EV charger cable size explained

If you're asking "what size cable should I get for my electric car charger", you're in the right place. Getting the cable size right for your electric vehicle (EV) charger is not just about performance; it is a critical safety consideration that protects your home and your investment.

Why correct EV charger cable size matters

The cable connecting your electric vehicle (EV) charger to your home's consumer unit is the lifeline of your charging system. Its size directly impacts how safely and efficiently electricity flows to your car.

Safety implications of incorrect sizing

Using an undersized cable for your EV charger can lead to serious problems. It risks overheating, which can damage the cable and charger, reduce energy efficiency, and, in severe cases, pose a significant fire hazard. The IET Wiring Regulations (BS 7671) are clear: all electrical installations, including EV chargers, must conform to these standards to prevent such dangers.

Efficiency and performance considerations

An undersized cable also causes a greater voltage drop over its length. This means less power reaches your charger, potentially slowing down charging times and wasting electricity as heat. While an oversized cable might seem like a safer bet, it adds unnecessary cost to the installation without providing proportional benefits in efficiency or performance. The goal is to find the optimal balance for your specific setup.

Cost implications of cable choice

Selecting the correct cable size avoids both the immediate cost of an unnecessarily large cable and the potential long-term costs associated with an inefficient or unsafe system. This includes higher electricity bills due to energy loss, or worse, expensive repairs from electrical damage or fire.

Key factors determining cable size

Choosing the right cable size for an EV charger involves several technical considerations beyond just the charger's power rating. These factors are crucial for safety, efficiency, and compliance with UK electrical standards.

Charger power output (kW)

The power output of your EV charger is the primary determinant of cable size. A higher power output (kW) means a higher current (amps), which requires a larger cross-sectional area for the cable to carry the electricity safely. Most home EV chargers in the UK are typically rated at 7.4 kW, drawing around 32amps (A) on a single-phase supply. Some installations, particularly in properties with a three-phase electricity supply, might use 22 kW chargers.

Cable length and voltage drop

The distance from your consumer unit to the EV charger significantly affects cable sizing. Longer cable runs lead to increased electrical resistance and a greater voltage drop. UK electrical standards, such as those outlined in BS 7671, typically recommend that the voltage drop for circuits, including EV chargers, should not exceed 5% to ensure efficiency and safety. If the voltage drop is too high, the cable size must be increased to compensate, even for a given power output.

Installation method and environment

How the cable is installed also impacts its current-carrying capacity. A cable clipped directly to a surface can dissipate heat more effectively than one enclosed in conduit, buried underground, or grouped with other cables. The installation method influences the cable's maximum permissible operating temperature and, consequently, its safe current rating. For instance, a cable buried underground will require a larger size than the same cable run clipped direct, due to reduced heat dissipation.

Ambient temperature and grouping factors

The ambient temperature of the installation environment and whether the cable is grouped with other current-carrying cables further reduce its current-carrying capacity. Higher ambient temperatures mean the cable starts hotter, leaving less margin for temperature rise from the current it carries. Similarly, grouping cables together restricts heat dissipation, necessitating a larger cable size to maintain safety.

Cable size recommendations for common EV chargers

While specific cable sizing requires a qualified electrician's assessment, general recommendations can help homeowners understand the requirements for common EV charger types.

7.4 kW single-phase chargers

For a typical 7.4 kW home EV charger, which draws approximately 32amps, a 6mm² twin and earth cable is often sufficient for shorter runs, typically under 15 metres. However, for longer distances or less favourable installation methods (such as being enclosed in insulation or conduit for a significant portion of its run), a 10mm² cable may be required to prevent excessive voltage drop and overheating. The exact requirement depends on the factors discussed previously, including cable length, installation method, and ambient temperature. Understanding your electricity bill can also help you see the impact of efficient charging.

22 kW three-phase chargers

22 kW EV chargers are significantly more powerful and typically require a three-phase electricity supply, which is less common in UK domestic properties but can be installed. For these chargers, which draw around 32amps per phase, a 6mm² three-core armoured cable is often specified, especially for outdoor runs. Again, cable length and installation conditions will influence the final selection, potentially requiring larger sizes for very long runs.

Understanding cable types: twin and earth vs SWA

The two most common cable types for EV charger installations are Twin and Earth (T&E) and Steel Wire Armoured (SWA) cable.

• T&E: This is a common cable type used for internal domestic wiring. It consists of two insulated current-carrying conductors (live and neutral) and an uninsulated earth conductor, all encased in a grey PVC sheath. It is suitable for indoor installations where it is protected from mechanical damage.
• SWA: SWA cable is designed for outdoor or buried installations. It features a robust steel wire armouring beneath its outer sheath, providing excellent mechanical protection against impact and crushing, as well as protection from rodents. This makes it ideal for runs that are exposed to the elements or buried underground.

