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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?

  • Former Community Member
    0 Former Community Member
    John Peckham. Thank you for your contribution. As I understand it the MCB’s primary role is to protect the cable. It’s activation time should be in line with the time the excess current heats the cable. For our MCB it seems that will be some 6 minutes at 50 A. Even at 60 A this will be between one and 15 minutes. So I shouldn’t be overcautious about nuisance tripping.
    I take your point about responsibility and of course an electrical fire in my home is a serious matter for me to ponder. I need to rely on the electrician who installed the cable when the house was constructed has fitted the appropriate MCB and the electrician who carried out the inspection last year properly certified it as so.

  •  I need to rely on the electrician who installed the cable when the house was constructed has fitted the appropriate MCB and the electrician who carried out the inspection last year properly certified it as so.

     


    We must not forget that a cable run is largely not visible and can run through thermal insulation. Also the inspection and tests are often just  small percentage samples of the whole installation, except final circuit testing.  What may have been o.k. then may not be o.k. now with new loads added.


    Z.


  • Further thoughts as I was cooking my chips last night. ?


    My method is to use a chip pan with 2 litres of oil. Heat for 10 minutes over small burner full on. Put chips in oil and turn burner down to low setting. Cook for 7 minutes. Remove chips and turn burner full on. Heat oil for 3 minutes. Immerse chips and carry on for a further 4 - 5 minutes with burner full on.


    So there is an element of diversity just with one pan.


    I am sure that the rating of these modern cookers is a selling point. It's a bit like cars - mine's a 2 litre, well mine's got 3 litres; mine has 200 horsepower ... When does anybody ever use 600 bhp on a public road?
  • 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.
  • Zoomup:

    32 Amp circuit  recommended for the hob.

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


    Z.


    Where did you see the 32A recommendation Zoom? I can't see any mention of it on that page or in the installation instructions - http://media3.bsh-group.com/Documents/9001093674_E.pdf.

    I know some retailer sites make such recommendations, but their choices seem to be limited to either 13A socket or 32A hard-wire.


      - Andy.


  • Former Community Member
    0 Former Community Member
    David – thank you. When I tentatively posted my questions it was to seek advice from trained and experienced members of the Institution. I was looking for evidence either for or against my assumption that the installation was both safe and sensible. If electricians with years of experience had evidence or had come across real life problems it would have helped me to decide whether to persist with our choice. None of the replies has presented any evidence of problems though some have suggested caution. It is troubling to find that each of the three kitchen fitting companies we approached were aware that electricians would expect an extra cable to be laid for our choice of cooking appliance (we didn’t find this out until we had made our choice). The electrician who made a site visit’s comment about indemnity and guarantee was made after I had questioned his reasoning and I took it to be a threat or attempt to intimidate. I have sent email requests for a quote giving a brief outline of our request to 13 local electricians who have websites. Four have replied. Three told me that it wasn’t possible. One was prepared to consider it and seems to have backed out once I gave more details. Most householders would simply accept the statement from the electrician – we did not have that option.  Those householders will have wasted both money and resources. I have found one electrician who has said he agrees with my assessment of the problem whom I am hoping will call back on Tuesday. If he doesn’t I have a lot more research to do to find someone who will accept the job.
    I am not an electrician, I have not been trained, not passed an exam nor do I have years of experience. Questioning someone who does must be done tactfully if you want them to actually do the job. I hope that my question has allowed David’s experience and knowledge to be more widely shared throughout the community.
    My values for the MCB tripping time were taken from https://docs.rs-online.com/f0bf/0900766b8047c04e.pdf as we have a Wylex SBB40 fitted. I didn’t know about the source that you quote.

  • Former Community Member
    0 Former Community Member
    Sorry - a Wylex NSB40
  • Hi Louie

    I'm afraid you have come across the problem we have with many electricians, they are keen to give opinions but only to their advantage! The problem we have is that the training is pretty poor and the exams that need to be passed are pretty trivial really and need very little understanding of the science behind the electrical system as a whole. This is illustrated by the EICR, electrical installation condition report, which is needed now for privately rented properties. Many of those produced (and I have a big pile of them here) are rubbish, the person issuing them has little understanding of the wiring regulations (BS7671) and in many cases report non-problems to get installation or repair work. There are a large number of electricians who think that a cable running at a few percent above the regulation rating will instantly catch fire, and have no understanding of the physics behind the magic number tabulated by BS7671. Whilst deliberate undersizing of conductors is unwise, the way that this should be approached is by means of the load profile, and the outcome of reasonableness is known as diversity. Deciding on this is very much an engineering judgement, although various suggestions are made in the "On-site Guide" and various other places. It is important to understand the features of the load, and its usage, as I have attempted to describe to you above. The whole subject is likely to become a matter of public difficulty soon, as the electrical supply to houses depends on a high degree of diversity, the supply available if all houses are using power is only 7-8 amps maximum, and electric car charging uses considerably more than this. The result will be a lot of blown substation fuses, power cuts and cable damage under the streets unless a great deal of new supply work is carried out. It has been estimated that £3 trillion will have to be spent if we shift to entirely electric vehicles, and there end up a similar number to cars etc. now. The Government do not seem to understand the problem, therefore is at least as bad as the electricians! I'm afraid they think that electricity grows on Green trees.


    David CEng etc.
  • biglouie:
    I am not an electrician, I have not been trained, not passed an exam nor do I have years of experience.


    OK, the majority viewpoint in here is that the existing circuit is suitable.


    So what work needs to be done?


    And who is contracted to do what?


  • AJJewsbury:
    Zoomup:

    32 Amp circuit  recommended for the hob.

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


    Z.


    Where did you see the 32A recommendation Zoom? I can't see any mention of it on that page or in the installation instructions - http://media3.bsh-group.com/Documents/9001093674_E.pdf.

    I know some retailer sites make such recommendations, but their choices seem to be limited to either 13A socket or 32A hard-wire.


      - Andy.




    I found this listing.....


     




    Bosch PXV851FC1E Induction Hob, Black



     

    (5)

    Induction hobs heat the pan not the hob using an electrical current. Induction hobs can heat a pan faster than a traditional gas or electric hob, and are more energy efficient. They’re also easier to clean, as you can wipe the hob shortly after using it. Remember, not all ceramic hobs are …

     

    "Amperage: 32A

    Aperture needed: H5.1 x W75 x D50cm

    Brand: Bosch

    Burner Power: 1x 1.8kW, 1x 1.4kW, 1x 2.2kW, 1x 3.3kW or 2x 2.2kW

    Cable Included: YES

    Control Position: Front"


    Z.