Voltage Drop Calculator per BS 7671 for Commercial Buildings
Commercial building voltage drop calculations under BS 7671 must account for cascaded distribution boards where the total drop is cumulative from the main switchboard. Appendix 4, Table 4Ab permits 3% for lighting and 5% for other uses. Sub-main feeders often dominate the overall voltage drop budget in multi-storey buildings.
Quick Reference Table
| BS 7671 Voltage Drop Parameters — Commercial — BS 7671 (18th Edition + Amendment 2:2022) | ||
|---|---|---|
| Parameter | Value / Requirement | Clause Reference |
| Lighting circuits | 3% maximum (6.9 V at 230 V) | Appendix 4, Table 4Ab |
| Power circuits | 5% maximum (11.5 V at 230 V / 20 V at 400 V) | Appendix 4, Table 4Ab |
| Permissible drop scope | Origin of installation to current-using equipment | Regulation 525.1 |
| Three-phase mV/A/m values | Per conductor CSA and installation method | Table 4D1B, Column 4 |
| Busbar trunking systems | Manufacturer's declared voltage drop per metre | Regulation 525.2 |
| Motor starting transient | Short-duration voltage dip excluded from steady-state assessment | Regulation 525.1 Note |
How to Calculate Voltage Drop for Commercial Buildings
- 1
Map the distribution architecture
Identify the path from the main switchboard through sub-distribution boards to the final circuit. Commercial buildings often have 2-3 levels of distribution, and voltage drop accumulates at each stage.
- 2
Allocate voltage drop budget per stage
Divide the total permissible voltage drop (e.g. 11.5 V for power at 230 V) between the sub-main feeder and final circuits. A common split is 60% for the sub-main and 40% for final circuits.
- 3
Calculate sub-main feeder voltage drop
Using the maximum demand current and cable route length, apply VD = (mV/A/m × Ib × L) / 1000 with three-phase values from Table 4D1B Column 4 for 400 V feeders.
- 4
Calculate final circuit voltage drops
For each final circuit from the sub-distribution board, compute the voltage drop independently. The total drop at the load is the sum of the sub-main and final circuit drops.
- 5
Verify cumulative drop at worst-case outlet
Identify the circuit with the longest run and highest load. Confirm the total voltage drop from the main switchboard to this point stays within the Table 4Ab limit.
- 6
Consider busbar trunking alternatives
For risers in multi-storey commercial buildings, busbar trunking may offer lower voltage drop per metre than cables. Use the manufacturer's declared values per Regulation 525.2.
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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 |