craig1410
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| posted on 9/11/14 at 04:47 PM |
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Electrical advice pls. Cooker installation.
Hi,
I have just ordered a new cooker and hob and need a bit of electrical advice. We currently have a single built-under electric oven with a 4 ring gas
hob above it. We are replacing this with a double electric oven (Belling BI70F) and an electric hob (Belling CH60RX) so obviously the electrical load
is increasing quite a bit. I think the existing wiring is fine for the oven but the hob will require additional capacity. I'll probably get an
electrician in to commission the installation but would like to do as much prep as possible including running the new cable.
So, current wiring includes a 32A MCB in the consumer unit and what appears to be 4mm^2 T&E cable (red and black wires are 4.6mm diameter
including the insulation (measured with digital callipers). It's definitely not 2.5mm^2 cable and I don't think it's as heavy as
6mm^2 cable. This goes to a 45A cooker switch which then goes to the terminal block behind the cooker where the cooker cable connects to it.
I think the above cabling is probably suitable for my new oven which is rated at up to 5.5kW but would appreciate confirmation of this. I'm not
100% sure of the cable route but it appears to go up the partition wall and into the space between ground and first floor where, if it follows a
sensible route, it is approximately a 5m cable run. House was built in 2001 to give you an idea of building regs in play at the time. It's in
Scotland in case that matters. The new hob is up to 6.2kW and clearly I can't just wire that into the same circuit unless I'm happy to run
back and forward to the breaker box every time we use the oven and hob at the same time...
So, what is the normal way to resolve this situation? Is it a case of installing another 32A MCB and 4mm cable (or 6mm cable?) to a separate 32-45A
switch in the kitchen or should I use the old cable to pull through a larger cable? (I realise it is probably clipped to the joists so this is
probably simplistic). I don't think a 6mm cable would do the job for both appliances so maybe it's 10mm if I go with a single cable? Or
would you just use the 4mm existing cable to pull through 2 new 4mm cables? Looking for practical advice and tips of the trade really. I've done
quite a bit of electrical work myself before and I've got a degree in electronics and electrical engineering with (more importantly) a lot of
practical experience so I'm pretty competent but I know there are a lot of little details (and perhaps some major ones) which I might not be
aware of. So I may still get an electrician in to commission the hob but I'm hoping I can install the oven myself and at least run the cable for
the hob.
Another question I have is whether I need to fit the isolation switch for the hob in view or if I can hide it in an adjacent cupboard? The oven switch
is next to the hob but is fixed into a tiled wall so I'd rather not have to fit another switch beside it and risk messing up the tiles as I
don't have spares. I know I can't mount the switch directly behind the hob for obvious reasons.
Here are the installation manuals for the appliances:
PDF for Oven: http://www.belling.co.uk/media/62447/Belling%20BI60FP.pdf
PDF for Hob: http://www.belling.co.uk/media/206605/ch60rx.pdf
Many thanks for any advice you can offer.
Craig.
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JoelP
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| posted on 9/11/14 at 05:12 PM |
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It's very rare for people to use 4mm cables. I would pull one stand out and multiply up. If it does turn out to be 6mm, you are fine to connect
both to it. 6mm 32a can take 15kw with diversity applied.
Switches can be hidden so long as accessible.
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whitestu
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| posted on 9/11/14 at 06:27 PM |
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Isolators always used to be wall mounted and visible but it is common practice these days to hide them away.
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craig1410
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| posted on 9/11/14 at 07:47 PM |
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Thanks for the responses guys.
Joelp,
I take it the only way I can check if it's 6mm or 4mm is to measure the diameter of the conductor itself or pull out enough of the cable to be
able to see the grey insulation and read the text? I didn't bother doing this because I thought that even 6mm wouldn't be enough but if
you're sure 6mm can carry 15kW then it's definitely worth checking that.
How confident are you that 32A is enough to run a 5.5kW oven and 6.2kW hob? Obviously it's unlikely that all the loads will be on at the same
time (I guess that's what you refer to as diversity) but I can imagine the oven, grill and a couple of hobs would be a relatively common
occurrence. Then again the thermostats will be coming on and off on the oven and grill so the load won't be constant. What I don't want to
happen though is for the circuit breaker to be tripping all the time, especially on something like Christmas day when it will be in heavy use. Would a
5.5+6.2kW setup be installed with 6mm cable as a matter of course these days or would that be considered cutting it fine?
