Cable Size Calculator UK — BS 7671 Guide IET Wiring Regulations 2026
Cable sizing in the UK follows BS 7671 (IET Wiring Regulations), which specifies current-carrying capacity, voltage drop limits, and correction factors for installation conditions. The most common domestic cables are 1.0mm² for lighting (6A), 2.5mm² for ring finals (32A), and 6.0mm² for cookers and EV chargers (32-45A). This guide covers sizing for all common UK domestic circuits.
BS 7671 Cable Current Ratings (Table 4D5A)
The current-carrying capacity of cables depends on the installation method. For twin & earth (T&E) cable clipped direct (Reference Method C per BS 7671): 1.0mm²: 15.5A. 1.5mm²: 20A. 2.5mm²: 27A. 4.0mm²: 37A. 6.0mm²: 47A. 10.0mm²: 65A. 16.0mm²: 87A. These ratings apply at 30°C ambient with no grouping or insulation correction. In reality, you must apply correction factors that reduce these ratings.
Correction Factors — Ca, Cg, Ci
BS 7671 requires applying correction factors to account for real installation conditions: Ca (ambient temperature): At 30°C = 1.00 (standard). At 35°C = 0.94. At 40°C = 0.87. Loft spaces in summer can reach 40°C+. Cg (grouping): Multiple cables bundled together reduce capacity. 2 cables = 0.80. 3 cables = 0.70. 4+ = further reduction per Table 4C1. Ci (thermal insulation): Cable touching insulation: 0.75. Cable enclosed in insulation: 0.50. The corrected current = Table rating × Ca × Cg × Ci. Use our Wire Size Calculator.
Common UK Domestic Circuits
Lighting circuit (6A MCB): 1.0mm² T&E. Max 10-12 light points per circuit. CPC (earth): 1.0mm². Ring final (32A MCB): 2.5mm² T&E forming a ring. Serves general sockets in one floor/area. Maximum floor area: 100m². Radial socket (20A MCB): 2.5mm² T&E. Alternative to ring for smaller areas (up to 50m²). Cooker circuit (32-45A): 6.0mm² T&E on 32A MCB (most cookers) or 10.0mm² on 45A MCB (large range cookers). Electric shower (40-50A): 10.0mm² on 45A or 50A MCB for 9.5-10.5 kW showers. EV charger (32A): 6.0mm² on 40A Type C MCB per IET Code of Practice.
Voltage Drop Calculations
BS 7671 limits voltage drop to 5% of nominal voltage (230V × 5% = 11.5V) for the total circuit, with 3% recommended for lighting. Voltage drop per metre per amp is given in the cable tables: 1.0mm² T&E: 44 mV/A/m. 2.5mm² T&E: 18 mV/A/m. 6.0mm² T&E: 7.3 mV/A/m. 10.0mm² T&E: 4.4 mV/A/m. Formula: VD = (mV/A/m × Design Current × Length) ÷ 1000. For a 32A cooker on 6.0mm² at 15m: VD = (7.3 × 32 × 15) ÷ 1000 = 3.5V (1.5% — acceptable).
| Cable Size | Current Rating | Typical Circuit | MCB Size |
|---|---|---|---|
| 1.0mm² | 15.5A | Lighting | 6A Type B |
| 1.5mm² | 20A | Immersion heater | 16A Type B |
| 2.5mm² | 27A | Ring final / Radial | 32A / 20A |
| 4.0mm² | 37A | Large radial | 32A |
| 6.0mm² | 47A | Cooker, EV charger | 32-40A |
| 10.0mm² | 65A | High-power shower, sub-main | 45-50A |
| 16.0mm² | 87A | Sub-main, large loads | 63-80A |
EV Charger Cable Sizing per IET Code of Practice
The IET Code of Practice for EV Charging specifies: a 7kW (32A) home charger requires a dedicated radial circuit with 6.0mm² T&E cable, a 40A Type C MCB, and a Type A or Type B RCD (30mA). The cable must be sized accounting for correction factors — if the cable passes through insulated areas (loft, wall cavities), use Ci = 0.75, which may require upsizing to 10.0mm². PME earth considerations: if TN-C-S (PME) supply, additional earthing requirements per BS 7671 722.411.4.1 apply. Many installers use an earth rod as additional protection.
Disclaimer: For educational reference only. Consult a licensed professional.