Cooker Circuit Through Insulated Wall
A kitchen refurbishment adds a 7.2 kW induction hob and a 3.6 kW built-in oven, both 230 V. The existing cooker circuit is 6 mm² copper twin-and-earth protected by a 32 A Type B MCB. For most of its 18 m run the cable is clipped direct to a masonry wall, but for 4 m it passes through a thermally insulated stud wall inside conduit, bunched with two other loaded circuits. The manufacturer’s data give 6 mm² clipped-direct current-carrying capacity as 47 A; the installation method in the insulated wall has a derating factor of 0.50, and grouping with two other circuits gives a further factor of 0.80. A 13 A socket is not incorporated in the cooker control unit. The client wants to keep the existing cable and MCB to save money. Diversity may be applied to cooking appliances: 10 A of the first appliance plus 30% of the remainder, plus 5 A if a socket is included. Ambient temperature is 30 °C.
1. Apply the diversity rule to calculate the design current for the hob and oven.
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Total connected load = 7.2 + 3.6 = 10.8 kW. Current = 10,800 / 230 = 46.96 A. Diversity: 10 A for the first appliance plus 30% of the remainder, with no socket allowance because the control unit has no 13 A socket: Ib = 10 + 0.30 × (46.96 − 10) = 10 + 11.09 = 21.1 A. The design current is therefore about 21 A, not 47 A. The diversity rule is a design allowance, not a guarantee that both appliances will never run together; the final circuit and cable must still be selected for the calculated design current and the manufacturer’s instructions.
2. Calculate the effective current-carrying capacity of the 6 mm² cable in the insulated section and decide whether the existing 32 A MCB and cable are acceptable.
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Effective capacity in the insulated section: Iz = 47 × 0.50 × 0.80 = 18.8 A. The design current is 21.1 A, so the cable is overloaded in that 4 m section before any allowance for the 32 A MCB. The 32 A MCB also cannot protect a cable whose effective capacity is 18.8 A. The existing arrangement is not acceptable. Options: reroute the cable out of the thermal insulation; reduce grouping; use a cable with a higher tabulated capacity, such as 10 mm²; or provide a separate circuit. If 10 mm² clipped direct is rated, for example, 63 A, the derated capacity would be 63 × 0.50 × 0.80 = 25.2 A, which exceeds 21.1 A, but the exact value must be taken from the cable manufacturer’s table and the protective device must still be selected to protect the cable. Do not simply keep the 32 A MCB and hope the diversity figure covers the insulation problem.
3. State what you would record and how you would explain the decision to the client, including one alternative that preserves the existing cable.
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Record: the connected load and diversity calculation; design current; cable size, route and length; the derating factors and their source; tabulated and effective Iz; protective device rating; Zs and insulation resistance results; and the date and signature. Explain to the client in plain language that the cable in the insulated wall can carry only about 19 A, while the calculated demand is about 21 A, so the cable could overheat and damage the insulation or start a fire. The 32 A breaker will not prevent this because it is set above the cable’s derated capacity. Offer the reroute as the least disruptive option, or a 10 mm² replacement with verification. Put the advice and the client’s decision in writing.