Domestic Maximum Demand and a New 7 kW Car Charger
A 230 V single-phase domestic consumer unit is supplied through a 100 A main fuse and 25 mm² tails. The installed circuits are: lighting totalling 1.2 kW; two 32 A ring final circuits; a 10 kW cooker; an 8.5 kW instantaneous shower; and a 3 kW immersion heater. The owner wants to add a 7 kW car charger and argues that because the main fuse has never blown, the supply is adequate. Using the standard diversity guidance: lighting 66%; socket outlets 100% of the first ring and 40% of the second; cooking appliances 10 A plus 30% of the remainder; instantaneous water heaters and immersion heaters 100%. The car charger is a continuous 7 kW load and no load management is installed.
1. Calculate the diversified maximum demand before and after adding the car charger.
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Lighting: 1.2 × 0.66 = 0.792 kW. Sockets: first ring 32 A × 230 V = 7.36 kW; second ring 7.36 × 0.40 = 2.944 kW; total 10.304 kW. Cooker: 10 000/230 = 43.48 A; diversified current = 10 + 0.30 × (43.48 − 10) = 20.04 A = 4.61 kW. Shower = 8.5 kW; immersion = 3 kW. Total before charger = 0.792 + 10.304 + 4.61 + 8.5 + 3 = 27.21 kW. At 230 V: 27 206/230 = 118.3 A. After adding 7 kW: 34.21 kW, or 148.7 A. Even before the charger the diversified demand exceeds the 100 A fuse, and with the charger it is far above it.
2. Assess the 100 A main fuse and 25 mm² tails against the calculated demand. Is the owner's argument valid?
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The calculated diversified demand of 118.3 A already exceeds the 100 A fuse, and 148.7 A with the charger is well outside its rating. The 25 mm² tails are also loaded beyond their normal clipped-direct capacity at 148.7 A. The owner's argument is not valid: a fuse not blowing does not prove the installation is adequately designed, because diversity assumes not all loads run simultaneously and because fuses have thermal tolerances. Diversity guidance is a design aid, not a licence to exceed the supply capacity. Continuous loads such as the car charger are particularly onerous and must be assessed at 100% of their rating, not averaged away. The installation requires a supply upgrade, load management or a change in the load pattern before the charger is connected.
3. Recommend compliant options for installing the charger, and state what must be confirmed with the supply authority.
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Options include: (1) install a load-management system that measures the total incoming current and limits or interrupts the charger so the supply is not exceeded; (2) use an interlock so the charger cannot operate with the shower or immersion heater; (3) replace the immersion heater with a lower-demand or off-peak source; or (4) upgrade the supply, for example to a three-phase or higher-capacity single-phase service. The charger circuit itself must be designed with the correct cable size, a 30 mA RCD suitable for DC leakage where required, and a protective device rated for the charger. The supply authority must confirm the maximum permitted import, the fuse rating and the earthing arrangement. A competent person must also verify that the existing consumer unit has spare capacity and that the earthing and bonding can carry the increased fault current.