How to wire in oven and hob - advice please

Many thanks for the reply to my recent post regarding kitchen electrical wriring regs. All issues are now sorted except for the oven and hob - which has now arrives and 'er indoors is nagging to try it out :-)
To summarise - the oven is rated at 5.18kW (22A @ 230v) (double under counter oven) and the hob at 5.4kW (23A @ 230v).
The boss wants to have only one DP isolator above the worktop, which I agree with, so the question is how to wire this lot in.
The oven and hob manuals state that a 30A fuse should be used with a minimum of 4mm T&E cable.
My idea was 10mm T&E cable from the CU to the DP isolator then 6mm T&E to two cooker outlet plates then to the appliances. The CU currently has a 32A MCB on the cooker radial circuit, but I could change this to a 45A MCB. However my concern is that the overall fuse is 45A, but the individual appliances should really be 30A. Can you get fused cooker outlet plates or something similar, or must I go for two completely separate circuits from the CU with 32A MCBs.
This cannot be an uncommon application, but what is the best way to proceed, with the regulations.
BTW the CU is only a few metres from the oven hob, max 4m or cable from CU to DP switch then 1m to the appliances.
Any help greatly appreciated so I can get it done this weekend
Thanks
Duncan
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I personally would wire two 30A supplies to two isolators. If you wanted to hide the switches you could put them in the cupboard at the side of the oven housing, but not right at the back where you cant get to them. You won't need to switch them off other than maintenance so you're not going to need to access them that often. Also keeps both switches from being above the worktop, double happy for the boss! Your original way wasn't really up to scratch, the 6mm wouldn't be protected other than by the 45A fuse, not good. You could get 32A fuses to go by the hob & oven but you'd take up more room than the 2 switches so would have to put them in the cupboard next to the oven anyway, so if you have to put something in there you may as well make it the switches, cheaper, easier and more convenient. If the hob goes You've still got the oven, without having to take it all out to isolate it.
SJW A.C.S. Ltd.
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The Regs allow you, sensibly, to treat the hob + oven as comprising a single cooking appliance, to which you apply standard Domestick diversity rules for a cooker. The combined Hamperage from your ratings is 45A. The Diversity allowance for domestic use is "10A + 30% of the rest +5A if you've a socket on the control panel". In your case, ass-U-ming no such socket, the diversity-corrected load is (10 + 35*0.3) = 20.5A. Thus it's well within Regs to use a single circuit, with a 32A MCB, wired in 6mmsq as far as the switch (4mmsq kind-of OK on short run without thermal insulation, but don't arse about, use 6mmsq). From the DP switch make the connections to the hob and oven in 2.5mmsq if you already have it, or 4mmsq if you're buying new (might as well allow for the odd Christmas lunch peak load).
I do hope your hobnoven come with thermally resistant oversleeving for the L and N conductors where they're connecting in to the hob, though - it does get warm in these appliances (no s**t Sherlock ;-) and plain PVC gets soft and less useful at higher temps (70C upwards is the usual guide)...
HTH - Stefek
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On 14 Jan 2004 21:25:14 GMT, snipped-for-privacy@hp.com wrote:

Have you ever wired anything up before. Stating regulations willy nilly does not constitute a safe and constructive answer to help the OP along. If I came across a cooker and hob wired the way you suggest I would probably condemn the whole installation on the grounds anyone who plays around with one part of the electrical installation regardless of knowledge or competence will probably have had a go at the rest of the wiring. It's generally not good practice to run multiple supplies from one isolator, especially without providing any protection in the way of upstream CPD's.
SJW A.C.S. Ltd.
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snipped-for-privacy@hp.com scribbled :

