
- • Three-phase values assume balanced load. For unbalanced loads, use single-phase values.
- • Long cable runs may require upsizing to meet voltage drop limits.
- • Regulation 525.1 permits higher limits where the supply is from a private LV supply.