Armoured cable for outdoor EV charger installations

When installing an EV charger outdoors or in an exposed location, the choice of cable type becomes even more critical. Armoured cable, particularly Steel Wire Armoured (SWA), is frequently the preferred and safest option.

When to use armoured (SWA) cable

Armoured cable should be used whenever the cable run is exposed to potential mechanical damage, UV radiation, or moisture. This includes:

• Outdoor runs: If the cable is routed along an exterior wall, across a garden, or to a detached garage.
• Buried underground: Any portion of the cable that is buried directly in the ground must be armoured to protect it from spades, garden tools, and ground movement.
• Areas of high traffic or risk: In carports, driveways, or other areas where the cable could be accidentally struck or crushed.

Benefits of armoured cable protection

The primary benefit of SWA cable is its enhanced protection. The steel wire armouring provides a robust physical barrier against accidental damage, which is crucial for safety and longevity in exposed environments. It also offers an additional earth path, further improving the electrical safety of the installation. This robust construction ensures that the cable can withstand the rigours of outdoor conditions, preventing costly damage and potential hazards.

Sizing considerations for armoured cable

Sizing armoured cable follows the same principles as other cable types, but with additional considerations for its installation environment. The current-carrying capacity of SWA cable can be affected by whether it is buried directly in the ground, run in conduit, or exposed to direct sunlight. A qualified electrician will factor in these specific conditions, along with the charger's power output and cable length, to determine the appropriate cross-sectional area for the armoured cable.

UK regulations and professional installation

Ensuring your EV charger installation complies with UK electrical regulations is paramount for safety and legal compliance. This is not a task for untrained individuals; professional expertise is essential.

Compliance with BS 7671 (IET wiring regulations)

All electrical installations in the UK, including EV chargers, must conform to the IET Wiring Regulations (BS 7671). These regulations set the standards for electrical safety and ensure that installations are designed, installed, and tested to prevent electric shock, fire, and other hazards. Adhering to BS 7671 means that the cable sizing, protection devices, earthing arrangements, and overall installation method meet the highest safety benchmarks.

Part p building regulations and certification

Beyond BS 7671, EV charger installations also fall under Part P of the Building Regulations. This means that certain electrical work in dwellings must be carried out by a competent person and certified to the local authority. Upon completion, a qualified electrician will issue an Electrical Installation Certificate, confirming that the work complies with all relevant safety standards. This certification is vital for insurance purposes and when selling your property. Additionally, Approved Document S provides guidance on EV charge point installation and location.

The role of a qualified electrician

Given the complexities of cable sizing, voltage drop calculations, earthing requirements, and regulatory compliance, EV charger installations must be carried out by a qualified electrician. They possess the necessary expertise and certifications to:

• Assess your home's existing electrical infrastructure.
• Calculate the correct cable size based on all influencing factors.
• Select the appropriate cable type and installation method.
• Ensure compliance with BS 7671 and Part P of the Building Regulations.
• Issue the required certification for the work.

Attempting to install an EV charger yourself without the proper qualifications and understanding of these regulations can lead to dangerous and non-compliant installations.

Future-proofing your home EV charging setup

The electric vehicle landscape is evolving rapidly. When installing a home EV charger, it is wise to consider not just your current needs but also potential future developments to avoid costly upgrades down the line.

Considering future EV models and charging speeds

While a 7.4 kW charger might be sufficient for your current EV, future models may offer faster charging capabilities. If your budget and electrical supply allow, installing a cable that can accommodate a higher power output (e.g., suitable for a 22 kW charger, even if you initially install a 7.4 kW unit) could save you from needing a complete cable replacement later. This is particularly relevant if you anticipate owning multiple EVs or upgrading to a vehicle with a larger battery.

Assessing potential grid upgrades

Most UK homes have a single-phase electricity supply, limiting domestic charging to around 7.4 kW. However, some properties, or those undergoing significant renovation, might consider upgrading to a three-phase supply to enable faster 22 kW charging. If this is a long-term possibility, discussing it with your electrician during the initial EV charger installation can help plan for future cable routes and capacities, even if the three-phase upgrade is not immediate.

Planning for smart charging integration

Smart charging allows your EV to charge at optimal times, often when electricity is cheapest or when renewable energy generation is high. While smart chargers handle the communication, ensuring your electrical infrastructure, including the charging cable, is robust and reliable is a foundational step for seamless integration. By ensuring correct and safe cable sizing, Fuse helps homeowners achieve an abundant, worry-free home charging experience without trade-offs in safety or efficiency. This is a foundational step in integrating electric vehicles into a modern, efficient home energy system.

Choosing the correct cable size for your electric car charger is a critical decision that impacts safety, efficiency, and future adaptability. By understanding the key factors involved and entrusting the installation to qualified professionals, you can ensure your home EV charging setup is robust, compliant, and ready for the road ahead. Fuse Energy's professional installation services are backed by 24/7 human customer support, offering peace of mind for complex technical decisions like cable sizing.