What Size Cable for EV Charger

When installing an electric vehicle charger at home, one of the most important technical considerations is the cable size. The size of the cable, or more accurately, its cross-sectional area measured in square millimetres (mm²), determines how much current it can safely carry to charge your vehicle. Using the wrong cable size can cause voltage drops, overheating or even potential electrical hazards. Getting this right is not just about performance, but also about safety and compliance with UK electrical regulations. This article explains everything you need to know about what size cable to use for your EV charger, how it is calculated, and what standards must be followed when setting up a home charging point.

Understanding Cable Sizing for EV Chargers

In electrical installations, the term “cable size” refers to the diameter of the copper or aluminium conductor inside the insulation. Larger cables can carry more current without overheating or causing excessive voltage drop, which is particularly important for high-power devices like EV chargers.

Electric vehicle chargers operate at different power levels, most commonly 3.6kW, 7.4kW and 22kW. The higher the power output, the more current the charger draws, and therefore the larger the cable required. For example, a 7.4kW charger typically draws around 32 amps on a single-phase supply, whereas a 22kW charger can draw up to 32 amps per phase on a three-phase supply.

Cable size selection depends on several factors, including the charger’s power rating, the distance between the consumer unit and the charger, the installation method, and environmental conditions. Longer cable runs, for instance, may require thicker cables to prevent voltage drop.

Common Cable Sizes Used for Home EV Chargers in the UK

For most home installations in the UK, a 7.4kW single-phase charger is the standard. The most commonly used cable for this setup is 6mm² twin and earth (T&E) or 6mm² SWA (steel wire armoured) cable, depending on whether the cable is installed indoors or outdoors.

If the charger is located a long distance from the main consumer unit, installers may use 10mm² cable to reduce voltage drop and maintain efficiency. The thicker cable can carry more current over longer distances without compromising performance.

For smaller chargers rated at 3.6kW, which draw around 16 amps, a 2.5mm² or 4mm² cable may be sufficient for short runs. However, most electricians prefer to use 6mm² cable even in these cases for future-proofing and added safety.

In commercial or three-phase installations using 11kW or 22kW chargers, cable sizes can range from 6mm² to 16mm² depending on current load, length, and installation method.

Factors That Influence Cable Size Choice

Selecting the correct cable size is not as simple as matching the charger’s current rating. Several factors influence the final choice:

Distance to Charger: The longer the cable run, the higher the resistance, and the greater the potential voltage drop. To compensate, electricians often increase the cable size. For example, a charger located 20 metres from the fuse board might require 10mm² cable instead of 6mm².

Installation Method: Cables that are buried underground, run through insulation, or enclosed in conduits cannot dissipate heat as efficiently as those installed in open air. In these cases, a larger cable size is chosen to prevent overheating.

Ambient Temperature: High ambient temperatures can reduce the cable’s ability to carry current safely. Installers may need to use cables with higher temperature ratings or increase the cross-sectional size to compensate.

Load Diversity and Future Expansion: Many homeowners choose to install thicker cables even if not strictly necessary, in case they upgrade to a higher-powered charger later. This avoids the cost of rewiring.

Voltage Drop Regulations: UK regulations under the IET Wiring Regulations (BS 7671) stipulate that the voltage drop for fixed wiring must not exceed 3% for lighting circuits and 5% for other circuits, including EV chargers. The cable size must therefore be sufficient to keep the voltage drop within acceptable limits.

Single-Phase vs Three-Phase Supplies

Most UK homes have single-phase electricity supplies, which means power is delivered through one live conductor and one neutral. These systems are limited to around 7.4kW for EV charging. For these installations, 6mm² cable is generally standard, although 10mm² may be used for long runs or to future-proof the installation.

Three-phase supplies, found mainly in commercial premises or larger residential properties, can support higher charging speeds of up to 22kW or more. Because each phase carries part of the load, three-phase systems distribute current more evenly, but the total current can still be substantial. Cable sizes for three-phase chargers vary depending on layout, but 6mm² to 16mm² armoured cable is typical.

Armoured Cable vs Twin and Earth

The type of cable is just as important as its size. For indoor installations where the cable is protected within walls or conduit, twin and earth cable is commonly used. However, for outdoor or underground runs, steel wire armoured (SWA) cable is required.

SWA cables have an extra layer of steel strands around the inner cores, providing mechanical protection against damage from impact, rodents, or garden tools. They are also weather-resistant, making them ideal for connecting an external wall charger or one mounted on a detached garage.

Safety and Compliance Requirements

All EV charger installations in the UK must comply with Part P of the Building Regulations and BS 7671 (IET Wiring Regulations). Only qualified electricians registered with a competent person scheme, such as NICEIC or NAPIT, should carry out the work.

The installation must include a dedicated circuit for the charger with its own circuit breaker and residual current device (RCD). The RCD protects against electric shock and must be type A or type B, depending on the charger’s manufacturer requirements. Some modern chargers have built-in RCD protection, but this must always be verified.

Installers must also ensure proper earthing and bonding to prevent electric shock risks. In some cases, especially where the charger is installed outdoors, an earth electrode may be required.

Typical Cable Size Examples

Although exact requirements vary depending on installation conditions, the following are typical examples used in UK home charger setups:

A 3.6kW charger (16 amps) within 10 metres of the consumer unit generally uses 4mm² T&E cable.

A 7.4kW charger (32 amps) within 10-15 metres of the consumer unit typically uses 6mm² T&E cable.

A 7.4kW charger with a longer cable run of 20-30 metres may require 10mm² SWA cable to maintain voltage drop limits.

A 22kW three-phase charger typically uses 6mm² to 16mm² SWA cable depending on the installation.

Each situation must be assessed individually, so cable size calculations should be confirmed using official wiring tables from the IET.

Professional Installation and Testing

Cable sizing and charger installation are not DIY tasks. An incorrectly sized or fitted cable could cause electrical faults, tripped breakers or fire risks. Professional installers perform load calculations, select the correct cable type and size, and carry out insulation and continuity testing to confirm the safety of the installation.

Once complete, they will issue an Electrical Installation Certificate confirming that the system meets UK safety standards. This certificate should be kept for insurance and warranty purposes.

Conclusion

Choosing the right size cable for your EV charger is crucial for safe and efficient operation. In most UK homes, a 7.4kW charger will require at least 6mm² cable, increasing to 10mm² for longer runs or to future-proof for higher loads. Outdoor installations must use armoured cable for durability and safety.

While it might be tempting to cut corners or guess cable sizes, only a qualified electrician can determine the correct size for your specific property and charger model. Proper cable selection not only ensures compliance with UK electrical regulations but also protects your investment and your home. By getting the fundamentals right from the start, you’ll enjoy fast, reliable and safe EV charging for years to come.