So you advise to wire from the DP switch in 2.5mm cable even though the whole circuit is backed up by a 32A MCB? Don't play with something you know nothing about.
--
Gary
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"Advise" is over-strong, but it's both within the letter of the regs and consistent with safe procedure. The two partloads are - if you read the poster's original query - each going to draw no more than 24A (10 of your best British keelowatts); and that's only under peak conditions when all 4 themostats/simmerstats on the hob cut in at once, or when both ovens have their element thermostats switched on (e.g. when warming up) and their fancy-dancy side-mounted auxilliary browning elements switched on (or whatever else is in there to get the full load up to 10kW). Like I said, if buying new anyway then 4mmsq would give a bigger margin for safe operation; full paranoia would be 6mmsq all the way through, Just In Case some other occupant at a later date decided to attach an all-in-one older-style freestanding cooker to just one arm of the circuit beyond the DP switch; but 2.5mmsq for the short (earth loop impedance considerations) downstream sections to separate hob & oven *is* permissible, safe, and practised, where the loads are as the original poster described.
These are thermostatically controlled loads, which have no way of causing an overload beyond the rated power, and even in "full" use draw well under the full rated power when considered over timescales of minutes: very different from either a socket circuit, where multiple unknown loads can be plugged in, or motor circuits where a stall or excess mechanical load can cause a higher current to flow for a sustained period. And in just such cases, the Regs foresee and allow for - and qualified penny-penching circuit design engineers looking to cut another 60p off the materials costs for the next Barratt box take full advantage of - sizing of conductors according to the peak load, rather than according to the protective device. You may not find it pretty; you may argue that it's cutting close to the edge; and that it's a safe simplification to adopt the simpler principle "all conductors should be capable of carrying for an indefinite period the nominal current of the protective device" (In in Regs-speak). But that's *not* the full story on either what the Regs say or what economical engineering practice does to balance cost against compliance; and the tweaks about "nature of the load" allowing sizing by design current rather than nominal are used internally in equipment wiring (you don't think those hob elements have 6mmsq conductors leading to them, do you?), and in the widespread Continental practice of 0.75mmsq appliance flex, unfused, connecting into 16A radial circuits with no closer fusing, as well as this particular case of splitting out feeds to Modern Kitchen Hob-n-Oven splitlevel units.
Still, don't mind me: I probably don't know what I'm talking about; and probably never wired nuttin'...
Stefek
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I see no reason why this is not acceptable, assuming no insulation. Indeed, it is the standard method, as I understand it.
The 2.5mm cable is rated to 27A (IIRC). The nature of the load prevents a greater load than 22.5A being drawn. Diversity allows both parts to share a 32A circuit. The earth loop impedence and voltage drop on a short run like this is very unlikely to forbid it, unless the run of 6mm is long.
It is no different in principle from running an unfused spur in 2.5mm on a 32A ring circuit, where the spur cable is also only rated for 27A (20A under certain installation conditions).
Christian,
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"Christian McArdle" wrote in message

All quite correct, except for one thing no-one's mentioned yet: ambient temperature correction factors (see Table 6A1 in your On-Site Guide). The 27A 'clipped direct' rating of 2.5mm^2 T&E only applies up to 30 degC, at 40 degrees the derating factor is 0.87, at 50 degrees 0.71. So in this situation 2.5mm^2 cable is only suitable if the temperature won't exceed about 40 degrees, and it's quite possible that that could be exceeded where the cable approaches and enters the appliance.
So with 2.5mm^2 PVC cable Stefek's design is likely to be marginal or non-compliant. With a 2.5mm^2 heat resistant flexible (3093Y etc.) you'd probably be OK, provided you've got a bootlace ferrule crimper for making the ends off in the wiring accessories.
NB (for the pedantic) your statements "nature of the load prevents [overload]" and "diversity allows [...]" don't sit comfortably together in the same paragraph without further explanation. Diversity is being applied, quite validly, to the 'feeder' circuit from the MCB to the point where the circuit splits after the cooker control switch. Since diversity has been applied, overload, relative to the design current (Ib), _can_ occur -- but that's OK since the 32A MCB will provide overload protection for the 6mm^2 cable. After the split point, if we drop down to 2.5mm^2, there's no overload protection, only s/c fault protection provided by the MCB. Hence no diversity can be applied after the split and the cables have to be rated for the FLC of the appliances -- which of course is what we've done.

