EV Charger Cable Size Calculator — Size the Circuit for 7.4kW and 22kW Chargers

Designing an EV charger installation involves two separate calculations, and it pays to keep them distinct:

The cable sizing question matters more for EV chargers than for most loads because the load profile is unusual: near-constant full current for 4-8 hours at a time. There is no diversity to apply on a single charger — the cable must be sized for the full circuit rating, with correction factors for how and where it is installed, and the voltage drop checked over what is often a long run to a driveway or detached garage.

The standard UK domestic EV charger is a 7.4kW single-phase unit. The design current is 7400 / 230 = 32.2A, and the charger is built for a 32A dedicated circuit protected by a 32A device.

Typical cable choices for a 32A EV circuit:

• 6mm² twin and earth — the typical choice for shorter internal runs in favourable installation methods. Check the derated capacity if the route passes through insulation or is grouped with other cables.
• 10mm² twin and earth — the usual step-up for longer runs where voltage drop approaches the limit, or where derating pulls 6mm² below 32A.
• 6mm² or 10mm² SWA — for external, buried, or surface-run sections to a detached garage or driveway pillar. See the SWA cable size calculator for armoured runs.

Because the charger draws its full current for hours, treat marginal cases conservatively — a cable running at its exact tabulated capacity for a full overnight charge, every night, has no margin. The calculator above shows the derated capacity next to the design current so you can see the headroom.

Where a three-phase supply is available — commercial premises, workplaces, and a minority of homes — a 22kW charger triples the charging speed. The per-phase current is:

So a 22kW charger is, from a cable perspective, a 32A three-phase circuit — typically wired in 4-core SWA (three phases plus neutral, with the armour as the protective conductor where it qualifies) for the external run, protected by a 32A three-pole device.

Three-phase voltage drop uses the tabulated three-phase mV/A/m values against the 400V line voltage, which the calculator handles when you select a three-phase circuit. Note that many EVs can only accept single-phase AC charging even when connected to a three-phase unit — that affects the customer conversation, not the cable sizing, which is always done for the full charger rating. The three-phase power calculator covers the power and current relationships in more detail.

A 7.4kW charger is to be installed on a garage wall, 25 metres of cable from the consumer unit, run clipped and then through the garage in 6mm² twin and earth:

The pattern to remember: on a 32A EV circuit, 6mm² starts to run out of voltage drop headroom at around 40-45 metres, and derating can rule it out much earlier. The calculator does both checks together for your exact conditions.

EV charging circuits have specific protection requirements under BS 7671 Section 722 (electric vehicle charging installations), and the practical points for the circuit design are:

• 30mA RCD protection — the charging point requires RCD protection, and because EV charging equipment can produce DC fault currents, a Type A RCD (or better) is the standard specification rather than Type AC. Many chargers include built-in 6mA DC fault detection which pairs with a Type A RCD.
• Dedicated circuit — one circuit per charging point, protected by a 32A device for a 7.4kW unit.
• Earthing arrangement — on a PME (TN-C-S) supply, additional measures are required for EV charging, such as a charger with PEN fault detection or a local earth electrode. This is covered in depth on the EV charger load calculator page.
• Disconnection times — verify the earth fault loop impedance at the charging point with the disconnection time calculator, particularly on long runs where Zs accumulates.

Most EV charger installations involve a mixed route — twin and earth inside the house, then an external section to the charger position. Common approaches:

• Surface-run SWA on external walls — robust and straightforward; clip at appropriate intervals and gland properly at both ends.
• Buried SWA to a detached garage or driveway pillar — BS 7671 requires a buried cable to incorporate an earthed armour or metal sheath (or be in a duct providing equivalent protection), to be marked by cable covers or marker tape, and to be buried deep enough to avoid foreseeable disturbance. Accepted practice is around 450-600mm depth depending on ground use.
• Ducted runs — a duct makes future replacement or upgrade (say, from a 32A circuit to something bigger) far easier, which is worth considering given how quickly EV equipment is evolving.

When the route changes installation method — clipped, in insulation, buried — the cable must be sized for the worst-case section. Run the calculator for the most onerous method on the route. For the armoured section specifics, see the SWA cable size calculator.