NigeEss
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| posted on 8/3/09 at 10:25 PM |
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Q for the domestic wiring gurus
Putting in a new kitchen and have bought one of the new fangled induction hobs
and a separate double oven.
Combined rating of 50amps and I have one spare slot on the consumer board.
So what's best to do ?
Run a single 10mm cable and split off to two isolation switches, some sort of add
on to the con unit or a new unit.
Any other suggestions ?
Time is an illusion. Lunchtime doubly so.................Douglas Adams.
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Liam
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| posted on 8/3/09 at 11:31 PM |
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A single cooker switch should be OK, feeding an outlet plate with two cable outlets - one for hob and one for oven. Lots of other things to consider
though...
Are you aware this is notifiable work?
What's your CU like - old style fuse board or modern CU with MCBs/RCBOs? Can it handle the extra load?
Where is the cable going to be routed (effects its rating)?
Will most probably need RCD protection. On RCD/RCBO in existing CU, or seperate mini CU for the oven/hob?
Have a search on www.diynot.com in the electrics UK forum - lot's of info and knowledgable sparks.
Hope that helps,
Liam
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NigeEss
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| posted on 9/3/09 at 12:23 AM |
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Cheers Liam.
It's a modern CU, two years old, split, half RCD with MCB's. Cable routed under suspended
floor for 4 metres then 2metres through wall. Still have the option of conduit plastered
in or a bit more chiselingto run it through the voids in the hollow block wall.
The amount of work is not an issue, doing it right is.
Time is an illusion. Lunchtime doubly so.................Douglas Adams.
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nitram38
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| posted on 9/3/09 at 04:24 AM |
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You say half your CU is RCD controlled. New regs now require cables buried less than 50mm (which means most house wiring) have to have RCD
protection.
So most houses need full RCD protection.
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JoelP
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| posted on 9/3/09 at 07:23 AM |
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quote:
Originally posted by nitram38
You say half your CU is RCD controlled. New regs now require cables buried less than 50mm (which means most house wiring) have to have RCD
protection.
So most houses need full RCD protection.
Entirely correct but only as and when he changes each circuit, as they regs arent backdated.
Thats set up above is fine on a 32a mcb with 6mm cable, after diversity. You are allowed 15kw of domestic cooking appliances on a 32a cooker circuit.
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Liam
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| posted on 9/3/09 at 01:05 PM |
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You have to apply diversity to the two appliances separately, and in this case you get 29A, or 34A if the cooker switch has a socket. If the hob has
a 'boost' function the diversity calcs are different. Pushing it for a 32A breaker in any case. In addition, many electricians would
agree that the diversity allowed by the regs on a cooker circuit is a bit slack as there's a high chance the appliances will see close to full
load. Not much point having dual ovens/fancy hob if you can't use it all, and no fun tripping your breaker when you're trying to cook
christmas dinner!
I've rated my cooker circuit cabling and protective device to handle full load - no diversity (only 30A in my case as the hobs are gas).
You'd be well advised to stick with the 10mm and use at least a 40 or 45A protective device. This would ideally be on an RCBO on the
unprotected side of the CU so that it has a 30ma RCD all of its own.
Your cable route sounds OK - i.e. no nasty de-rating factors should apply. Dont put it in the void of a cavity wall - if this gets filled with
insulation at some point the cable will be de-rated significantly. Clipped direct into a chase will be best.
Also, you ignored my comment on it being notifiable work. Are you aware that if you're not getting it done by a registered spark, you're
supposed to notify building control, pay their fee, and have them inspect and test. If you dont have the right bits of paper you may have hassle at
house selling time! You did say you wanted to do it right 
I'd still recommend a look on DIYnot.com if you haven't already. You'll get advice from real electricians (I'm not one by the
way).
Liam
[Edited on 9/3/09 by Liam]
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JoelP
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| posted on 9/3/09 at 07:51 PM |
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i know what you are saying liam, about 32A being a bit close to the wind, but i have NEVER had a call back regarding a cooker circuit. The 15kw figure
is in the OSG.
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Liam
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| posted on 9/3/09 at 11:11 PM |
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quote:
Originally posted by JoelP
...but i have NEVER had a call back regarding a cooker circuit.
Probably cos you switch off the phone on christmas day 
I'm sure as you say it's fine in practice, and for house bashing or price work for customers you might well do the minimum acceptable for
ease and cost. I'd still say NigeEss may as well future-proof his install by running 10mm instead of 6mm since he's going to all the
effort of chasing etc etc. Never know when insulation might get added to the cable run for example, or him or a new owner want to put in an even
bigger cooker etc etc. Almost silly not to run it in 10mm and fit a protective device big enough that it isn't likely to nuissance trip.
IMHO of course.
Liam
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NigeEss
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| posted on 10/3/09 at 10:44 PM |
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Liam, I didn't ignore your comment, simply sighed and resigned myself to more forms and
besurocrasy.
My plan is to run 10mm from the CUunder the suspended floor to a 60a Jbox then two runs of
6mm, one for each appliance so I have two independant isolation.
The wall chase can either be trunked or a bit deeper into the cavity of the blockwork
as they are some sort of hollow terracotta sh*t.
So not into any voids that would get insulated. It's an internal wall anyway.
Might run everything in and get a spark to make the connection if he's happy with the setup.
Time is an illusion. Lunchtime doubly so.................Douglas Adams.
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Liam
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| posted on 12/3/09 at 01:04 PM |
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Where would the JB be? Not allowed to have unaccessible JBs anymore, and for good reasons. I believe you are also not supposed to go down in cable
size until a point with a fuse that will protect the smaller cable from the potential fault current that may be carried by the larger cable. This
isn't really a very conventional solution anyway.
Best practice would be 10mm all the way to a 45A cooker switch above worktop height and to the side of the cooker. Then 10mm from the load terminals
of the cooker switch to a double cable outlet plate (eg
this one) behind the oven that the two appliances would be wired to.
If you really want independant isolation (not really needed) you'd still run the 10mm all the way to the first cooker switch, then continue from
the supply side of that to a second cooker switch (i.e. a radial circuit). Then cables from the load side of the two cooker switches to two single
outlet plates.
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