Cable Sizing Calculator per BS 7671 for EV Charging Installations
Cable sizing for BS 7671 EV charging installations applies Section 722, which mandates dedicated circuits per Regulation 722.311, load management diversity from Table 722.1 for multiple charge points, and cables rated for continuous duty at the full charger output current — typically 32 A (7 kW) or 63 A (22 kW) single-phase or three-phase.
Quick Reference Table
| BS 7671 EV Charging Cable Sizing References — BS 7671 (18th Edition + Amendment 2:2022) | ||
|---|---|---|
| Parameter | Value / Requirement | Clause Reference |
| Dedicated circuit requirement | Each EV charge point on its own final circuit | Regulation 722.311 |
| Multiple charger diversity | Diversity factors for 2 or more charge points | Table 722.1 |
| Continuous load rating | Cable rated at 100% of charger output current | Regulation 722.531.1 |
| PME earthing restrictions | TN-C-S earth requirements for outdoor chargers | Regulation 722.411.4.1 |
| Voltage drop | Total drop for the EV circuit within 5% limit | Appendix 4, Table 4Ab |
How to Calculate Cable Sizing for EV Charging Installations
- 1
Confirm charger specification
Identify the EV charger output — typically 7.4 kW (32 A single-phase) or 22 kW (32 A three-phase). The cable must carry 100% of this current as EV charging is a continuous load.
- 2
Design the dedicated circuit
Per Regulation 722.311, each charge point requires its own dedicated final circuit from the distribution board with appropriate overcurrent protection.
- 3
Apply diversity for multiple chargers
When sizing the upstream supply cable for multiple chargers, apply diversity from Table 722.1. For example, four chargers may have a diversity factor of 0.7, reducing the sub-main design current.
- 4
Select cable size with derating
Apply ambient temperature (Table 4B1), grouping (Table 4C1), and installation method derating to the continuous load current. A 7 kW charger typically needs 6 mm² cable for short runs.
- 5
Verify voltage drop
Check voltage drop from the distribution board to the charge point. For a 30 m run at 32 A, the drop must stay within the 5% limit of Appendix 4.
- 6
Address PME earthing requirements
For outdoor installations on a TN-C-S system, Regulation 722.411.4.1 requires additional earth electrode or use of the SWA cable armour as the CPC to limit touch voltage.
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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 |