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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?

  • 0
    I am with the electricians on this. Putting both appliances on one cable that is protected by a 40A MCB could cause the circuit breaker to trip under full load conditions. Quite rightly they want to protect themselves against any claim and want to pass that liability to you. You appear not to want to accept that risk but expect them to do so.


    The over riding Regulation in BS 7671 requires the designer and installer to take notice of the manufacturers instructions. What does the manufacturer say about the cable and the supply circuit breaker for each appliance?


    How is the cable run, in conduit or trunking?


    Diversity should not be applied to circuits supplying individual appliances to select the supply cable. The cable should be sized in accordance with the manufacturers instructions or the plated rating. Diversity factors should be used for distribution circuits or the whole installation. Yes I know that the OSG says something that reads differently but I would not mind betting that will change in the next edition.
  • 0
    Induction hobs are not any different as far as loading goes. I would point out to John that all multiple socket circuits do size the cable based on diversity (in that the total load is completely unknown, but often if all the sockets are fully loaded could be many times the breaker rating). In the case of fixed appliances, this is slightly less common but not prohibited. In the case of hobs and cookers there is a huge diversity effect from the thermostatic temperature control, and whilst in this case, it may be possible to trip a 40A breaker this would take a considerable period. Induction hobs take power in response to the size of the pan and the temperature set, so several 'rings' at full power for long is unlikely, as is the oven being on maximum for more than 5-10 minutes. I think that trips are pretty unlikely. I suppose a 10mm cable and 50A breaker is possible, but not worthwhile unless your cook is unusual in the methods (turn everything on at once with a full complement of large pans of cold water!). If you find the slight risk of the occasional trip using the above method, I suggest that one cable as described is adequate, unless your wall is stuffed with insulation. As you appear to have no cable route, fitting an extra cable is either via an outside route, or impossible so you will have to go with one anyway. It would take somewhere around an hour to trip with full load continuously, this would be extremely difficult to achieve with the appliances you have specified. I would have no difficulty with this arrangement, even in a commercial kitchen.
  • 0
    biglouie:

    Half way between Keighley and Skipton?


    Very nice indeed, but a bit far for me to go from the south coast!


  • 0
    davezawadi (David Stone):

    Induction hobs take power in response to the size of the pan and the temperature set, so several 'rings' at full power for long is unlikely, as is the oven being on maximum for more than 5-10 minutes.


    I take JP's point and if the manufacturer's instructions say it should be on something bigger than 6 mm², there is a problem. However, I very much doubt that they will say 6 mm², but not shared with anybody else.


    I have two cookers, an old commercial range (70 kW), which came with the house, and a commercial combination microwave (3 kW). I allow 20 minutes for the big oven to get Yorkshire pudding-hot. It's nearly Easter, so if I put in a lamb, the temperature would drop and the burners would probably go full on for another 10 minutes, by which time they will turn down and maintain the temperature. At equilibrium, the burners are only chugging along at the rate at which the insulation allows heat to escape. Similarly, once your pan of potatoes is boiling, the only heat required is to match the losses through the sides and top of the pan (+/- lid).


    I think that the OSG diversity figures are pretty reasonable.


  • 0
    I am not convinced that the standard diversity calculations for an electric cooker are applicable.in this case.

    The oven alone has a calculated loading of about 27 amps.

    Consider the following, What size circuit would you instal for a single load of 6.3 KW or 27 Amps ? I would say 32 amps.

    Now consider if some EXTRA load (the hob) is added to the circuit, does this really mean that a SMALLER circuit is now acceptable ?


    So common sense rules out a 25 amp circuit, as being insufficient for the oven alone.

    Common sense also rules out a 32 amp circuit since that only allows 5 amps for the hob if the oven is used.


    In my view SOME diversity can be allowed, but dont overdo it.

    A 40 amp circuit will probably be fine under all but exceptional use. 27 amps for the oven and 13 amps for the hob. It is not that likely that the hob will draw more than 13 amps, other than briefly, and even less likely that such intensive use of the hob will conicide with oven being being run at maximum temperature.


    A 50 amp circuit would be better if heavy use is anticipated.

    A 60 amp circuit would be OTT.
  • 0
    What size circuit would you instal for a single load of 6.3 KW or 27 Amps

    In this case though it won't be a single 6.3kW load - a single domestic oven doesn't take anything like that - most likely a double oven, perhaps including a grill in one of them - so more like 3off 2kW loads or at worst 2off 3kW loads - so I'd say there's a good case for diversity even within the one "oven".


       - Andy.
  • 0
    32 Amp circuit  recommended for the hob.

    https://www.bosch-home.co.uk/product-list/cooking-baking/hobs/induction-hobs/PXV851FC1E


    Z.
  • 0
    Diversity should not be applied to circuits supplying individual appliances to select the supply cable. The cable should be sized in accordance with the manufacturers instructions or the plated rating. Diversity factors should be used for distribution circuits or the whole installation. Yes I know that the OSG says something that reads differently but I would not mind betting that will change in the next edition.

    That sounds like a retrograde step to me. The 30% after 10A rule for a final circuit feeding a domestic cooking appliance has been in existance since at least the 15th Ed (it was actually part of the regs in those days - table 4A) - so has certainly stood the test of time. Most of the manufacturer's installation instructions I've see don't give any advice about diversity at all - just vague requirements for a "suitable" circuit, "according to national wiring rules" (which is understandable as the methods of connection varies greatly from nation to nation - some would use a 2 or 3-phase 16A circuit, rather than everything on a single phase as we do). If the suggestion is that we can't use the cooker diversity rule for the final circuit, but can for a distribution circuit that feeds it, you can end up in the situation where the distribution circuit can have a lower rating than the final circuit it supplies - which would invite ridicule to say the least.


       - Andy.
  • 0
    "Half way between Keighley and Skipton?"


    Not far from me, I`m over the border near Pendle Hill;.

    Unfortunately I`m partly (mostly) out of action due to arthritis and sciatica for the foreseeable future
  • 0
    AJJewsbury:
    What size circuit would you instal for a single load of 6.3 KW or 27 Amps

    In this case though it won't be a single 6.3kW load - a single domestic oven doesn't take anything like that - most likely a double oven, perhaps including a grill in one of them - so more like 3off 2kW loads or at worst 2off 3kW loads - so I'd say there's a good case for diversity even within the one "oven".


       - Andy.


    I was presuming a single large oven of 6.3 Kw loading. If in fact this is  two ovens or two ovens and a grill with a total 6.3Kw loading, then I agree with your remarks.

    Two ovens and a grill, and several cooking rings is indeed comparable to a standard domestic cooker, even if the components are in seperate cases.

    32 amps might be reasonable in such a case, such circuits have a good record in practice. 40 amps might be better if heavy use is expected, and 50 amps is almost certainly OTT.


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