Installing induction hob+electric oven...separate circuits or same?

[les24preludes]

New kitchen - induction hob at 7.4kW (32amp) and oven at 3.6kW (16amp) max ratings. Existing outlets are:
1. 45amp fused cooker circuit
2. fused socket off ring main (32A fuse)

Can both oven and hob be connected off the one 45amp cooker circuit? That's right up to the maximum.

Or can the oven be connected to the ring main?

Or does the oven require its own circuit - that would be an extra installation and need rewiring to the fuse box.

 

[Midwest]

Firstly OP, are you an electrician? If not how is this work being carried out?

 

[les24preludes]

Hi - I'm not a qualified electrician but I build a lot of electronics, so familiar with calculations and wiring stuff up. What I want here is a correct plan so I know what the installation is likely to be and how it affects the layout of the rooms and units.

I've read a lot of posts on this already, and you might be surprised how much they vary. For example, even electricians say they're fine with twin and earth, where cooker manufacturers always specify heat resistant cable. I already bought Butyl coated 6mm cable for the induction hob, which meets the standards of heat resistance.

I'm hoping this forum can offer me a more definitive answer to this fairly common question.

 

[HandySparks]

Hob and oven will be fine together on a 45A circuit (providing that the existing cable is properly protected by a 45A OCPD).

 

[Midwest]

There are some variables, and sitting from here its to easy to take things for granted. OP there is a factor called diversity that can be applied to these circuits & appliances, but there is also a recommendation about ovens/hobs etc, with a load of more than 2kw having their own radial.

The first question would be exactly what does your 45amp fused cooker circuit consist of?

 

[Taylortwocities]

The first question would be exactly what does your 45amp fused cooker circuit consist of?

Absoluutely. 45Amp is quite an unusual rating for a circuit (not unknown, but unusual). What makes you think it is 45 amps?

So, need to know
1. the size of the fuse or MCB in the consumer unit that protects this "cooker" circuit
2. the size of the cable

Two other items

That's right up to the maximum.

Well no it isn't/ When calculating the maximum demand for circuits we use a technique called 'diversity'. This allows us to reduce the assumed max current that a domestic oven etc will use in practice. this is based on the fact that everything in an oven/hob etc is thermostatically controlled so that, statistically, the appliances never pull the quoted maximum current.
There's an interesting article on this at TLC Electrical Supplies - https://www.tlc-direct.co.uk/Book/6.5.2.htm

Or can the oven be connected to the ring main?

Ah, no.

But it is likely that both can go on your existing circuit - depends on your answers to the questions, above...

 

[les24preludes]

Good points. The 45 amp circuit comes from the fuse box, where there's a 45 amp fuse, into a wall mounted switch with neon (no fuse). The wiring here is 5mm OD overall including insulation - I haven't looked up specifications yet to determine if this is 6mm or 10mm. Should be 10mm, so I hope that's right. The same diameter wiring goes down to what looks like a 45 amp junction box on the wall under the worktop. This is a usual single size junction box, i.e. 850x850mm.

If the supply were split into hob and oven, I'm thinking it would need a larger 3-way junction box. I'm expecting the 5mm OD cable to be 10mm, and from there it would go to a 6mm for hob and 2.5mm for oven. The butyl cable to the hob is 15mm OD including insulation - pretty solid.

 

[hightower]

Send me a photo of that 6mm cable connected to the hob/oven when you're finished, when many are designed (and ask in MI) for 2.5mm. You'll do well to terminate a 6mm cable in to most of the hobs/ovens I've come across.

 

[les24preludes]

Send me a photo of that 6mm cable connected to the hob/oven when you're finished, when many are designed (and ask in MI) for 2.5mm. You'll do well to terminate a 6mm cable in to most of the hobs/ovens I've come across.

Sigh.... absolutely. This is an AEG hob and while they specify 6mm heat resistant cable it wouldn't surprise me if it's too thick to fit properly, just like you say. Haven't come to that bit yet.

 

[Midwest]

Sigh.... absolutely. This is an AEG hob and while they specify 6mm heat resistant cable it wouldn't surprise me if it's too thick to fit properly, just like you say. Haven't come to that bit yet.

OP, not sure how it works, but you would have been better posting in the DIY section. Bit late now.

There's too much guessing (5mm OD cable to be 10mm) to be sure you have a 45amp supply, i.e. 10mm cable. Most cooker radial supplies are 32amp MCB & 6mm 6242y cable. It may be you have a 10mm cable, but the MCB might have been previously upgraded to 45amp, but not the 6mm cable.

Allowing for diversity & installation method, a 32amp supply may be suitable for you appliances, it may not. The heat resistance cable, is only necessary for the final connection to the appliance (if the manufacturer recommends it), and may not have to be 6mm in size, flexible cables have differing ccc to 6242y cables.

In all honesty, you might wish to consider employing an electrician to carry out this work (looks like a kitchen refurb anyway). Otherwise you might have a supply that's too small, or end up installing a larger cable that is unnecessary. If you do end up with a new circuit, this is notifiable to the LBC. And if the any additions/alterations cables are installed in walls, it may require additional protection by RCD. Use an electrician.

