Wiring an induction hob and two ovens on a single cooker circuit

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I currently have a double oven fed from a 45A MK cooker control unit with integrated 13 amp socket. The CCU is fed by a 10mm2 cable and protected by an RCD and 32A MCB about 8m from the CCU.

I’m planning my new kitchen and have chosen the appliances.
  • The gas hob will change to an induction hob that can pull up to 30A (7.2kW)
  • The double oven will be replaced by:
    • a single main oven (16A)
    • built-in combination oven/microwave (fitted with a 13A plug)

I’m trying to plan the best way to connect these up. Running additional cables neatly from the CU to the kitchen is not easy so I'd really like to avoid that. I suspect with diversity, it is reasonable to assume that the existing 10mm2 feed will be adequate to supply both ovens and the hob though I probably need to up the 32 MCB to 40A?

Then the problem is connecting them up. I want to keep the CCU to allow me to isolate all the appliances easily from above the worktop.

I was thinking the neatest and safest solution would be to use a small 3 way (+ isolator) consumer unit in a cupboard close to the ovens & hob. Take the switched load output from the CCU in 10mm2 cable into the 3 way CU and from there feed the appliances:
  • 6mm2/32A MCB for induction hob
  • 2.5mm2/16A MCB to main oven
  • 2.5mm2/16A MCB to 13A socket for combi oven/microwave

Can anyone see anything wrong with that proposal or have any better suggestions?
 
Last edited:
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You can connect all of then to your cooker control unit, no 3 way CU required. Who on earth used 10mm cable on a 32 amp circuit?
 
You can connect all of then to your cooker control unit, no 3 way CU required. Who on earth used 10mm cable on a 32 amp circuit?

But don't you then have the problem that the cables to the appliances are then undersized for a circuit with a 40 amp fuse?
 
Keep your 32 amp MCB then. Your appliances will never exceed that as they cycle on and off.
 
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Keep your 32 amp MCB then. Your appliances will never exceed that as they cycle on and off.
I very much doubt that the (flexible) "cables to the appliances" would theoretically be 'up to' coping with 32A (since that would require 4mm² flex) - so one would be down to relying on the fact that the loads were "unlikley to result in an overload" (in which case any size of MCB would be OK).
 
What if-anything do the instructions for the single oven say about breaker ratings?
 
What if-anything do the instructions for the single oven say about breaker ratings?
An interesting question, but the answer is not necessarily all that helpful - since, even when they make some sense, it's often not clear whether they are talking about the minimum or maximum rating of the breaker.

There seem to be only two options. If the appliance is such that one feels one can argue that it is "not likely to result in an overload current" then, provided only that the (usually flexible) cable connected to the appliance is large enough to carry the maximum current drawn by the appliance 'in normal use', then the circuit's breaker can be be whatever is appropriate for the fixed wiring of the circuit, even if that is greater than the current-carrying-capacity of the appliance's flex.

However, if one does not feel that one can make that assumption, then the rating of the breaker is limited by the CSA of the appliance's flex - i.e. 25A for 2.5mm² flex or 32A for 4mm² flex (the latter probably fairly unusual) - and, in the former case, 25A may possibly not be enough for the appliance (e.g. if it is a large cooker).

Kind Regards, John
 
@JohnW2 does make a good point, however traditionally we use 32 amp for a cooker supply, so the manufacturer should have designed the product to work on a 32 amp supply, and I see no problem using a double cooker connection unit to supply both oven and induction hob, my stand alone cooker is rated 220 - 240V - 50Hz. 10466 - 12455 W so could draw over 50 amp, but the instructions say it needs a 32 amp supply.

If you look inside the cooker the cables are not 4 mm² it assumes should there be a fault it will trip either the 32 amp MCB or the RCD.

Fact that the cooker is divided into two parts does not really change what it can draw. However clearly designed for domestic use. So each heat area has limits as to how long it can draw power for, with mine it depends on heat settings, but it will auto turn down, or off with over heating and after a set time has elapsed so it would be near impossible to over load for long enough to cause damage.

However using over a 32 amp MCB may cause problems, I have seen 45 amp supplies, and I have seen nothing in any instruction set I have read giving maximum overload size, mine says "32 ampere minimum capacity" and "Connection should be made with 6.0mm² flexible twin and earth cable." it does not give a maximum size.

I note my mothers Zanussi Oven states 13 amp fuse should be used in UK and Europe. Not sure how they would get a 13 amp fuse in Europe? But downloading instructions very few give maximum supply size.
 
