Cable Sizing Calculator per BS 7671 for Commercial Buildings
Cable sizing for BS 7671 commercial buildings selects conductor sizes for sub-main and final distribution circuits using Table 4D2A (multicore SWA) or Table 4D1A (single-core) current ratings, applying Regulation 523.1 grouping derating and verifying sub-main voltage drop remains within the 5% Appendix 4 limit for the total installation.
Quick Reference Table
| BS 7671 Commercial Cable Sizing References — BS 7671 (18th Edition + Amendment 2:2022) | ||
|---|---|---|
| Parameter | Value / Requirement | Clause Reference |
| SWA current ratings | Multicore armoured cable ratings for various methods | Table 4D2A |
| Grouping derating | Factors for cables on perforated trays and ladder | Regulation 523.1, Table 4C3 |
| Sub-main voltage drop | Total drop from origin must not exceed limits | Appendix 4, Table 4Ab |
| Discrimination | Upstream/downstream device coordination | Regulation 536.4 |
| Earth fault loop impedance | Maximum Zs for automatic disconnection | Table 41.3 |
How to Calculate Cable Sizing for Commercial Buildings
- 1
Assess the total maximum demand
Calculate the total connected load for the commercial building using diversity factors from BS 7671 Appendix 1 for lighting, small power, HVAC, and specialist loads.
- 2
Design the sub-main distribution
Size sub-main cables from the main switchboard to distribution boards — typically multicore SWA rated per Table 4D2A for cable tray or direct burial installations.
- 3
Apply environmental derating
Account for ambient temperature (Table 4B1), grouping on cable trays (Table 4C3), and any thermal insulation proximity per Regulation 523.7.
- 4
Verify voltage drop across tiers
Check cumulative voltage drop from intake to final outlet. For commercial buildings, apportion the 5% total budget between sub-mains and final circuits.
- 5
Confirm fault protection
Verify earth fault loop impedance (Table 41.3) ensures disconnection within 5 s for distribution circuits or 0.4 s for final circuits.
Try the Cable Sizing Calculator
Run compliant BS 7671 calculations for commercial buildings — free, instant results with full clause references.
Calculate Cable Sizing 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 |