What sized cables must I use?

[drdr]

I am about to install an oven, a combination oven/microwave and an induction hob. They draw 3.45kw, 3.65kw and 7.2kw respectively.
I need to use 2 core plus earth cable for each. BUT I have been given conflicting information about the size of the cables. One person said 6mm and the other 10mm. What is the correct size? Also could the two ovens be fed off the same cable?
Thanks

 

[EFLImpudence]

Assuming clipped to surface or buried in masonry:

Theoretically you could use 2.5mm² and a 25A MCB.

However, as it is a new circuit, you may as well use 6mm² and a 40A MCB. 10mm² is not necessary.
Yes, they can all run off this one supply but you may wish to have separate circuits for one or two appliances.

Also, as it is a new circuit, it is notifiable to the Local Authority before you begin OR you may employ a registered electrician.

 

[ban-all-sheds]

One person said 6mm and the other 10mm. What is the correct size?

That will be the size chosen by the person who signs this declaration on the Electrical Installation Certificate (with my highlighting of the part relevant to this topic):

I being the person responsible for the design, construction, inspection & testing of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the design, construction, inspection & testing hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to 2015 except for the departures, if any, detailed as follows:

Seriously - nobody here is going to sign that, and you can't ask the person who does to follow the advice of an anonymous stranger here, nor can you present them with a fait accompli of a cable you've installed and expect them to pretend that they did it.

The circuit needs certifying, it needs testing etc, and as EFLI said it requires Building Regulations approval. If you do the work then you'll either have to apply for approval in advance, which means paying a fee, and satisfying your LABC that you'll be able to comply with Part P, or you'll need to engage a registered third party certifier, if you can find one, which again means paying a fee, and negotiating with them how they want to proceed wrt design decisions, inspections etc.

 

[drdr]

Thanks for that
I am using a qualified electrician but the two I have spoken to have given different answers. There is a considerable difference in price between using 6 and 10mm cables. That is the reason for my question.
I am having a completely new kitchen, as part of that I will go through the whole certification exercise.
Personally I cannot see a problem if you are an electrician and know the rules; if you know the loading then I would have thought the cable sizes should be obvious. But then I am not - that is why I asked the question. I do know that 10mm cable is fairly horrible to work with.

 

[JohnW2]

I am using a qualified electrician but the two I have spoken to have given different answers. There is a considerable difference in price between using 6 and 10mm cables. That is the reason for my question.

How big a difference? - the difference in price of the cable is only about £1.50 per metre, and the extra effort involved in installing 10mm² cable is not that enormous.

In any event, as has been said, it is all-but-impossible that you would actually need 10mm², so I think you probably should be hesitant to employ an electrician who said that was necessary, anyway!

Kind Regards, John

 

[drdr]

John
well it is quite a distance from the board, and with two cables I figure it is twice the price as if I used 6mm
I think you are right.

 

[JohnW2]

well it is quite a distance from the board, and with two cables I figure it is twice the price as if I used 6mm. I think you are right.

As EFLI said, one 6mm² cable would almost certainly be more than enough to supply all three of your cooking appliances. However, if you wanted to split it into two circuits, then 10mm² would be quite ridiculous - in fact 2.5mm² for each of them would almost certainly be more than enough, and even 4mm² would probably represent 'overkill'

Kind Regards, John

 

[ban-all-sheds]

well it is quite a distance from the board,

Distance is irrelevant to comparative costs.

and with two cables

Ditto number of cables.

I figure it is twice the price as if I used 6mm

Mathematically impossible unless 10mm² is more than double the price of 6mm². It isn't.