Induction hob cable size

Discussion in 'Electricians' Talk' started by CraigMcK, Feb 27, 2017.

  1. CraigMcK

    CraigMcK Screwfix Select

    Looking for an opinion on cable size for new induction hob

    Total load is 7400w @ 230v = 32.17A, distance from CU about 10m

    So... 32A 6mm or 40A 10mm

    Thanks
     
  2. Dr Bodgit

    Dr Bodgit Super Member

    If the total is 32A, then with diversity you'll be fine with 6mm on a 32A MCB. However if it were me, and its a new run of cable, then I'd put in 10mm to make it future proof.

    Is there also an electric oven to wire in?
     
  3. PaulBlackpool

    PaulBlackpool Screwfix Select

    Just go off the manufacturers recommendations as per the instructions.
     
  4. KIAB

    KIAB Super Member

    I would run 10mm with 32A mcb, you then have option of 40A if upgrade cooker in future.
     
    Dr Bodgit likes this.
  5. CraigMcK

    CraigMcK Screwfix Select

    Thanks gents

    Only hob on this run, oven is a separate circuit
    Funnily AEG's instructions only specify the 7.4kw and cable type, nothing else:

    upload_2017-2-27_11-1-40.png
    upload_2017-2-27_11-2-4.png
     
  6. fire

    fire Well-Known Member

    An 32Amp MCB has a bimetallic strip in it which heats up when it hits or exceeds 32Amp to cause the breaker to disconnect. If you nominally draw 32.2Amp then the over-current bimetallic strip in the 32Amp breaker will be constantly heating up, maybe not enough to open the breaker but it will be warm during full load. Your current loading is too close to a 32Amp breaker.

    With that in mind, use 10mm2 40Amp to handle the overhead if needed. As an appliance ages it tends to draw slightly more power, falls out of spec so to speak.
     
    KIAB likes this.
  7. unphased

    unphased Screwfix Select

    I really don't understand why people think that 32A standard cooker circuits wired in 6mm2 need to be changed. It is extremely unlikely you would need a 40A circuit wired in 10mm2. It just isn't necessary in the vast majority of cases. Better to have two 32A circuits than one 40A if you want to 'upgrade'. It's impractical to terminate three wires that size in one outlet.
     
    Mr Milliamp likes this.
  8. unphased

    unphased Screwfix Select

    Fire you are forgetting that mcbs will quite happily carry 1.4 X their rated tripping current before tripping. You are making up that rubbish aren't you.! Forum opinion rather than fact. You aren't a sparky are you, it's obvious.
     
  9. Lectrician

    Lectrician Screwfix Select

    MCBs have two tripping mechanisms, thermal and magnetic. The magnetic part will operate instantaneously at 5x for a B type, 10x for a C type and 20x for a D type. This is were the max Zs figures come from. The thermal trip will operate at a current above the rated current, up to the "instantaneous" trip point where the magnetic part takes over. If you draw over the maximum rating, but less than the instantaneous tripping point, it will take a long while to trip, while the thermal slug heats up and operates - You look at the current curves to determine how long this would be. However, you would never allow an MCB to be regularly overloaded, and this is a requirement in the regs. The 1.45X mentioned above refers to how much you may "overload" a cable before operation of the MCB, and goes hand in hand with two other statements which state the protective device must not be less that the design current and that the protective device must not be greater than the current carrying capacity of the cable. An MCB is designed to comply with all three requirements when installed as a standard circuit.

    I would be more than happy with the hob on a 32amp circuit, assuming no oven with it, although if it was a new circuit and not existing, foolish not to up it.

    Cooking habits and appliances have changed since the OSG diversity guide lines were written.
     
    fire and KIAB like this.
  10. KIAB

    KIAB Super Member


    Spot on.

    I knew a women a few years ago had a 3 phase supply in her domestic kitchen, the combintion oven was huge, was around a 18Kw- 20Kw load.:eek:
     
    Last edited: Feb 28, 2017
  11. Dr Bodgit

    Dr Bodgit Super Member

    I stand by my initial comments - 6mm and 32A will be fine but if you're putting in a new cable, why wouldn't you up it to 10mm. Same labour cost, just a bit extra for the cable and future proofed (or extra flexibility).
     
    KIAB likes this.
  12. unphased

    unphased Screwfix Select

    Its not future proofed at all. You simply use a standard cooker circuit. Those who increase the cable size to 10mm2 are unfamiliar with domestic cooker circuits and are wasting money; it wont achieve any benefit whatsoever.
     
    seneca likes this.
  13. seneca

    seneca Screwfix Select

    Agreed UP, 10mm for a standard domestic cooker circuit is a total waste of money (and copper!)
     
  14. Mr Milliamp

    Mr Milliamp Member

    As UP said, 10mm is a nightmare to terminate. Especially in a little CCU with another cable. Don't know why they don't create an 8mm cable, 6 to 10 is nearly double.
     
    seneca likes this.
  15. fire

    fire Well-Known Member

    Picture speaks a thousand words.

    [​IMG]

    although we are only talking mA over the rated current, it does not change the way a MCB is designed and how it's thermal over-current protection operates. Manufacturers have to make MCB's to a particular tolerance, that said that tolerance causes slight different characteristics across MCB's.

    They call it BiMetal and i call it Bimetallic, they mean the same thing. A continuous all be it small over-current will inevitably heat up the Bimetallic strip, it may take longer than a minute to trip if it ever trips as the heating element may not be in operation long enough but again depending on the quality and tolerances the MCB is made to will effect the behaviour.

