UK Cable Sizes & Current Ratings — BS 7671 Reference
Selecting the correct cable size is a fundamental part of electrical installation design in the UK. This reference page covers the standard cable sizes used in domestic and light commercial installations, their current-carrying capacities under BS 7671 (18th Edition), and the typical circuits they serve.
Twin and Earth (6242Y) Current Ratings
Flat twin and earth cable (6242Y) is the standard cable for domestic wiring in the UK. Current ratings below are for Reference Method C (clipped direct to a surface) from BS 7671:2018+A2:2022 Appendix 4 Table 4D5, column 6:
| Cable Size (mm²) | Current Rating (Method C) | CPC Size (mm²) | Common Circuit |
|---|---|---|---|
| 1.0 | 16 A | 1.0 | Lighting circuits (6 A MCB) |
| 1.5 | 20 A | 1.0 | Lighting circuits (longer runs), immersion heater (on short run) |
| 2.5 | 27 A | 1.5 | Ring final circuits (32 A), radial sockets (20 A) |
| 4.0 | 37 A | 1.5 | Cooker circuit (small), immersion heater, storage heater |
| 6.0 | 47 A | 2.5 | Cooker circuit (standard), shower (up to 9.5 kW) |
| 10.0 | 64 A | 4.0 | Shower (10.5–11.5 kW), large cooker, EV charger |
| 16.0 | 85 A | 6.0 | Sub-main to outbuilding, large EV charger, meter tails |
Current ratings shown are for Reference Method C (clipped direct). Ratings reduce for Method A (enclosed in insulation) or Method B (enclosed in conduit/trunking). Always apply correction factors for grouping, ambient temperature, and thermal insulation.
Common Domestic Circuits
| Circuit | MCB Rating | Cable Size | Notes |
|---|---|---|---|
| Lighting | 6 A Type B | 1.0 mm² or 1.5 mm² | Max 12 points per circuit (common practice) |
| Ring final (sockets) | 32 A Type B | 2.5 mm² | Max 100 m² floor area served |
| Radial sockets | 20 A Type B | 2.5 mm² | Max 50 m² floor area served |
| Immersion heater | 16 A Type B | 2.5 mm² | Dedicated radial circuit, unswitched fused spur |
| Cooker | 32 A Type B | 6.0 mm² | Via cooker control unit with 13 A socket |
| Electric shower (9.5 kW) | 40 A Type B | 6.0 mm² or 10.0 mm² | Depends on run length and voltage drop |
| EV charger (7 kW) | 32 A Type B | 6.0 mm² or 10.0 mm² | Depends on run length; PEN fault protection required |
Key Points for Cable Selection
- Current-carrying capacity — The cable must carry the design current without overheating. Apply correction factors for ambient temperature, grouping, and thermal insulation.
- Voltage drop — BS 7671 limits voltage drop to 3% for lighting and 5% for other circuits. Longer cable runs may need upsizing to stay within limits.
- Fault current — The CPC (earth) must carry the earth fault current long enough for the protective device to operate. Check adiabatic equation where required.
- Installation method — How the cable is installed affects its current rating. Cables buried in insulation can carry significantly less current than those clipped to a surface.
Frequently Asked Questions
What is Reference Method B in BS 7671?
Reference Method B in BS 7671 (18th Edition, Appendix 4) covers cables enclosed in conduit or trunking — for example, run on a wall — whereas the ratings in the table above are for Reference Method C, cable clipped direct to a surface. Enclosed cables dissipate heat less effectively, so BS 7671 Table 4D5 tabulates lower current ratings for Method B than Method C for the same cable size, and generally lower still where the cable is surrounded by thermal insulation. That is why the installation method must be fixed before a rating is read from the tables. Our Cable Sizing Calculator applies the rating for your chosen installation method rather than assuming clipped-direct figures.
Calculate Your Cable Requirements
- Cable Sizing Calculator — determine the correct cable size to BS 7671
- Voltage Drop Calculator — check voltage drop compliance
- Circuit Breaker Sizing Calculator — determine correct MCB rating
- EV Charger Cable Sizing Calculator — cable sizing for EV charger installations
Current ratings are extracted from BS 7671:2018+A2:2022 (18th Edition). This reference is for guidance only and does not replace the need for proper cable sizing calculations by a qualified electrician. All domestic electrical work must comply with Part P of the Building Regulations.