.... my stand alone cooker is rated 220 - 240V - 50Hz. 10466 - 12455 W so could draw over 50 amp, but the instructions say it needs a 32 amp supply.
Diversity. It would only ever use anything approaching 50A for very short periods (if everything was switched on simultaneously from cold). As far as a likely 'average' (over any reasonable period of time), the standard diversity calculation assumes that, if the peak load is 50A, then that 'average' (over reasonable periods of time) would only be about 23.3A, so a "32A supply" is more than adequate - a "25A" one would actually do.
If you look inside the cooker the cables are not 4 mm² it assumes should there be a fault it will trip either the 32 amp MCB or the RCD.
As I said, that is fine so long as one is confident that the load "is unlikely to result in an overload current" (as opposed to a fault current). If it could 'result in an overload', then a B32 MCB could allow around 46A to flow for an hour (or higher currents for shorter periods), which might be enough to 'melt' (or at least, damage or seriously overheat - hence fire risk) conductors which were a lot smaller than 4mm².
... I have seen nothing in any instruction set I have read giving maximum overload size, mine says "32 ampere minimum capacity" ...
As I said, the instructions often just say simply "a 32A supply", without any indication of whether they are talking about minimum and/or maximum.

Kind Regards, John
 
I note my mothers Zanussi Oven states 13 amp fuse should be used in UK and Europe. Not sure how they would get a 13 amp fuse in Europe?
Possibly in Malta, but basically yet another example of manufacturers instructions being wrong.
 
My opinion differs from many and I dislike cables not being protected by a suitable fuse/MCB as I've had too many experiences of fault currents well below the Fuse size but well over the current carrying capacity of the cable.=
I presume that you are talking about 'overload' currents, since true 'fault currents' (as per usual definitions) will inevitably be considerably greater than the OPD rating ('fuse size') as well as being greater than the CCC of the cable?

What you suggest is, of course, always the 'safest' and is, indeed, virtually always the case in terms of the truly 'fixed wired' of a circuit. We've been through this before and, whilst you are able to produce some anecdotes which appear to contradict this, I would say that there are certain types of loads which are extremely unlikely to result in 'overload' (as opposed to 'fault') currents and, even in relation to your anecdotal 'exceptions', an RCD (now required for virtually all circuits) would often clear the fault.

Also, to be pragmatic, in those very rare 'exception' case, I doubt that, say, a short length of 2.5mm² flex would actually come to harm as a result of carrying the current that a B32 could allow to flow (about 46A for an hour - in extremely unlucky circumstances).

I would add that "...currents well below the Fuse size but well over the current carrying capacity of the cable" can, theoretically, arise in the absence of any faults (everyday sense) in a ring final circuit, unless the circuit has been designed very conservatively (no sockets anywhere near either end of the ring).

Kind Regards, John
 
I presume that you are talking about 'overload' currents, since true 'fault currents' (as per usual definitions) will inevitably be considerably greater than the OPD rating ('fuse size') as well as being greater than the CCC of the cable?...
Kind Regards, John
I don't care what terminology is used, however in that case it was a faulty appliance that caused an excessive current to flow in a cable. Therefore I'll call it 'fault current'
The cable was correctly rated for the appliance but due to negligence (in MY OPINION) of omitting suitable OCD, it was able to run at at least 3 times it's CCC.

In my book that is not acceptable but allegedly within the regs.
 
I would add that "...currents well below the Fuse size but well over the current carrying capacity of the cable" can, theoretically, arise in the absence of any faults (everyday sense) in a ring final circuit, unless the circuit has been designed very conservatively (no sockets anywhere near either end of the ring).

Kind Regards, John
The only time I've had a serious example of this issue had been a 32A socket for a welder within 1m of the CU, which is well outside the correct design parameters for a ring circuit using 2.5mm² cable.
I'm not saying the problem doesn't otherwise exist, indeed I've known the first three points on a ring being WM, DW, Tumble drier, virtually at the CU then within 2m were toaster, kettle, coffee machine plugged in. The ring then meandered around the rest of the house.

The symptom was regular MCB tripping as the householders typical early morning routine was to start all three machines then make drinks and toast. Yes that did warm the cable, from memory I believe it was something like 30A Vs 15A on the legs. In my experience that one was fairly unusual and fortunately easy to alter.
 
I don't care what terminology is used, however in that case it was a faulty appliance that caused an excessive current to flow in a cable. Therefore I'll call it 'fault current'
The cable was correctly rated for the appliance but due to negligence (in MY OPINION) of omitting suitable OCD, it was able to run at at least 3 times it's CCC.
In my book that is not acceptable but allegedly within the regs.
Three times is a lot and not normally the case in omission of overload examples; there can't be many such examples in a consumer unit.

I find it more incredible that the manufacturer of the appliance involved used internal wiring that could withstand, what, four, five or six times its design load.
 

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