EV Charger Load Calculator — Demand Assessment for Electric Vehicle Installations

Electric vehicle chargers place a substantial and sustained load on the electrical supply. Unlike most domestic appliances that cycle on and off or draw variable power, an EV charger draws a near-constant current for several hours — often overnight. Understanding these loads is essential for correct maximum demand assessment and supply capacity verification.

The most common domestic EV charger in the UK is a 7kW single-phase unit, which draws approximately 32A at 230V. This is a dedicated circuit requiring a 32A MCB or RCBO, 6mm² cable for most run lengths, and a Type A (or better) 30mA RCD. The charger typically runs for 4-8 hours per charge, depending on the vehicle battery capacity and state of charge.

For commercial and workplace installations, three-phase 22kW chargers are common. These draw approximately 32A per phase at 400V and can charge an EV in 1-3 hours. Rapid DC chargers (50kW and above) are typically limited to forecourt and public charging locations and require specialist power supplies beyond the scope of standard BS 7671 installation design.

The cable sizing calculator in Elec-Mate includes specific EV charger presets that account for the sustained nature of the load when sizing cables and selecting protective devices.

The choice between single-phase and three-phase EV charging depends on the available supply, the desired charging speed, and the cost implications. Most UK domestic properties have a single-phase 100A supply, which limits EV charging to 7kW maximum (32A at 230V).

Not all electric vehicles can accept three-phase charging. Many popular models in the UK (including some Tesla variants) have single-phase onboard chargers limited to 7kW, even when connected to a three-phase supply. It is important to check the vehicle specification before recommending a three-phase charger installation. The three-phase power calculator can help verify the per-phase current for balanced and unbalanced loads.

The Electric Vehicles (Smart Charge Points) Regulations 2021, which came into force on 30 June 2022, require all new domestic and workplace EV chargepoints to be "smart" — capable of adjusting their charging rate in response to signals. From an electrical design perspective, load management is critical because it allows the charger to reduce its current draw when the total site load approaches the supply capacity.

There are two main types of load management:

Dynamic load management is the preferred approach because it maximises charging speed while preventing supply overload. The Elec-Mate calculator models both approaches and can determine whether load management is needed based on the existing maximum demand and the supply capacity.

When installing multiple EV chargers — for example, in a car park, housing development, or fleet depot — it is unrealistic to assume all chargers will be drawing full current simultaneously. Diversity factors allow the electrical design to account for the statistical likelihood that chargers will be used at different times and at varying power levels.

The IET Code of Practice for Electric Vehicle Charging Equipment Installation provides diversity factors based on the number of chargers:

These factors significantly reduce the total demand calculation for multi-charger installations. For example, ten 7kW chargers without diversity would require a 70kW supply (approximately 300A at 230V). With a 0.50 diversity factor, the assessed demand drops to 35kW (approximately 150A), which may be feasible on an existing supply. The diversity factor calculator in Elec-Mate applies these factors automatically for multi-charger installations.

One of the most critical considerations for EV charger installations is the earthing arrangement. The majority of UK domestic properties use a TN-C-S (PME) earthing system, where the neutral and earth are combined in the supply cable (the PEN conductor) and separated at the origin of the installation.

PME earthing presents a specific risk for EV charging because the vehicle is simultaneously connected to the electrical supply (via the charging cable) and to true earth (via its tyres on the ground). If the PEN conductor were to become open circuit (a PEN fault), the vehicle body could rise to a dangerous potential relative to true earth.

The IET Code of Practice and BS 7671 Regulation 722.411.4.1 require additional protective measures when EV charging equipment is connected to a PME supply. The acceptable solutions include:

The Elec-Mate calculator flags PME earthing requirements automatically when you specify a TN-C-S earthing system and an EV charger installation. For more information on earthing arrangements, see our dedicated guide.

Before installing any EV charger, you must verify that the existing electrical supply has sufficient spare capacity to accommodate the additional load. A standard UK domestic supply is typically 60A or 100A single-phase, provided by the distribution network operator (DNO).

Adding a 7kW (32A) EV charger to a 60A supply is particularly challenging because the charger alone consumes more than half the total supply capacity. If the existing installation already draws 40-50A during peak demand (evening cooking, heating, lighting), the combined load would exceed the 60A supply. In this case, options include:

• Requesting a supply upgrade from the DNO (from 60A to 100A, or from single-phase to three-phase)
• Implementing dynamic load management to limit the charger when other loads are high
• Reducing the charger current from 32A to a lower setting (e.g., 16A / 3.6kW)
• Scheduling charging to off-peak hours when other loads are minimal

The maximum demand calculator helps you assess the existing installation demand, and the EV charger load calculator determines whether there is sufficient headroom for the charger. The results can be documented on the EIC certificate for the charger installation.