Splitting 10mm cable for multiple ovens

[James Carmichael]

Hi,

I have a 10mm cable, fed from a dedicated 45 RCBO in my consumer unit, feeding my current range cooker. Everything (mains incomer, consumer unit, all internal wiring) done in 2012.

SWMBO has redesigned the kitchen. She has ordained that we shall have a 16 amp rated oven; a separate 13 amp rated oven; a warming drawer rated at about 4 amps; a gas hob with whatever small feed it needs to make the piezo spark lighters work.

She has further ordained that all of the above will be routed through a grid switch array; one switch per appliance.

Three material things:
(1) GAFAIK, grid switches are rated at a maximum of 20 amps.
(2) I don't fancy trying to manoeuvre 10mm cable around to feed all of these things.
(3) I don't fancy unplastering and decarpeting my house to rip lots of wiring out and start again.

So, basically, I need to find a gizmo that will enable me to take the 10mm 45 amp feed, fuse it down five ways with 5 20amp (or 16 amp) fuses, so that I can then run 2.5mm cable from there to each grid switch then appliance.

Best I can come up with is a small 'garage style' consumer unit, 10mm to main switch, 20 amp MCBs.

Anyone got a better answer? Sticking a slave consumer unit in the kitchen is a bit of a butt ugly answer and this can't be that uncommon a scenario ...

Thanks

James

 

[EFLImpudence]

So, basically, I need to find a gizmo that will enable me to take the 10mm 45 amp feed,

There is such a thing. It is called a junction box. Fit in a concealed but accessible location.
Replace the RCBO with a 32A one.

fuse it down five ways with 5 20amp (or 16 amp) fuses, so that I can then run 2.5mm cable from there to each grid switch then appliance.

You do not need to fuse it down for the ovens.

You can fit sockets for the smaller appliance using the fuse in plug for fusing down.

 

[James Carmichael]

@EFLImpudence -

Thanks for the reply.

I guess I am struggling to find a junction box that isn't some sort of surface mounted monstrosity that will take a 10mm cable and 5 or so 2.5mm cables!

If I don't fuse down when I break the 10mm cable into multiple 2.5mm cables before I get to the sockets, how am I protecting those cables? Or, do I not need to and it's quite ok to have a 2.5mm cable feeding a single 16amp (or whatever) appliance on a 45 amp feed?

 

[JohnW2]

If I don't fuse down when I break the 10mm cable into multiple 2.5mm cables before I get to the sockets, how am I protecting those cables? Or, do I not need to and it's quite ok to have a 2.5mm cable feeding a single 16amp (or whatever) appliance on a 45 amp feed?

You don't need to (particularly if, as EFLI suggested, you reduce the 45A RCBO to a 32A one) - since cooking appliances are, to all intents and purposes, not capable of creating 'overloads', so 'overload protection' is not required. 'Fault protection' (protection against short circuits) would be adequately provided by the RCBO (certainly if it were 32A, and probably if it were 45A).

Kind Regards, John

 

[EFLImpudence]

I guess I am struggling to find a junction box that isn't some sort of surface mounted monstrosity that will take a 10mm cable and 5 or so 2.5mm cables!

It will be surface mount but put it somewhere you can't see it.

If I don't fuse down when I break the 10mm cable into multiple 2.5mm cables before I get to the sockets, how am I protecting those cables?[ Or, do I not need to and it's quite ok to have a 2.5mm cable feeding a single 16amp (or whatever) appliance

You do not need to.
The appliances cannot cause an overload - but anyway 2.5mm² can carry 27A.

on a 45 amp feed?

The supply will be 32A if you change the RCBO

 

[bsr]

What's wrong with a surface mounted junction box? Mount it behind one of the ovens.

 

[James Carmichael]

What's wrong with a surface mounted junction box? Mount it behind one of the ovens.

Rationally, nothing, I guess.
Just feels a bit messy but I guess it's what everyone else does.

