Cable Sizing Calculator per BS 7671:2018+A2 for United Kingdom
Cable sizing in the United Kingdom follows BS 7671:2018+A2 (IET Wiring Regulations, 18th Edition). Engineers select conductor cross-sectional areas using Table 4D1A current-carrying capacities, apply grouping factors from Table 4C1, ambient temperature corrections from Table 4B1, and verify voltage drop against Appendix 4 limits of 3% lighting and 5% power.
Quick Reference Table
| UK Cable Sizing Key References — BS 7671 — BS 7671:2018+A2 (18th Edition + Amendment 2:2022) | ||
|---|---|---|
| Parameter | Value / Requirement | Clause Reference |
| Supply voltage | 230 V single-phase / 400 V three-phase (TNS or TN-C-S) | Regulation 132.4 |
| Frequency | 50 Hz | Regulation 132.4 |
| Current-carrying capacity | Thermoplastic (70 °C) and thermosetting (90 °C) ratings | Table 4D1A / 4E1A |
| Voltage drop limits | 3% lighting, 5% other circuits from origin | Appendix 4, Table 4Ab |
| Grouping correction factors | Reduction for circuits in same enclosure or trunking | Table 4C1 |
| Common conductor sizes | 1.0, 1.5, 2.5, 4, 6, 10, 16, 25, 35 mm² | Table 4D1A |
| Maximum demand calculation | Diversity factors per Appendix A for domestic installations | Appendix A, Table A1 |
| Protective device coordination | Ib ≤ In ≤ Iz and I2 ≤ 1.45 × Iz | Regulation 433.1 |
How to Calculate Cable Sizing for United Kingdom
- 1
Determine design current (Ib)
Calculate the maximum load current for the circuit. For domestic circuits, apply BS 7671 Appendix A diversity. For commercial, use assessed maximum demand.
- 2
Select protective device rating (In)
Choose an MCB, RCBO, or fuse rating such that Ib ≤ In. Common UK ratings: 6, 10, 16, 20, 32, 40, 50, 63 A.
- 3
Identify UK installation method
Determine the installation method per BS 7671 Table 4A2 — clipped direct (C), enclosed in conduit (B), on tray (E/F), or in thermal insulation (100/101).
- 4
Apply correction factors
Apply grouping (Table 4C1), ambient temperature (Table 4B1), and thermal insulation (Reg. 523.9) factors. Divide In by the product of all factors to find the required tabulated Iz.
- 5
Check voltage drop against UK limits
Calculate voltage drop using mV/A/m values from Table 4D1B. Verify total drop stays within 3% (6.9 V) for lighting and 5% (11.5 V) for power at 230 V.
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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 |