Oven & Hob radial circuit

[HughH]

Hi all,

I would like to get some advice please on the following circuit for our new kitchen. The proposed layout has a 3.2kw electric oven and a 7.4kw induction hob. These would connect via 45A isolation switches to a 6mm2 radial circuit. I am running the cables myself and would like to know if: is it better to connect these appliances in series on the same cable or preferable for each appliance to have its own dedicated 6mm2 cable and terminate these into the same MCB?
Many Thanks

 

[winston1]

If you connect them in series they won’t work properly. The 240v will be split between them in inverse proportion to the power requirement. You need to connect them in parallel, and yes you can use one cable. That is normal.

 

[ericmark]

There has been some talk about ovens on a large supply, the standard UK supply to a cooker is 32 amp and there is no problem connecting an oven to a 32 amp supply unless the manufacturer says other wise.

However at what point there is a problem is a subject for debate, clearly with the cable sizes in an oven you would not use a 100 amp supply, but as to a 40, or 45 amp supply I am really not sure, what we want is should a cable work loose and fall off or an element go short circuit it will fail safe.

In real terms it would likely touch some thing earthed and trip the RCD, however we are told we should not rely on the RCD, so personally I would not want to supply an oven with over 32 amp, but can't find any regulations to support that.

But be it a twin cable or single cable if they come from same protective device then make no difference, the only reason for two cables is so an oven over 3 kW can be protected with an overload of a reasonable size, under 3 kW we can use a FCU but over it needs to come from the CU as you can't get an overload device local of a reasonable size. My mother had a second CU in the kitchen so kitchen supply independent to rest of house.

You can get twin cooker connection units so you can power hob and oven from same supply, my stand alone cooker can draw 60 amp however it is on a 32 amp supply as the manufacturer recommends and the MCB (old house) and RCBO (new house) has never tripped, so clearly a 32 amp supply is ample.

The induction hob can deliver more power than a hot plate, but it wastes less power, so in real terms the total use of power drops, as heats the pan not the house, so personally I would supply both from 32 amp supply.

You could daisy chain the cables using a cooker supply unit, clearly wired in parallel but I understand what you mean wired in series.

It does not matter size of isolator as long as larger than size of supply, it is the fuse/MCB/RCBO size that matters. I think no problem in England to do as you say, in Wales the LABC charges would mean not worth while, better to use a scheme member electrician.

 

[AndyPRK]

or preferable for each appliance to have its own dedicated 6mm2 cable and terminate these into the same MCB?
Many Thanks

So you wouldn't normally go to the effort of having two 6mm cables and then terminate them in the same MCB.
Two cables, Two MCBs.
The only exception maybe that there aren't any spare ways and you intend to upgrade the CU at some point

 

[AndyPRK]

In answer to your question, of two 6mm cables or one.
I think either is acceptable.

If the CU is close to the kitchen, and it's easy to run 2 cables in, I would run two in. (and use 2 MCBs) (and 2 isolator switches)

This has the advantage that if an element fails and trips the RCD, you can isolate it via the isolation switch and still have one piece of cooking equipment.
Beans for tea, rather than takeaway or microwave

 

[EFLImpudence]

4mm² is adequate for a 32A MCB.

New circuits are legally notifiable to the Local Authority before you start the work.

 

[HughH]

If you connect them in series they won’t work properly. The 240v will be split between them in inverse proportion to the power requirement. You need to connect them in parallel, and yes you can use one cable. That is normal.

Thanks Winston1, yes I understand. In 'series' was the wrong terminology to use. One cable from CU to 1st isolating switch (say, for Induction Hob), and cable continuing to Oven isolating switch.

 

[HughH]

So you wouldn't normally go to the effort of having two 6mm cables and then terminate them in the same MCB.
Two cables, Two MCBs.
The only exception maybe that there aren't any spare ways and you intend to upgrade the CU at some point

To AndyPRK, its relatively easy for me run two cables but unfortunately there is only one space available. From what I have read elsewhere ii is frowned upon to have two cables -> one MCB. Given this, it seems one cable is the way to go.

 

[HughH]

In answer to your question, of two 6mm cables or one.
I think either is acceptable.

If the CU is close to the kitchen, and it's easy to run 2 cables in, I would run two in. (and use 2 MCBs) (and 2 isolator switches)

This has the advantage that if an element fails and trips the RCD, you can isolate it via the isolation switch and still have one piece of cooking equipment.
Beans for tea, rather than takeaway or microwave

One cable = takeway for me

 

[ericmark]

In answer to your question, of two 6mm cables or one.
I think either is acceptable.

If the CU is close to the kitchen, and it's easy to run 2 cables in, I would run two in. (and use 2 MCBs) (and 2 isolator switches)

This has the advantage that if an element fails and trips the RCD, you can isolate it via the isolation switch and still have one piece of cooking equipment.
Beans for tea, rather than takeaway or microwave

Since a MCB does not isolate, that will not work, a MCB with a normal consumer unit is single pole so will only turn off the line, there are RCBO's which will switch both lives, line and neutral, but only really needs with TT supplies.

The major problem is a consumer unit used in UK, is the grouping together of many circuits on the same RCD, with a 100 mA RCD this was fair enough, it was rare they tripped, but with 30 mA it does become a problem.

The same applies to RCD types, however since a new circuit needs either LABC or a scheme member to raise the completion or compliance certificate there is no real point talking about it for a DIY project, as he will have some one on site to advise.

 

[AndyPRK]

This has the advantage that if an element fails and trips the RCD, you can isolate it via the isolation switch and still have one piece of cooking equipment.
Beans for tea, rather than takeaway or microwave

You will note I said use the isolation switch as its DP. Knowing the MCB would not do an adequate job.

OP - I wouldn't let the 1 cable per MCB put you off. But as you wish.

 

[JohnW2]

Since a MCB does not isolate, that will not work, a MCB with a normal consumer unit is single pole so will only turn off the line ...

I would probably have said "... will sometimes not work", since, at least in my experience, RCD trips are much more commonly due to L-E faults that N-E ones, so it would, in practice, probably 'usually work' (i.e. opening a SP MCB would 'usually' prevent an RCD tripping).

Kind Regards, John