Kitchen Hob Replacement in a 230 V Apartment
An apartment renovation in Tel Aviv has a single-phase 230 V supply, a 25 A main incoming circuit breaker and a 30 mA RCD at the origin. The owner wants to replace a gas hob with a 7.2 kW induction hob and keep the existing 3.0 kW water heater on the same kitchen final circuit. The existing circuit is a 2.5 mm² Cu radial in 20 mm conduit, 18 m long, protected by a 16 A MCB. It is grouped with two other loaded circuits in the same trunking, and the ambient temperature in the service shaft is 35 °C. The hob instructions require a dedicated 32 A supply. The contractor proposes simply changing the 16 A MCB to 32 A and using the existing cable to save time and avoid chasing walls. The client asks whether this is acceptable and what the minimum compliant alternative is.
1. Calculate the design current of the hob and check whether the existing 2.5 mm² circuit, after correction for ambient temperature and grouping, can carry it. State the outcome clearly.
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The hob design current is Ib = 7200 W / 230 V = 31.3 A. The existing 16 A MCB is already smaller than the load current, so the circuit is overloaded even before cable correction. For the cable, a typical 2.5 mm² Cu PVC-insulated conductor in conduit has a base Iz of about 24 A at 30 °C. Apply Ca ≈ 0.94 for 35 °C and Cg ≈ 0.80 for three grouped loaded circuits: Iz = 24 × 0.94 × 0.80 ≈ 18.0 A. Even without grouping, the corrected value is only about 22.6 A. Therefore a 32 A MCB would not protect the 2.5 mm² cable, and the cable cannot carry 31.3 A. The proposal is non-compliant. The existing circuit may remain for the water heater only if its own load and protection are verified, but the hob needs a new dedicated circuit.
2. Propose a compliant dedicated circuit for the hob and verify its conductor size by voltage drop (use 6 mm² Cu, 18 m, 32 A; mV/A/m ≈ 7.3). Include protection and isolation requirements.
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Install a dedicated 32 A single-phase circuit from the apartment board to a local isolator near the hob. Use 6 mm² Cu in a separate conduit so that the grouping factor does not apply. Its base Iz in conduit is about 40 A; at 35 °C, 40 × 0.94 ≈ 37.6 A, which exceeds the 32 A breaker rating and the 31.3 A load. The voltage drop is approximately 31.3 × 18 × 7.3 / 1000 = 4.1 V, or 1.8% of 230 V, well within the usual 5% limit. Protect with a 32 A MCB and ensure 30 mA RCD protection is present; the existing origin RCD may be used only if it is verified and selective with any downstream RCD. Provide a local isolator accessible to the user and label the circuit clearly. Do not connect the water heater to this circuit.
3. The water heater remains on the old 16 A circuit. Explain why the 30 mA RCD does not remove the need for correct overcurrent protection and why selectivity with the main 30 mA RCD matters.
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An RCD responds to residual current to earth; it does not detect overload between live conductors. A 3.0 kW water heater draws about 13 A, so the 16 A MCB and 2.5 mm² cable may be adequate if the corrected Iz exceeds 16 A, but the breaker must still protect the cable against overload and short circuit. The RCD cannot substitute for that. Selectivity matters because if the main 30 mA RCD and a downstream 30 mA RCD are in series, a fault may trip either or both. That can remove power from unrelated circuits and make fault finding difficult. Use a time-delayed selective upstream device where required, or separate RCD groups, and verify discrimination by test. The final circuit must remain protected for both overcurrent and earth fault.