Combi Oven Cable Sizing in a Restaurant Kitchen
A restaurant kitchen is adding a 24 kW, 380 V three-phase combi oven with a power factor of 0.95. The oven will run for about four hours per day. The proposed supply is a 10 mm² four-core XLPE/SWA cable, 28 m long, installed in a steel trunking with four other loaded circuits. The trunking is in a ceiling void where the ambient temperature is 38 °C. The existing sub-board has a spare 63 A Type C MCB. The installer has allowed for the oven's design current but has not applied ambient-temperature or grouping factors, arguing that the 63 A MCB will protect the cable. The kitchen also has a 12 kW three-phase fryer and a 6 kW single-phase water heater on the same sub-board. The client wants the oven connected without changing the sub-board, because the kitchen must reopen the next morning. The cable route passes through a 2 m section of thermal insulation near the oven.
1. Calculate the oven design current and evaluate whether the proposed 10 mm² cable is adequate. State the correction factors and the cable size you would require.
Read the worked answer
The design current is Ib = 24000 / (√3 × 380 × 0.95) = 38.4 A. Using tabulated values for the selected cable type and installation method, assume a 10 mm² four-core XLPE cable has It = 57 A. The ambient correction factor at 38 °C is about Ca = 0.87, and the grouping factor for five loaded circuits in trunking is about Cg = 0.6. The derated capacity is therefore Iz = 57 × 0.87 × 0.6 = 29.8 A, which is below the 38.4 A design current, so 10 mm² is not acceptable. A 16 mm² cable with It = 76 A gives 76 × 0.87 × 0.6 = 39.7 A, which is marginal once the thermal insulation section and actual grouping are considered. A 25 mm² cable with It = 96 A gives 96 × 0.87 × 0.6 = 50.1 A, which is adequate. The voltage drop for 25 mm² is approximately 38.4 × 28 × 1.5 / 1000 = 1.6 V, or about 0.42% of 380 V, so voltage drop is not the limiting factor. The 2 m thermal insulation section must also be assessed, and the cable rerouted or further derated if necessary.
2. The installer wants to retain the existing 63 A Type C MCB. Is this acceptable? Explain the overload and fault-current requirements.
Read the worked answer
The 63 A MCB cannot be retained. For overload protection, the circuit must satisfy Ib ≤ In ≤ Iz. The oven design current is 38.4 A, but after derating the selected 25 mm² cable has Iz of about 50 A. A 63 A device is greater than Iz, so the cable could be overloaded before the MCB operates. A 40 A Type C MCB is appropriate for Ib = 38.4 A and Iz ≈ 50 A. Its earth-fault loop impedance must be verified to ensure disconnection within the required time. The 2 m thermal insulation section must be assessed; if the final derated Iz falls below 40 A, a larger cable or a different route is needed. The 63 A MCB may also fail to discriminate with upstream devices and may not protect the cable against sustained overload. The immediate answer is: replace the 63 A MCB with a 40 A device and verify Zs, or use a 50 A device only if the final derated Iz is at least 50 A.