    The point is as i mentioned in my last post, over-current within the thermal zone of the MCB's characteristics will cause the bimetallic strip to heat up. As you can see from my sourced linked in picture, it is fact and not opinion.

    Magnetic operastion is based on the Type B, C, D... which is either 3-5x Ir 5-10x Ir etc.

    Thermal operation occurs based on time at any current over Ir the more over-current the faster it will open, the less current the longer it will take to open.
     
    Lectrician likes this.
  16. CraigMcK

    CraigMcK Screwfix Select

    Fire
    Just to clarify, from some point of authority as I worked in a circuit breaker manufacturing company for 20 years. The bimetal starts to bend after any load is applied, the MCB is calibrated through the use of standard bimetals per rating and an adjustment screw to then trip at the correct time within the constraints of the standard B,C,D (and others) curves. For others clarity the thermal element of the curves is identical across all the types
    MCB's will generally not trip at anything below 112% of their rating, so a 32A MCB will never trip below ~36A'ish. Most are calibrated a bit higher.


    The main reason for the question was at what point over 32A would people say it needs to go up to the next rating. I do understand the chances of an induction hob ever running at 7400w for any period is slim, but I'm not overly familiar with how they work and whether they are in essence adjustable load or the adjust the heating effect through on / off switching
     
    BiancoTheGiraffe likes this.
  17. fire

    fire Well-Known Member

    I completely agree with that.

    Hobs particularly induction hobs don't turn on and off like a ceramic hob does. They don't have the conventional method of temp sensing as an induction hob is a large electro-magnet. An induction hob is very energy efficient, it transfers over 90% of the electrical energy it uses into the heating up of your food/water etc.
    If it was a 4 induction hob and all 4 were used simultaneously at the maximum setting i'd have said you would be pulling the maximum. Many high quality induction hobs can boil 1 litre of water in less than or near 60 seconds.

    Would you be using all 4 hobs at max for an hour lets say? Chances are probably not, 4 pans each with 2 litres of water in them on full blast run for less than 2 minutes then yes you would need to pul 36Amps too trip the breaker in that condition.
    I brew my own beer so i use quite allot of energy all be it Gas but if i used electric then there could well be a possibility i would be hitting the max Ir for a prolonged period of time. 20 Litres of wart would take far longer than 2 minutes to a rolling boil.

    You have approx 5w power loss(depending on manufacturer) through a 32Amp breaker run at 32Amps, again you have your cable resistance over a 10m run.

    The actual standards for the multiple for over-current is ~x1.1 to disconnect in less than 60 minutes and x1.5 for 1 minute disconnection and less than 5 seconds at 3x for a type B.

    So baring in mind as you correctly stated, the Bimetallic strip is always warm bellow the Ir any current above Ir regardless of it being mA or higher would heat up even more, waste more energy in heat inside the consumer unit, higher heat in a copper conductor = higher resistance which = more heat still. Ok all well within specs but as my original post was intended to bring the OP awareness to what is going on inside a MCB as so he could make an educated decision.

    Little did i expect an insult to follow from a fellow forum community member... lol.

    Larger cable = less resistance for same length meaning less power loss and larger breaker run at lower current than it's Ir also = less heat = less power loss. <- of course this is overthinking it and extreme idea but just thought i'd mention it.

    Furthermore, manufacturers guidelines state that the breaker should not be continuously loaded approaching it's nominal rated current when mounted in an enclosure. Also Wylex state that a 66% diversity policy is applied to breakers which will operate at their nominal current rating for longer than 1 hour. Also Wylex rating for ambient temperature of 20'c is x1.0 so 32.2Amp would be against the manufacturers instructions if you used Wylex.

    So again regardless of the calculated facts, if the answer is that the Hob will be continuously operated at full power you would be better off using 10mm2 into a 40Amp breaker... If you can safely call it so that it won't be operated like this then the smaller size would be fine, making this decision on a forum is not wise, It is for the installer to call.

    A good rule of thumb here would be, if your load is rated at 32Amps and above but less than 40Amp then it is a good idea to use a 40Amp on 10mm2.
    Of course unless you can correctly ***** the load and circuit correctly as mentioned above.
     

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  18. metrokitchens

    metrokitchens Screwfix Select

    Just a thought. If you did heat your 20 litre beer water up would that only use one hot plate? Or in this ridiculous scenario did you imagine heating up four 20 litre containers at one time? Not sure tripping the MCB is the biggest problem in that case as the glass hot plate is only rated to take 25KG in weight.

    Just use a 6mm like every other house. Hob only on the cable and no socket on the switch.

    It's like going round the house adding up all the possible appliances plugged into a ring final and being amazed it adds up to 50 amps!
     
  19. fire

    fire Well-Known Member

    No i have a dedicated catering hob that takes my 35 litre stockpot which works wonders when i use my still to make moonshine.
    I was looking at a induction catering single hob for a large stockpot a few weeks ago but did not bother as Gas heating is more controllable, more what you used in past and know than trying something new, cheaper to setup too. Large catering induction hobs are astronomical in price and that is before you even get to installing one.
     
  20. e30mark

    e30mark Member

    Combined Oven & Hob load of 15kw on a 32A / 6mm cable is fine, assuming volt drop due to length isn't ridiculous.
     

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