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Induction Hob again

Former Community Member
Former Community Member
I hope that you don’t find me impudent but I am having difficulty finding an electrician to explain why they will not install the induction hob and double oven that we are looking to fit in our new kitchen.
We have chosen a hob 7.4 kW and oven 6.3 kW. The cooker radial circuit has 6mm2 cable with 40A MCB. The run is some 12 m. The house was built (converted) in 2002 and inspected last year (DPN18C). We cannot run an extra cable because the CU is separated from the kitchen without a horizontal floor or roof space and presumably runs the existing cable through the stud walls.
The kitchen fitting company electrician has visited but won’t carry out the work unless we sign an indemnity as it would invalidate our insurance and the appliance guarantee. He suggested instead two plug-in ovens which we could add to the ring main. I have contacted four other local electricians but they all use 13.7 kW means 59.7 A which means 10 mm cable and ignore my request to consider diversity.
Should I give up and accept the two oven solution or perhaps a gas oven?
Do you know of a way of finding someone who understands diversity?
Am I simply wrong?

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    I refer you all to fig 3A4 in the regs. where you will see that at 60A your (presumably a 40A BS60898 type B) will take 1000 seconds to trip, so around 17 minutes. This figure will be far from exact, and this number should be the worst-case so may be somewhat shorter. At 50A it will probably never trip. I don't know where these numbers you are quoting come from Louie, but you are actually quite safe. To start a fire the cable will need inflammable material and a temperature of 4-500 degrees centigrade (red heat), you can never achieve that unless completely surrounded by excellent insulation and overload at 60A for a long time. Cables do not start fires, they will melt and short out long before anything else, and particularly if the circuit is RCD protected sufficient temperature is not possible. As the overload cannot be more than 17 minutes and this temperature cannot be achieved, even with insulation. It may damage the cable (very unlikely) but the only outcome is shorter cable life.


    Looking at table 4D2B you will see that the volt drop of 6mm cable is 7.3 mV/A/m so at 60 A and cable at 70 degrees C the volt drop per metre is 438 mV. The power loss is then 26.3W /m, at this current, which as you can see is not great. Compare the temperature with a 25W filament bulb glass, which is much more compact and you will get the idea. Even with good insulation, you will see that the rating reduces by about 25%, and this is for a continuous load for a long period. (Ref method A).


    From the above, you will see that the diversity calculation is perfectly reasonable, and it has proved satisfactory for a very long time. Even at the maximum temperature your ovens will still be thermostat limited, so not be continuous, as otherwise, the oven would take a very long time to heat up. In reality, the circuit should not trip, and will not be subject to excessive temperature.


    I hope this explains diversity sufficiently to make everyone happier with all types of circuits and loads. Danger comes from long term smallish overloads, the long term being many hours. A few minutes here or there is not a problem.
Reply
  • 0
    I refer you all to fig 3A4 in the regs. where you will see that at 60A your (presumably a 40A BS60898 type B) will take 1000 seconds to trip, so around 17 minutes. This figure will be far from exact, and this number should be the worst-case so may be somewhat shorter. At 50A it will probably never trip. I don't know where these numbers you are quoting come from Louie, but you are actually quite safe. To start a fire the cable will need inflammable material and a temperature of 4-500 degrees centigrade (red heat), you can never achieve that unless completely surrounded by excellent insulation and overload at 60A for a long time. Cables do not start fires, they will melt and short out long before anything else, and particularly if the circuit is RCD protected sufficient temperature is not possible. As the overload cannot be more than 17 minutes and this temperature cannot be achieved, even with insulation. It may damage the cable (very unlikely) but the only outcome is shorter cable life.


    Looking at table 4D2B you will see that the volt drop of 6mm cable is 7.3 mV/A/m so at 60 A and cable at 70 degrees C the volt drop per metre is 438 mV. The power loss is then 26.3W /m, at this current, which as you can see is not great. Compare the temperature with a 25W filament bulb glass, which is much more compact and you will get the idea. Even with good insulation, you will see that the rating reduces by about 25%, and this is for a continuous load for a long period. (Ref method A).


    From the above, you will see that the diversity calculation is perfectly reasonable, and it has proved satisfactory for a very long time. Even at the maximum temperature your ovens will still be thermostat limited, so not be continuous, as otherwise, the oven would take a very long time to heat up. In reality, the circuit should not trip, and will not be subject to excessive temperature.


    I hope this explains diversity sufficiently to make everyone happier with all types of circuits and loads. Danger comes from long term smallish overloads, the long term being many hours. A few minutes here or there is not a problem.
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