 

[James Carmichael]

The supply will be 32A if you change the RCBO

If I were perverse, for reasons I can't think of right now, and insisted on keeping the 45A RCBO would using 4mm cable for the links to each appliance be better than using 2.5mm?

Thanks, again, for your help.

 

[James Carmichael]

You don't need to (particularly if, as EFLI suggested, you reduce the 45A RCBO to a 32A one) - since cooking appliances are, to all intents and purposes, not capable of creating 'overloads', so 'overload protection' is not required. 'Fault protection' (protection against short circuits) would be adequately provided by the RCBO (certainly if it were 32A, and probably if it were 45A).

Hi John,

Thanks for the post.

I am clearly not a spark so excuse the dumb question, but if 2.5mm cable is rated to 27A maximum, I don't understand how a 32A RCBO would (and a 45A RCBO probably would) protect it from a short circuit induced high load? Is this the "can withstand approx. 1.5 x max load for an hour" rule I can never quite remember?

 

[AndyPRK]

You could use a dual plate cooker connection unit.

10mm in

And 4 or 5 2.5mms out

 

[JohnW2]

If I were perverse, for reasons I can't think of right now, and insisted on keeping the 45 amp RCBO would using 4mm cable for the links to each appliance be better than using 2.5mm?

There's really no need. As EFLI and myself have both said, those appliances essentially cannot create an overload - and even the 45A RCBO would probably provide adequate fault protection (particularly given the 10mm² cable)

Mind you, I personally find 4mm² cable easier to work with than 2.5mm² (a bit less rigid!).

Kind Regards, John

 

[EFLImpudence]

Your appliances cannot cause an overload.

Short circuit (L to N) and Earth fault (L to E) currents might be hundreds of amps and cause instant disconnection.

 

[James Carmichael]

Your appliances cannot cause an overload.

Short circuit (L to N) and Earth fault (L to E) currents might be hundreds of amps and cause instant disconnection.

Thanks.

I'm not trying to be deliberately obtuse (just trying to understand) but ...

Ok, get it. The devices cannot cause any sort of overload.

But, short circuit. If the fault current is likely to be 100s of amps, why are you suggesting I swap the RCBO for a 32amp one and/or why is John suggesting a "32A RCBO will definitely be ok, 45A probably"? Surely, both will trip pretty instantaneously in a short circuit?

To put it another way:
I *don't* need do reduce the RCBO to provide any sort of overload protection (for reasons you've kindly explained).
It seems pretty immaterial whether it's 32A or 45A in the event of a short circuit.

This is, now, more for my academic interest (genuinely want to learn) than anything else. You've collectively convinced me: Chunky junction box behind one of the ovens it is!

 

[EFLImpudence]

why are you suggesting I swap the RCBO for a 32amp one and/or why is John suggesting a "32A RCBO will definitely be ok, 45A probably"? Surely, both will trip pretty instantaneously in a short circuit?

Simply because you might as well and to avoid all these unnecessary questions as we cannot see the actual situation.

Your cooker circuit (assuming cable not in any thermal insulation or conduit) could have been 32AMCB with 4mm² cable in the first place.
10mm² is used for the supply to whole flats.

To put it another way:
I *don't* need do reduce the RCBO to provide any sort of overload protection (for reasons you've kindly explained).
It seems pretty immaterial whether it's 32A or 45A in the event of a short circuit.

This is, now, more for my academic interest (genuinely want to learn) than anything else. You've collectively convinced me: Chunky junction box behind one of the ovens it is!

 

[JohnW2]

Your cooker circuit (assuming cable not in any thermal insulation or conduit) could have been 32AMCB with 4mm² cable in the first place.

Indeed, and to further the OP's quest for education, we could perhaps add that, because of the concept of diversity (which takes into account that cooking appliances are not 'all on, all of the time'), a 32A/4mm² circuit is regarded as adequate to service cooking appliance loads with a total 'peak' power of about 83A.

Kind Regards, John