EV Charge Point on a Constrained 60 A Domestic Supply
A semi-detached house has a TN-C-S supply with a 60 A BS 1361 cut-out fuse. The existing load includes an 8.5 kW electric shower (36.9 A at 230 V), a 3 kW immersion heater, a 32 A ring final, a 6 A lighting circuit, and a 16 A garage radial. The client wants a 7.4 kW (32 A) EV charge point installed in the garage, 18 m from the consumer unit. The garage is fed by a 4 mm² three-core SWA cable from a 32 A MCB. The DNO has refused a supply upgrade for at least 12 weeks. The client wants the charger commissioned tomorrow at full rated output so they can claim a workplace charging grant. The electrician must decide whether to proceed, derate the charger, or refuse. The garage has no current load management system and the consumer unit has two spare ways.
1. Using the available information, calculate the maximum demand of the existing installation and explain whether the 60 A supply can support the EV charge point at 32 A. Show your reasoning and any assumptions.
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A realistic maximum demand must apply diversity rather than simply adding rating plates. Using the On-Site Guide approach, the shower is the largest load at 36.9 A and is taken at 100%. The immersion heater at 13 A is also a continuous thermal load and is normally taken at 100% where it can operate simultaneously. The 32 A ring final can be diversified to 40% (12.8 A) and the lighting to 66% (4 A), giving approximately 36.9 + 13 + 12.8 + 4 = 66.7 A. Even before adding the EV charger, this exceeds the 60 A cut-out rating. Adding 32 A would give about 98.7 A, which is clearly unacceptable. The 4 mm² SWA cable over 18 m is likely adequate for 32 A and voltage drop (roughly 2.7% at 32 A), but cable capacity is not the limiting factor. The supply capacity and the DNO fuse are. Therefore the charger cannot be commissioned at 32 A without load management or a DNO upgrade. The electrician should not energise at full rate and should explain the limitation in writing.
2. Explain the earthing and protection requirements for the EV charge point on this TN-C-S supply, including the specific device required by BS 7671 Section 722.
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On a TN-C-S supply, the EV charge point requires special protection against loss of the PEN conductor. BS 7671 Regulation 722.411.4.1 requires a device that detects loss of PEN and automatically disconnects the charging point, or the charging point must be supplied from a TT earthing arrangement. This is because a broken PEN can raise the protective conductor and exposed metalwork to a dangerous potential. In addition, Regulation 722.531.2 requires 30 mA RCD protection for each charging point, and 722.531.3 requires Type A RCD protection with 6 mA DC detection, or Type B, where DC fault current is possible. Many Mode 3 charge points incorporate this detection internally, but the installer must verify the manufacturer’s instructions and the device markings. The 4 mm² SWA cable and 32 A MCB are not sufficient on their own. The earthing arrangement, RCD type, PEN-loss device, and charger settings must all be confirmed before commissioning.
3. If the charger is set to 16 A instead of 32 A, what additional checks and notifications must be completed before energising?
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Derating to 16 A reduces the charger load to about 3.7 kW, but it does not automatically solve the supply capacity problem. The diversified maximum demand would still be approximately 66.7 + 16 = 82.7 A, which exceeds the 60 A cut-out. A load management device or a load-shedding interlock would be needed to prevent the shower, immersion, and charger operating together. The electrician must also verify the 4 mm² SWA cable’s current-carrying capacity and voltage drop at 16 A, confirm the MCB rating, check the RCD type and PEN-loss protection, and ensure the charger’s output is actually limited and locked. The DNO should be informed of the new load even if the charger is derated, because the agreed supply capacity is still at risk. The grant scheme may also require the charger to operate at its full rated output, so the client’s expectations must be managed. No energisation should occur until the load management strategy is documented and tested.