Re the isolator switch, I don't mind having the one visible switch as it's tucked behind the microwave anyway but I don't want to
have to install another one next to it.
I'll measure the cable conductor diameter shortly and see if it's 6mm. By my calculations, 6mm should be 2.764mm diameter and 4mm should
be 2.257mm diameter. I hope you're right and it's 6mm!
Thanks again,
Craig.
Edit: Looks like 4mm cable. There are 7 cores each of which is between 0.8 and 0.9mm diameter. So that's 0.425mm radius and PI.r^2 = 0.567 sq.mm
per core *7 = 4sq.mm or thereabouts. Damn!
[Edited on 9/11/2014 by craig1410]
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JoelP
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| posted on 9/11/14 at 08:09 PM |
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I believe the 15kw figure came from the onside guide, page 160 according to Google.
Diversity allows you to take the power rating of an appliance, and supply it 10a plus 30% of the rest. Ie 15kw is roughly 60a, so you supply it with
10+(0.3x50),which is around 25 or 26a.
If installing new though you'd just use 2 circuits probably. Your choice how you proceed!
Some hobs can be programmed to limit their max current draw, even working off a 13a plug.
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JoelP
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| posted on 9/11/14 at 08:14 PM |
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To expand on that, your double oven is rated 5.5kw but most of the time you'll only use one cavity. Even when on both, at maybe 2kw each, they
are going to be coming on and off on the thermostat. Total rating often isn't even possible as you often can't use the grill at the same
time as the oven.
Assuming that the 4mm cable is adequately protected by the 32a breaker (which depends on how the cable is installed and where it runs), you might even
be OK with both on a 4mm cable.
My entire house is fed from a 10mm cable.
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craig1410
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| posted on 9/11/14 at 09:37 PM |
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Hi,
Yeah I've been reading up on diversity - I was aware of the fundamental concept behind it but just didn't know the formula. I'm a
bit surprised at the 30% figure, I thought that would have been higher. I've also been reading up a bit on MCBs and how the rated current
isn't the instantaneous trip current but rather continuous load current with MCBs able to run at 1.45x rated current for up to an hour before
they trip.
So, as I see it my 5.5+6.2kw= 11.7kw load = 11.7/230 = 51A
Using diversity this is:
10 + (51-10)*0.3 = 22.3A (I believe by marking the outlet as "Cooker Only" I can avoid the extra 5A normally required.)
If I understand correctly, 4 sq mm cable has a capacity of at least 26A worst case (enclosed in a conduit in an insulated wall) or up to 37A if
clipped direct which I think mine will be. Since this circuit already has a 32A MCB on it and I see no evidence or need for conduits and there is no
insulation between the ground floor and 1st floor where this cable is routed, is it safe to assume the 37A capacity? On that basis, it should be
capable of running the cooker and hob at 22.3A with diversity applied with room to spare. I realise that the 37A means the cable is running at 70C
(30C ambient) but my ambient tends to be more like 23C and 22.3A is only 60% of the maximum 37A rated limit so I would expect the cable to be much
cooler than 70C. This is also a relatively short cable run of around 5 meters too.
So, from the above, does anyone think I am completely crazy to just connect oven and hob to the existing 4mm cable and 45A switch? I realise
it's not what you would use if you were designing the circuit today but is it safe and does it meet regulations if I leave it as is? And would a
qualified electrician happily use the existing 4mm cable or would they recommend an upgrade or refuse to use 4mm entirely?
Final question - is it legal for me to install my new appliances myself if I leave the existing wiring alone and just attach the new appliances to the
cooker connection point or do I need to go via the competent person scheme or building control?
Many thanks,
Craig.
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JoelP
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| posted on 9/11/14 at 10:54 PM |
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Correct on all points I believe Craig.
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craig1410
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| posted on 9/11/14 at 11:14 PM |
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Thanks, I realise this isn't ideal but it often comes down to philosophical arguments about future expansion etc which doesn't apply when
you've already got a 4mm cable installed and it's a major hassle to run a bigger cable.
I've got one of those eco-power meter things that I can use to see how much power is used for a typical dinner so I'll go ahead and
install and if I find that the diversity factor is a bit optimistic in practice then I can look at running a 6 or 10 mm cable before Christmas
dinner...
Thanks again!
[Edited on 9/11/2014 by craig1410]
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