Oven / Cooker Hob Diversity Question

Hi,

I would appreciate some views on the following issue I have.

The Cooker radial in a house has been wired in 2.5mm T&E, to save the expense of a larger cable. Without boring with the detail, I cannot pull out and replace the existing cable, nor wire in a new 32A circuit using another route. I know that sounds strange but trust me that the house has been modified in such a way as to make it only possible by running a cable round the front/side and rear of the house, and nobody really wants to go down that route.

Owner installed a 2.7kw electric oven with a gas hob that takes a 5A supply for lighting the rings. The oven is hard wired into a fused 13A outlet. The gas hob 5A supply is wired directly into the control switch on the kitchen wall (!) . A double socket has also been spurred off the circuit. The whole cct is protected by a 24A MCB

The desire is to replace the gas hob with a new 6kw electric hob. The new electric hob is one of the new ones which split the load via two separate 16A supplies, or if the two leads are connected, then via one 32A supply, therefore normally not a problem if you have a 32A cct, however in this case we don't.

Question is, can the new electric hob be accommodated on the existing 2.5 T&E cct with 24A MCB ?

Our total calculated cooking load is Oven 11A and Hob 26A = 37A

Looking at the diversity rules (Tables A1 and A2 in OSG) as I understand them (the harder you look the less clear they become !) diversity becomes 10A plus 30% of final load of connected cooking appliances in excess of 10A = 30% of 27A (11-10 + 26) = 8A

So 10A + 8A + 5A =23A, which should work IF my maths and understanding is correct, however it is pushing a 24A MCB to the limit.

So as I said at the start I would appreciate views on the feasibility of doing this.

Hi - what exactly does the new induction hob manufacturer’s installation instruction have to say about it’s electrical connection?

What make is the mcb to give 24A?

To be protected by a max 32A MCB

you Nead a new cable or a smellar load

Sorry Just checked again - protection is actually a 25A RCBO in a 2.5 T&E cable circuit. Apologies

Hi again - the extra double outlet spur, what loads will be on it? If it’s got a toaster and kettle like my kitchen outlet, then that’s another thing to consider.
Re the 25A rcbo on 2.5mm - only ok if it’s 100% clipped direct.
Anyway, I think your adding up is ok, but a 2.5mm cable doesn’t sound right for those loads, sorry.

45140 said:

To be protected by a max 32A MCB

Click to expand...

Interesting ... do you happen to remember the make and model?

Hi Wilko,

My first reaction was to say no cannot be done, but then I wondered about whether diversity could provide an answer.

Make of the hob is a Russell Hobbs RH60EH401B.

The cabling through the ceiling void is not clipped but run through the middle of the joists and there is no slack to clip up anyway. There is no insulation with it being the ground floor ceiling void. It has been plastered into the wall, it being a 1930s brick and plaster wall.

If nothing can be done, and to be honest it is coming back to my original thoughts really, the 5A gas hob supply needs to be routed via 5A outlet I would have said.

I've always only applied diversity to a singular appliance. In a single oven, the combination of what can be turned on at once is restricted.

A separate oven and separate hob can be used independently by the user, creating different loads to a combined oven & hob. Just my opinion.

And this is where things are not so clear. The OSG states that the calculation after the first 10A is "...30% of final load of connected cooking appliances in excess of 10A..." So I take that to mean what it says - connected cooking appliances. So you can either add everything up, take 10A off then add on 30% of what is left, or add what is left above 10A for the cooker, to 30% of the remaining total load. Two different ways of reading.

As the calculated load therefore comes out at 23A it appears that a 25A RCBO can be used or I remove the single double socket spur which makes the load 18A and so within the range of a normal 20A MCB.

How long is the 2.5 from c/u to cooker?
Does it go through any insulation?

45140 said:

Make of the hob is a Russell Hobbs RH60EH401B.

Click to expand...

Hi - if I’ve looked up the right one, on page 10 (the electrical connection notes) it does say a 32A mcb is to be used, just saying.