 

[les24preludes]

Thanks, Midwest. Evidently the first thing I need to know is what cable is in cooker radial supply. If the OD of the cableing is 5mm, looks like 6mm to me. I've been looking up specifications and the OD of a 10mm earth wire is at least 6.1mm for the earth wire:

20147721 | Prysmian 6491X H07V-R Conduit Cable, 10 mm² CSA, , 750 V Green/Yellow PVC 100m | Prysmian - http://uk.rs-online.com/web/p/conduit-trunking-cable/0381292/

In fact while pos and neg are 5mm OD, the existing earth is a bit less than that. So looks like I can forget running hob and oven off the existing cooker supply, which looks like 32 amp with a 45 amp fuse. But that's OK for the hob at least.

That leaves the oven - 16 amps. There's a mains socket under the worktop with a fused switch above which comes off the ring mains. So the question would be whether - considering diversity - it would be wise to run the oven off that. There's space in the fuse box for one more fuse, and it wouldn't be too disruptive to install another supply to the oven. I just want to know if it's necessary to meet wiring regs.

 

[Leesparkykent]

With diversity taken in to account the load comes to roughly 21A. How is the 6mm cable cooker supply installed?

 

[westward10]

If it is T&E the cpc (earth) for 6.0 will nearly always be a solid conductor. The 10.0 will be stranded.

 

[les24preludes]

If it is T&E the cpc (earth) for 6.0 will nearly always be a solid conductor. The 10.0 will be stranded.

Looked again, and you're right - the earth wire is solid. Pretty much confirms that the "cooker" supply is 6mm, so 32 amp. So hob only.

 

[Leesparkykent]

Looked again, and you're right - the earth wire is solid. Pretty much confirms that the "cooker" supply is 6mm, so 32 amp. So hob only.

Not necessarily...How is the 6mm installed. Does the route encounter any insulation?

 

[les24preludes]

With diversity taken in to account the load comes to roughly 21A. How is the 6mm cable cooker supply installed?

As I was saying above, there are 2 supplies under the worktop

1. Cooker supply, as described, 45 amp fuse in fusebox but looks like 6mm 32 amp supply with a switch and neon above the worktop, so suitable for hob only
2. Fused and switched supply off the ring mains which may or may not be suitable for a 16 amp 3.6kW (max) oven

 

[westward10]

Although it is almost certainly 6.0 it wouldn't be impossible for it to be 4.0.

 

[les24preludes]

Not necessarily...How is the 6mm installed. Does the route encounter any insulation?

You're talking about the "cooker" supply here? What do you mean by "does the route encounter any insulation"?

 

[westward10]

Are you talking about the final connection to the cooket or the cable from the board.

 

[Leesparkykent]

You're talking about the "cooker" supply here? What do you mean by "does the route encounter any insulation"?

How a cable is installed is called the installation method. The installation method is a factor that determines the current carrying capacity of the cable. One of the worst installation methods with regards to current carrying capacity is when the cable is run through or touching insulation. Do you know how your 6mm oven supply is installed so we can get a better picture of the current carrying capacity of the cable to aid further advice?

 

[les24preludes]

How a cable is installed is called the installation method. The installation method is a factor that determines the current carrying capacity of the cable. One of the worst installation methods with regards to current carrying capacity is when the cable is run through or touching insulation. Do you know how your 6mm oven supply is installed so we can get a better picture of the current carrying capacity of the cable to aid further advice?

You're talking about the existing "cooker" supply here? It will be used for the induction hob, not the oven. From the fuse box (45 amp fuse) there is about 8 metres of twin and earth going to the switch with neon. About 4 metres goes along brick wall and 4 metres in stud wall. Then from the switch above the worktop, about 1.3 metres to the junction box under the worktop. Is this what you want to know or can I gave you any more info?

This is all really helpful, guys.

 

[Dave OCD]

Good points. The 45 amp circuit comes from the fuse box, where there's a 45 amp fuse, into a wall mounted switch with neon (no fuse). The wiring here is 5mm OD overall including insulation - I haven't looked up specifications yet to determine if this is 6mm or 10mm. Should be 10mm, so I hope that's right. The same diameter wiring goes down to what looks like a 45 amp junction box on the wall under the worktop. This is a usual single size junction box, i.e. 850x850mm.

If the supply were split into hob and oven, I'm thinking it would need a larger 3-way junction box. I'm expecting the 5mm OD cable to be 10mm, and from there it would go to a 6mm for hob and 2.5mm for oven. The butyl cable to the hob is 15mm OD including insulation - pretty solid.

That's some junction box, probably bigger than the frontal area of an oven. I think you may mean 85mm x 85mm...

 

[Leesparkykent]

You're talking about the existing "cooker" supply here? It will be used for the induction hob, not the oven. From the fuse box (45 amp fuse) there is about 8 metres of twin and earth going to the switch with neon. About 4 metres goes along brick wall and 4 metres in stud wall. Then from the switch above the worktop, about 1.3 metres to the junction box under the worktop. Is this what you want to know or can I gave you any more info?

This is all really helpful, guys.

