Two ovens and induction hob wiring?

[ChrisZx]

Starting to think about my kitchen extension wiring. Before anyone asks, council are inspecting and testing so it's all legit!

I've only got a single slot available on the consumer unit (the existing cooker circuit that's unsuitable). Could I connect two ovens and an induction hob to one circuit?

I think the load is fine, 32A once diversity is applied so a 10mm2, 45A MCB circuit will be fine with the method(s) I can use. And I can't find anything that says I couldn't do it. I'm more thinking about the practicality of such an installation. I would like two 32A DP switches, one for both ovens and one for the hob. How could I acceptably split the 10mm2 feed into the two switches?

 

[winston1]

It is fine to put them all on one 32 A circuit wired in 4mm cable. 10 mm is way over the top. You don't need any double pole switches as it can be isolated at the CU. If you really want them run the 4mm cable to one and on to the other. Make the switches you use have big enough terminals for 2 x 4mm cables.

 

[ericmark]

If the LABC are inspecting, ask them what you can do. When I did work requiring notifying I had to do my own inspection and testing and the council were to come and see if I had done it right, they take a random reading and see if it's same as you got. In real terms they did nothing, they just sent the completion certificate in the post.

They did want at first to get some one to do an electrical installation condition report, some random electrician they knew.

Either way even if the LABC is responsible for the safety, in real terms it's down to you. So clearly to supply two ovens and a hob from a 100A supply would be wrong, as to what size would be acceptable that's down to your skill, personally I would say 32A, as that is the standard size for a cooker supply, 45A is OK when feeding a mini consumer unit, but should a wire come off in an oven we want it to open the trip not burn the cables.

I looked at what size manufacturers state, and often it only states minimum size, often no maximum stated.

As far as I am aware the LABC can charge you to have an EICR done, and if it fails then you also pay for a second one, and an EICR is some ones option, if you look at the safety council best practice sheets you will see where they say one should fail the fixed installation because a multi socket adaptor has been used. Personally I would say anything plugged in is not part of remit, however they say different.

So what you are asking is do we think the guy who does the EICR will accept three items on one supply. Well I would, but that does not mean he will.
As to splitting the supply you can get double cooker connection units which would do the job.

But what we don't know is will the guy testing use common sense, or is he a jobs worth? When I did a wet room the LABC inspector insisted on an extractor fan, even though the windows open. No good arguing, you have to do what they want.

 

[ChrisZx]

The council also inspected and tested my garage installation last year so I know the deal. You're right in that he really didn't look at much.

personally I would say 32A, as that is the standard size for a cooker supply, 45A is OK when feeding a mini consumer unit, but should a wire come off in an oven we want it to open the trip not burn the cables.

Not sure I follow, if 32A is standard size for a cooker supply, seems sensible to me to up that when adding an extra appliance to the circuit.

With the wire coming off example, are you thinking that one faulty oven might not trip it because the fuse is so big? And that might cause the ovens internal wiring/flex feed to fry, rather than the supply? Not something I had thought about.

I could sacrifice the boiler being on it's own supply to gain an extra mcb. Or could I add a mini CU next to my existing one? What do you think would be best?

 

[ericmark]

I have a stand alone cooker, it has two ovens and a 4 heat area induction hob, the lower oven has multi elements so you can heat from back with fan, the sides or top so the total current with all switched on is around the 50 amp mark, yet the manufacturer still says 32A supply.

The point is all the heat areas quickly start to regulate so the average current soon drops so it will not over heat the cable, and the 32A MCB is there to ensure the cable does not over heat, even if you do something daft, like turn the ovens on with doors open, worst case is it trips the MCB. Your set up is not different to mine, except the parts are separated out.

A twin cooker connection unit will allow two items to be connected to one supply, it is normal to have an isolator so some one servicing the units can switch them off when required, however if out of sight then really needs a means to lock off the supply. Standard cooker switch for all is all that's needed, in a restaurant where oven doors are opened and closed all the time, and pans are swapped across the heat areas then may be you could trip the supply, but with domestic very little chance.

