What size cable for a 7 kW EV charger?
A 7 kW home charger draws 32 A for hours at a time, so it needs a dedicated 32 A circuit. Clipped direct, 4 mm² twin and earth is enough up to about 32 m. Beyond that, or through insulation, use 6 mm² or more.
- V – Voltage
- I – Current
- R – Resistance
- P – Power
Step 1: the current
A "7 kW" charger is really 32 A × 230 V = 7.36 kW. It draws that for the whole charge, so no diversity is allowed. BS 7671 Section 722 also says each charge point needs its own dedicated final circuit.
Step 2: choose the cable for a 32 A circuit
Run lengths use the full 32 A, because the charger really does draw it:
| How the cable is run | Smallest twin and earth for 32 A | Longest run (voltage drop) | Longer run? Next size up |
|---|---|---|---|
| Clipped direct to a wall or joist | 4 mm² (rated 37 A) | up to 32 m | 6 mm²: up to 49 m |
| Above a plasterboard ceiling, insulation up to 100 mm | 6 mm² (rated 34 A) | up to 49 m | 10 mm²: up to 81 m |
| In an insulated stud wall, touching the inner face | 6 mm² (rated 35 A) | up to 49 m | 10 mm²: up to 81 m |
| In an insulated stud wall, not touching the inner face | 10 mm² (rated 32 A) | up to 81 m | 16 mm²: up to 128 m |
Runs outside or underground are usually done in steel wire armoured (SWA) cable, which has its own ratings. This table covers twin and earth only.
Other things to check
- RCD: each charge point needs its own 30 mA RCD of at least Type A. Protection against DC fault current over 6 mA is also needed, either built into the charger or as a Type B RCD. Check the charger's instructions.
- PME (TN-C-S) supplies: chargers used outdoors have extra earthing requirements in Section 722, because of the risk if the supply's combined neutral and earth breaks.
- Supply capacity: add 32 A to the house's existing load and check the main fuse and supply can take it. Many charger installs also need the DNO to be notified.
- Check the maximum Zs for the breaker: a B32 is 1.37 Ω (Max Zs table).
Assumes 70 °C flat twin and earth (6242Y), 230 V, an MCB or RCBO, 30 °C ambient, the circuit not bunched with others, and no thermal insulation round a clipped cable. Voltage drop limit 5 % (11.5 V), BS 7671 Table 4D5 values. Grouping, heat or insulation can need a bigger cable: check with the cable size calculator. For study and estimating only; the installing electrician must design and certify the circuit.