Voltage Drop
Voltage drop limits, mV/A/m values, and calculation formula from BS 7671 Appendix 4.
Voltage Drop Limits
Maximum Permitted Voltage Drop
From the origin of the installation: • Lighting circuits: 3% (6.9V at 230V) • Other circuits: 5% (11.5V at 230V)
Note: These limits apply from the supply terminals to the most distant point of the circuit. Where the supply is from a public distribution system, an additional 0.5% is allowed.
Flat Twin & Earth (70°C) mV/A/m — Table 4D5
Flat Twin & Earth (70°C) — Table 4D5 col 9
| Size (mm²) | 1Φ mV/A/m (all methods) |
|---|---|
| 1.0 | 44 |
| 1.5 | 29 |
| 2.5 | 18 |
| 4.0 | 11 |
| 6.0 | 7.3 |
| 10.0 | 4.4 |
| 16.0 | 2.8 |
Flat T&E is a single-phase cable tabulated to 16 mm²; its voltage drop is the same for every installation method.
Singles in Conduit (70°C) — Table 4D1B
Singles in Conduit / Trunking (70°C) — Table 4D1B
| Size (mm²) | 1Φ 2-cable (col 3) | 3Φ 3/4-cable (col 6) |
|---|---|---|
| 1.0 | 44 | 38 |
| 1.5 | 29 | 25 |
| 2.5 | 18 | 15 |
| 4.0 | 11 | 9.5 |
| 6.0 | 7.3 | 6.4 |
| 10.0 | 4.4 | 3.8 |
| 16.0 | 2.8 | 2.4 |
| 25.0 | 1.80 | 1.55 |
| 35.0 | 1.30 | 1.10 |
| 50.0 | 1.00 | 0.85 |
| 70.0 | 0.72 | 0.61 |
| 95.0 | 0.56 | 0.48 |
| 120.0 | 0.47 | 0.41 |
| 150.0 | 0.41 | 0.36 |
| 185.0 | 0.37 | 0.32 |
| 240.0 | 0.33 | 0.29 |
| 300.0 | 0.31 | 0.27 |
Reference Methods A & B (enclosed in conduit/trunking). Values in mV/A/m: r up to 16 mm², (mV/A/m)z above, which Appendix 4 §6 notes gives a voltage drop higher than the actual value.
Voltage Drop Formula
Voltage Drop
Use 1Φ (2-core) values for single-phase circuits. Use 3Φ (3-core) values for three-phase circuits. The formula is the same — only the mV/A/m value changes.
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