Two new single ovens - do I need two cables to the Fusebox?

[marc1959]

We are looking at installing two single ovens rated at 3.6kw each and 16A.

We currently have a double oven which is on a 30A fuse at the fusebox and is rated a max 5.1kw. A builder who will be doing some general works in the kitchen has said we need to have each oven separately wired to the fusebox and cannot use the existing cabling. Putting in new cabling would involve pulling up floorboards, new decorations, carpets etc, etc. and is simply something we do not wish to contemplate.

Is the builder correct or is there a way of using the existing cabling?

Any help appreciated - I don't intend doing the work myself but would like to know what needs to be done.

Many thanks.

 

[Albert]

I would connect them separately.
To be practical the very important question to answer is what is the existing cable size?
If it is at least 6mm, you could connect them but make sure that the CB is not more than 32A.
other option is to use the existing cable for one oven and run a new one for the other. I would advice you not to use a cable smaller than 6mm (although smaller cable would be ok).

The new regulations require that all new circuits will be protected with RCD/RCBO or run in metal conduit or be a cable with metallic protection, your builder (or his electrician) should know where to find more details).

He has to provide you with a certificate confirming that the work was done to the 17th edition of BS7671, and he has to notify the Council.

 

[dannyboi2003]

If it were me id use the existing cooker circuit to supply a mini-cu in a cupboard next to where the ovens are installed. The cu would be fitted with two 16amp breakers, one for each oven. The existing worktop isolator switch would be used to isolate both ovens at the same time.

I'm sure one of the more experienced guys will be along to confirm if what i've suggested is safe and within the regs and if not will be able to advise you further.

All the best
Dan

 

[Pensdown]

One 30amp circuit is ample for 2no 3.6kW ovens.

After applying diversity the design current of each oven is well within the rating of a 13amp fused connection unit.

From you existing cooker switch you can feed 2no fused connection units, one for each oven. 4mm cable will be fine.

You haven't mentioned your hob. Is it gas?

 

[marc1959]

It sounds, subject to confirmation that what has been suggested can be signed off under the various regs, that I don't a new circuit all the way back to the existing Fusebox (CU?) - so thanks, in particular, to Dannyboi and Pensdown. I now need to persuade an electrician to do it...

As for the hob, we were all set for replacing like with like - gas for gas -but then we were taken by a natty induction hob... however, if that queers the pitch and makes a further circuit to the main cu necessary, its back to plan A and a gas hob!

 

[Albert]

One 30amp circuit is ample for 2no 3.6kW ovens.
After applying diversity the design current of each oven is well within the rating of a 13amp fused connection unit.

You are right about the diversity factor, but are you sure that there are no de-rating factors to consider?

A 30A fuse/CB is OK as long as the cable carrying capacity is higher than the protective devices rating.

I suggested a min 6mm cable because this will allow flexibility in the future if
there will be a decision to upgrade the oven/cooker to more powerful ones.

My personal view; I always consider the max load of the oven/cooker thinking of Christmas. But again this is a personal approach. In the same thought I would never connect a 3.6kW load to a 13A FSU with or without diversity in mind, again this is me... These ovens can be connected via a 45A cooker control unit each...

 

[Pensdown]

Hi Albert,

They way I read the post.

Marc1959 has an existing 30amp circuit feeding a double oven so assuming the cabling etc has been installed in accordance with the regulations that were applicable at the time we are only taking about the final connection onto his two new ovens.

The builder has said that he needs to install a new circuit for one of the ovens

As each oven is rated at 3.6kW the total load after applying diversity is 11.7amps/oven so his existing 30amp circuit is suitable for both ovens with loads of spare capacity for Christmas.

I suggested one way he might connect the 2 ovens but as you’ve said there are others. But whatever way he connects them, he doesn’t need a new circuit.

 

[Albert]

As long all the "ifs" and "I thinks" were checked that they are safe, i have no problem. I wish Marc1959 all the best and good luck and I am sure that you will join me...

 

[securespark]

Assuming the cable was rated for 32A with the factors taken into account, if he were to swap the 3036 for a 32A Type B breaker, that would cover the flc of both ovens.

Put a 2 way CU on the end of the cable, with one 16A breaker for each oven.

 

[GaryMo]

A 30A fuse/CB is OK as long as the cable carrying capacity is higher than the protective devices rating.

In principle, as per the suggestion by Pensdown, the BS1362 fuse would provide overload protection and considering it would be 13A maximum then 2.5 or even 1.5 could be used as long as the cpc would be thermally protected under fault conditions (adiabatic).
Obviously the cable would require short circuit protection at the origin.

 

[sijaysee]

an oven of 3.6kw needs to go on its own 16A circuit. it will probably say that in the manual.
some ovens have a cleaning cycle that use full power for 2 hours.
thats why you cant apply diversity.

 

[roberta1000]

some ovens have a cleaning cycle that use full power for 2 hours. thats why you cant apply diversity.

Are you sure about that sijaysee? I thought that they simply used a very high temperature. An oven left on full power for 2 hours will certainly do that!!!