New hob + oven

Postby martin42 » Tue Jun 12, 2007 8:18 pm

I've bought an electric hob and electric oven, to replace an old gas hob and electric oven - but I'm starting to wonder if I can power both of the new items! I'd be very grateful for a clearer idea of my options...

New hob rating = 6.6 kW, new oven rating = 3.6 kW.

The old oven is fed from its own 32A type B MCB, running to a combined cooker switch / single 13A socket.

The old gas hob ignition is wired to a fused/switched spur unit on the kitchen ring. The ring looks to be wired with 2.5mmT+E, and is protected with a 32A type B MCB. The kitchen ring also powers the kettle, washer, fridge, freezer, toaster.

The new hob will draw 27.5 amps with all the rings on full, so that could easily be fed from the 32A cooker circuit. But the new oven would draw a further 15.0 amps, giving a total of 42.5 amps. So I'm not sure how to proceed. The cooker is on exactly the wrong side of the house to run a new cable back to the board, and with solid concrete floor downstairs, and uncut sheet flooring upstairs, running new cable all the way back is a last resort, though perhaps a professional spark would spot a possible route that I've missed.

FWIW, the house is 10 years old and has a split load consumer unit, with a 30mA 63A RCD protecting everything except lighting. The main switch and the meter fuse are rated at 100A. The meter seems to be rated at 80A. The central heating uses gas.

What are my options?

Presumably 42.5 Amps is far too much for the 32 Amp circuit, even allowing for how rarely the oven and all 4 rings will all be on full power?

I guess the 32 amp cooker cable *might* be protected conservatively: could I tell this with any accuracy by measuring the conductors with a vernier gauge?

Is it possible to power the 15 amp oven from the downstairs ring, or are fused spur units only rated up to 13 amps? That assumes that 17 amps would suffice for the other devices (kettle, washer, fridge, freezer, toaster) which I guess it would do at a pinch.

Many thanks for any pointers!

- Martin.
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Simply Build It

Postby thedoctor » Wed Jun 13, 2007 7:20 am

Firstly please read our project on Part P of teh building regulations and be aware that contrveining these regulations is both dangerous and now against the law. An electrician will look at your question later.
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Postby martin42 » Thu Jun 14, 2007 7:31 am

Thanks for the pointer to your Regs info - useful summary there. So any non-trivial work now needs to be done or approved by a Certified person.
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Postby sparx » Thu Jun 14, 2007 5:12 pm

Hi, I note your comments & would just like to clarify the position on domestic wiring thus;
It is not the 'triviality' or not, it's the location & type of work that has been a 'controlled service' since June 2005.
ANY electrical work in a bathroom, kitchen, or outside the main bldg. is covered as is the installation of a new circuit any where in the property,
also any fixed wiring for heating controls, solar panels & various other items.
Controlled means either registering the intended work with Building Control & paying a fairly hefty inspection fee before starting work yourself, or getting an approved electrician who is registered with one of the self-cert. scheme operators who can do the work & write the certs. & log the job with your local authority for you at minimal cost.
As for your existing circuit, to check if cable rating suitable, by the fact you have a 32A MCB then it must be at least a 6mm2 cable .
If you treat oven & hob as one appliance then regs say ;
First 10A, plus 30% of remainder, plus 5A if a socket on cooker panel.
So you have total of 27.5+15.0= 42.5, but allowing for diversity =
10a + 30% of remaining 32.5 ie 10.9 = 20.9A assumed average 5a for socket=25.9A on 32A circuit conforms.
It may however get a bit close on Xmas day!!!
regards SPARX
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Postby martin42 » Thu Jun 14, 2007 6:52 pm

Thanks - that's really useful! You sparks must be working flat out these days, as there seems little point in DIY now.

So my options are to treat both devices as a cooker, and just accept that it will trip if everything is on full blast together. Or, pay a professional to install a second 6 mm2 circuit from the board... Which will involve replacing the board, as there are no spare ways.

If having the board replaced, is it best practice now to have both sides protected by RCDs i.e. one for lighting, one for everything else? I've never really liked the lighting being unprotected. Is a 10mA RCD worthwhile for lighting, or does it cause nuisance trips?

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Postby sparx » Thu Jun 14, 2007 9:12 pm

Hi again Martin, sure are busy!
you've summed it up well, a second circuit would be ideal; as for consumer unit, as of next years regs. one of the changes is to have all equipment in bathrooms on an RCD-even lights, the trade off being no longer reqd to do any cross bonding! EU harmonisation! which will allow 13A sockets in bathroom if large enough to be 3 mtrs from bath!!!!
As you say prob. with nuisance tripping then, using a 100mA trip as main sw. doesn't help much either as they still trip with blown lamps etc.
The only way to go is to use a time delayed RCD for main sw. & standard RCD for power 1/2 of board. Unfortunately they are about twice the cost of standard ones, B.W. SPARX
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