Sorry I thought you wanted to know if the existing supply was adequate for both the oven and hob? The 4 meters that goes through a stud wall do you know if its packed with insulation?

 

[Octopus]

Have you got the oven? does it come with a cable? and if so does it have a plug attached?

 

[telectrix]

Looked again, and you're right - the earth wire is solid. Pretty much confirms that the "cooker" supply is 6mm, so 32 amp. So hob only.

NO. unless the 6mm cable is run through insulation, it will handle both hob and oven with ease.

 

[Dave OCD]

On the other hand if it's easy to run a seperate oven supply and ideally via separate RCDs or RCBOs you have the advantage of still having one cooking appliance operational should one develop a fault and 'trip' the RCD or OCPD.

 

[telectrix]

On the other hand if it's easy to run a seperate oven supply and ideally via separate RCDs or RCBOs you have the advantage of still having one cooking appliance operational should one develop a fault and 'trip' the RCD or OCPD.

always the negative waves, moriarty, always the negative waves. woof, woof.

 

[Dave OCD]

always the negative waves, moriarty, always the negative waves. woof, woof.

Are you p155ed ? I thought my post was positive, you'd want things designed as well as possible in your own house ?

 

[telectrix]

he cast out his hook and got a bite within seconds,

 

[Midwest]

How time flies.

 

[Midwest]

After all that kerfuffle, I still think the best advice is get an electrician in Les, before you have an accident

 

[les24preludes]

Just took a break to get other parts of the refit together.

Coming back to this, I think the answer is to use the existing 6mm "cooker" feed for the hob, and get an electrician to fit another 6mm feed from the fusebox for the oven, with its own fuse. That covers all the safety needs, and leaves a little extra capacity.

 

[kropaske]

Just took a break to get other parts of the refit together.

Coming back to this, I think the answer is to use the existing 6mm "cooker" feed for the hob, and get an electrician to fit another 6mm feed from the fusebox for the oven, with its own fuse. That covers all the safety needs, and leaves a little extra capacity.

Check the oven manual as well as often manufacturers require max 16/20amp PD for those ovens. So you may well comply with regs but not with manufacturer guidelines...

 

[Midwest]

Check the oven manual as well as often manufacturers require max 16/20amp PD for those ovens. So you may well comply with regs but not with manufacturer guidelines...

I might be wrong, but I thought ocpd were for the benefit of the cable they are supplying, not any appliance that's connected to it. Manufacturers instructions should now only be taken into consideration.

 

[kropaske]

I might be wrong, but I thought ocpd were for the benefit of the cable they are supplying, not any appliance that's connected to it. Manufacturers instructions should now only be taken into consideration.

Correct
But if they ask for one I would certainly fit one.

 

[Andy78]

Correct
But if they ask for one I would certainly fit one.

Even if it was completely unnecessary ?

 

[Midwest]

I'm not sure how providing a reduced ocpd for an oven for example might work, with all the different devices fitted internally, having different loads etc, to provide overload protection for the appliance. Have to say I've never seen this, only other than the manufacturer stating the minimum supply required?

 

[Andy78]

I'm not sure how providing a reduced ocpd for an oven for example might work, with all the different devices fitted internally, having different loads etc, to provide overload protection for the appliance. Have to say I've never seen this, only other than the manufacturer stating the minimum supply required?

The only reason I can think of would be to protect a flex supplied already connected to the appliance.

 

[Midwest]

Although an oven doesn't have a fixed load, one assumes the manufacturer would install a supply cable suitable for it's maximum load?

 

[Andy78]

Although an oven doesn't have a fixed load, one assumes the manufacturer would install a supply cable suitable for it's maximum load?

I have seen pre wired flexes with a CCC smaller than the total connected loads. I assume the manufacturer based the cable size on the diversity due to the cycling of the loads built in to the appliance.

 

[Midwest]

Yes agree, but with most ovens for example, you cannot have the main oven selected with the side, top or bottom element. And vise versa. So a suitable cable would be supplied for the maximum possible culmination of load. Same cannot be said of electric hobs, but I haven't seen any hobs with a manufacturers cable?

 

[Andy78]

Yeah it was a hob I was on about.

 

[Taylortwocities]

I haven't seen any hobs with a manufacturers cable?

I have a NEFF hob, that came fitted with a cable. It was a 5-core cable that can be configured to meet single/multi phase supplies..

Others i have fitted did need a cable to be supplied and fitted.

 

[kropaske]

I have a NEFF hob, that came fitted with a cable. It was a 5-core cable that can be configured to meet single/multi phase supplies..

Others i have fitted did need a cable to be supplied and fitted.

Like this one...

 

[Midwest]

Well there you go. Out of curiosity did it ask for derating ocpd?

 

[kropaske]

Well there you go. Out of curiosity did it ask for derating ocpd?

That's hob not oven.
I came across some from aeg and electrolux that ask for min 16 and max 20amp fuse.
It may be to stop people wire them in to the kitchen rings through 20amp isolator.

I went once to check one. Turn out it was 3.6kw oven and was connected with 13 amp plug to the socket. It last almost a 1 year before fuse went and burn socket slightly. Customer got new oven and this one looks really nice in my kitchen now...