The only worry is a pedantic inspector who sets out to be confrontational. The cooker supply is considered as a dedicated supply, clearly shared between three items it's not dedicated, but common sense says it's no different to a stand alone cooker, except a stand alone cookers oven can use more power, my bottom oven can have top, side and back elements all on together well over 16A just for oven, it is rare for a built in oven to exceed 16A as often plugged in (In Europe not UK) to a 16A supply.

 

[AndyPRK]

If it were me I would run a 6mm cable for it.

Run that into a switch for the hob, and then have a few inches of 4mm or 2.5mm to goto switches for the ovens.

This will allow you to have a working cooking appliance should the RCD start tripping due to a fault

 

[ChrisZx]

All makes sense, thanks. Total of the appliances I was looking at last night would be 82A, you still think that's alright? Like I said diversity says it's fine (just), but I wasn't sure if that was a bit restrictive.

Using 32A with smaller cables would make distributing easier as much easier to get two cables in one outlet.

Thanks for the help!

 

[AndyPRK]

well add up typically what you would have on.
2 hob plates
2 ovens.
And satisfy yourself the B32 wouldn't trip under those conditions
I would think that would be ok on 32A.

An alternative would be to have 10mm into a hidden? CU in the kitchen with maybe B32, b16, b16 but this does not isolate the neutrals should an element fail, no less useful

 

[winston1]

If it were me I would run a 6mm cable for it.

Why? 4mm is all that is required, cheaper, and easier to use.

 

[ericmark]

Some houses only have a 60A supply, 60, 80, 100 fuses all fit in same holder so no way to know. My cooker rated 220 - 240V - 50Hz 10466 - 12455 W so 52A so not as large as yours. I think I would go for two 32A supplies in that case. Are you sure 82A? my cooker has a 3.7 kW and 2.3 kW heat areas when using boost, however can't use both at the same time, so although adding up heat areas it is 12 kW in real terms it's only 6 kW.

 

[winston1]

All makes sense, thanks. Total of the appliances I was looking at last night would be 82A, you still think that's alright? Like I said diversity says it's fine (just), but I wasn't sure if that was a bit restrictive.

Using 32A with smaller cables would make distributing easier as much easier to get two cables in one outlet.

Thanks for the help!

82 amps is more that the main fuse to many properties.

Eric beat me to it!

 

[ericmark]

well add up typically what you would have on.
2 hob plates
2 ovens.
And satisfy yourself the B32 wouldn't trip under those conditions
I would think that would be ok on 32A.

An alternative would be to have 10mm into a hidden? CU in the kitchen with maybe B32, b16, b16 but this does not isolate the neutrals should an element fail, no less useful

If you really want isolation, you can get RBCO's with double pole switching, although current only monitored on the line.

 

[AndyPRK]

Why? 4mm is all that is required, cheaper, and easier to use.

While 4mm maybe adequate for B32 everyone uses 6mm, and it seams a little silly to install smaller than average when you know you are having an induction hob and 2 ovens on the end. Not exactly future proof

 

[winston1]

While 4mm maybe adequate for B32 everyone uses 6mm, and it seams a little silly to install smaller than average when you know you are having an induction hob and 2 ovens on the end. Not exactly future proof

Not everyone uses 6mm. Nothing can be future proof as we don't know the future.

 

[ChrisZx]

Some houses only have a 60A supply, 60, 80, 100 fuses all fit in same holder so no way to know. My cooker rated 220 - 240V - 50Hz 10466 - 12455 W so 52A so not as large as yours. I think I would go for two 32A supplies in that case. Are you sure 82A? my cooker has a 3.7 kW and 2.3 kW heat areas when using boost, however can't use both at the same time, so although adding up heat areas it is 12 kW in real terms it's only 6 kW.

If I'm honest I didn't look in loads of detail, just saw ovens at around 5.5kw and hobs at around 7.4kw, which with two ovens is around the 80A mark.

Diversity applied to all 3 appliances is actually 45A.

Very good point about house supply! We won't be picking the actual appliances for quite a while, I just wanted to make sure the feed can handle whatever we decide to go with, which could be two double ovens. But if that's an unfeasibly large supply then another option is to reel in our ambitions!