Induction hob on 6mm^2 cable? And oven?

[chris1982]

We are just planning a new kitchen and want to replace an old gas cooker with an electric hob and an electric oven. There is a cooker circuit on 6mm^2 cable that currently is on 16A MCB; an electrician came round for a quote and said he could replace this with a 32A MCB. I had hoped he'd say 40A, but now 32A is what we need to work with.
Now I am wondering whether this circuit can be used for both the hob, ideally induction, and the oven, and the electrician didn't really answer my question but just kept repeating "32A". How is diversity applied when connecting both appliances to the cooker circuit, i.e. diversity based on the combined use of the two?
Or should I forget about all that and ask for the oven not be wired from the cooker circuit but from the downstairs main ring (includes the kitchen socket)?
Thanks.

 

[EFLImpudence]

Yes, 32A is usual for historical reasons to do with 30A rewireable fuse - with MCBs you could have 40A or even 45A.

The circuit will be adequate for any hob and oven you may buy for domestic use.

 

[chris1982]

Thanks for the quick reply. So the "relevant" load is not the maximum load? Say we put on a 7.4kW induction hob and a 2.9 kW oven, that is a combined load of 44.8A at 230V. But you think that is find as one hardly ever uses all plates on the hob and the oven on max at the same time?
How about when installing a oven with self-cleaning function that might have an even higher load?

 

[EFLImpudence]

Thanks for the quick reply. So the "relevant" load is not the maximum load? Say we put on a 7.4kW induction hob and a 2.9 kW oven, that is a combined load of 44.8A at 230V. But you think that is find as one hardly ever uses all plates on the hob and the oven on max at the same time?

That is correct.
For cooking appliances, you may count the first 10A plus 30% of the remainder.
I expect the ratings are given at 240V so a bit less than your figure and then even less at 230V.

How about when installing a oven with self-cleaning function that might have an even higher load?

Not sure what they use when self-cleaning but the hob is a much greater load than the oven.

 

[chris1982]

Great, thanks. So I take it's the first 10A of the hob and oven taken together (10A of (7.4kW+2.9kW)/240V=42.9) and then 30% of the rest (0.3*((7.4kW+2.9kW)/240V-10A)=9.9A), right?

 

[Taylortwocities]

That is the basis of diversity. As EFLI says, with 32A on that circuit you will be fine.

Just have a look at your consumer unit, add up the trip values of all of the MCBs/fuses. Mine adds up to nearly 200 amps. The fuse from the electricity supplier is only 100amps.

When the electricity supplier calculates the ACTUAL MEAN demand for a new housing estate, they budget on as little as 2KW (thats about 10 amps) per dwelling. If they didn't we would be over-run with power stations.


Interesting graph taken from Loughborough Uni's study on domestic energy use
https://dspace.lboro.ac.uk/dspace-jspui/bitstream/2134/6997/1/Domestic electricity demand model paper _2 00_ A.pdf

Anyways, you'll be fine, even on Christmas Day, or when on self clean, or induction hob boost mode.!

 

[chris1982]

Thanks a lot. But from the calculations I still need to get someone in to replace the 16A MCB with a 32A on that 6mm^2 cable and can't leave the 16A MCB in place.

 

[Taylortwocities]

Of course. Depending on the installation method, it could be a 40A MCB.

 

[chris1982]

Depending on the installation method, it could be a 40A MCB.

That's what I thought. I used the calculator on TLC-Direct and thought the cable was suitable for a 40A MCB, but the electrician who gave the quote disagreed. I think I will get a second opinion.

 

[EFLImpudence]

So I take it's the first 10A of the hob and oven taken together (10A of (7.4kW+2.9kW)/240V=42.9) and then 30% of the rest (0.3*((7.4kW+2.9kW)/240V-10A)=9.9A), right?

Yes, total of 19.9A.

 

[Iggifer]

Not sure of the source of this, looks like the IET font to me. It came up in my Facebook news feed, nice little cheat sheet

 

[chris1982]

Very handy! Thanks.

 

[JohnW2]

That's what I thought. I used the calculator on TLC-Direct and thought the cable was suitable for a 40A MCB, but the electrician who gave the quote disagreed. I think I will get a second opinion.

Yes, but even if the cable could support a 40A MCB, you don't need it - a 32A one can support a pre-diversity load of about 83A!

[if total 'maximum' load is 83A, after diversity it is 10A + (30% of 73A) = 31.9A]

Kind Regards, John