Voltage Drop Calculator per BS 7671 for Residential Installations
Voltage drop in BS 7671 residential installations must not exceed 3% for lighting circuits and 5% for other circuits from the origin of the installation, as defined in Appendix 4, Table 4Ab. Regulation 525.1 sets permissible limits referenced to nominal supply voltage (230 V single-phase).
Quick Reference Table
| BS 7671 Voltage Drop Limits — Residential — BS 7671 (18th Edition + Amendment 2:2022) | ||
|---|---|---|
| Parameter | Value / Requirement | Clause Reference |
| Lighting circuit limit | 3% (6.9 V at 230 V) | Appendix 4, Table 4Ab |
| Power circuit limit | 5% (11.5 V at 230 V) | Appendix 4, Table 4Ab |
| Permissible voltage drop | From origin to load | Regulation 525.1 |
| mV/A/m copper conductors | Tabulated per conductor size and installation method | Table 4D1B |
| Diversity allowance | May reduce design current for domestic ring finals | Regulation 525.2 |
How to Calculate Voltage Drop for Residential Installations
- 1
Determine circuit type and supply voltage
Identify whether the residential circuit is lighting (3% limit) or power (5% limit). Confirm 230 V single-phase or 400 V three-phase supply from the consumer unit.
- 2
Establish design current and circuit length
Calculate the design current (Ib) from the connected load. Measure the cable route length from the consumer unit to the furthest point of utilisation, including vertical runs.
- 3
Select cable and find mV/A/m value
Choose the conductor size from BS 7671 Table 4D1B (copper thermoplastic) or Table 4D2B (copper thermosetting). Read the mV/A/m value for the installation method.
- 4
Calculate voltage drop
Apply the formula: VD = (mV/A/m × Ib × L) / 1000. For single-phase circuits use the two-conductor column; for three-phase use the three-phase column values.
- 5
Verify against Table 4Ab limits
Compare the calculated voltage drop percentage against the 3% or 5% limit from Table 4Ab. If exceeded, increase conductor size or reduce circuit length by splitting the circuit.
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Calculate Voltage Drop NowBS 7671 vs IEC 60364 Cable Sizing Comparison
| Parameter | BS 7671 | IEC 60364 |
|---|---|---|
| Scope | UK & derivatives | International (adopted by 60+ countries) |
| Voltage drop limit | 3% lighting / 5% other | 4% lighting / 5% other (typical) |
| Reference ambient temp | 30°C (air), 20°C (ground) | 30°C (air), 20°C (ground) |
| Installation methods | Reference Methods A-G (Appendix 4) | Reference Methods A-G (Table B.52.1) |
| Grouping factors | Table C.3 (BS specific) | Table B.52.17 (international) |
| Disconnection time (230V) | 0.4s final / 5s distribution | 0.4s final / 5s distribution |