Exactly.
--
Andy



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Thanks for a calm exposition of just how and why the design is marginal. "Marginal" is just right - i.e. (as I claimed) it's the sort of just-compliant watch-the-pennies just-about-good-enough no-headroom apporach used in practice on many mass-market builds; for one's own use at home 4mmsq would be sounder, but the 2.5mmsq may be justifiable. Though I made passing mention of the higher temperature of the oven/hob environment, I didn't make explicit the necessary derating of the cable. Such higher ambient temp is particularly likely to occur in the space behind the "built-in" oven, where there might well be a metre or so of wiring slack of your (carefully, gently folded ;-) final T&E. And I'd also briefly thought to mention the use of a heat-resistant 2.5mmsq flexible, and rejected it on the grounds of the poster being (a) unlikely to have a bootlace (or ring) crimper to hand, to make the connection to the terminations reliably, and (b) 2.5mmsq silicone-insulated or similar flex being harder to source than T&E - the OP might've ended up with 1.5mmsq "immersion heater" cable from some Saturday-morning trade-counter "sod the weekend cowboys" type, which would be well under reasonable current-carrying capacity...

Again, thanks for the clarification. With the increased popularity of split hob/cooker arrangements, will (or have) wiring-accessory mfrs brought out a "dual-control" cooker control switch, with two 45A or 32-A-rated DP switches in the one (probably double-width) accessory? Obviously you can mount two 45A control switches in a dual box, but it's more of a pain wiring them up, with the heavier incomer needing to go both in and out of the first one.
(Oh, and maybe the IEE will issue a guidance note on wiring split hob-n-ovens from one existing 45A cooker feed, discussing the limits on going down to smaller cable sizes for the post-control-switch portion, in as concise and authoritative terms as Andy managed in the text quoted above? ;-)
Stefek
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Andy, as I understand it the, temperature rating of the cables applies to the whole cable being at the higher temperature. When a small local ambient temperature variation occurs, the derating factors do not, IMO, apply, provided the temperature at which the insulation is breached does not occur. The best example I can think of, is the domestic iron, where there is a major temperature gradient between the sole plate and the cable cores. I do agree however that at the back of a built in oven, care in cable type is required. I believe that in the majority of built in oven installations, the back is actually fan cooled, to alleviate this problem. Anyway, that's why there is a fuse??!! The cable size will make zero difference in the majority of installations, as the I2R losses are too low to be significant against heat loss from the oven.
Regards Capitol
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which
agree
minimum
32A
proceed,
I kind of confused now, one answer saying I should use two seperate supplies and another talking about diversity rules and that I can use one 6mm T&E and a 32aA MCB.
Even if I apply Stefek's logic and the DP does have a socket I only get 25.5A or roughly 6kW of load. I have had an 8.5kW shower wired into a 32A MCB via 6mm T&E for years and never had a problem.
Further answers and/or hints / tips / advice urgently and of course greatfully recieved.
Duncan
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32A
I think you'd struggle to get a single 10mm2 and 2 x 6mm2 into the isolator.
When I did mine a few years ago, I ran 6mm2 to the Isolator and then 2 x 4mm2 to the Hob/Oven - and its been absolutely fine.
Tim.
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isolator.
Like the man says
mike
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My thoughts as well. The 6mm from 32amp MCB in the consumer unit to a 45amp DP switch on the wall. Then 4mm's to each appliance from the DP switch. The DP switch will be tight with two 4mm in the connectors, but they are meant to take 8mm without taking any of the strands off the cables. You're also better to sink a 32mm deep back box in the wall to make the cabling easier for yourself. A standard inch deep back box is just a bit too tight.
As was said in another reply, the oven and hob combination can be taken as one free standing stove, so the diversity of the thermostats working on the hob will allow the appliances to be run on the 32amp fuse rating. Unless your instruction manual says different. Anything higher than 32amp and you'll need to upgrade the supply from the consumer unit to 10mm and use a 60amp rated DP switch on the wall.
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What I have done is to pass one of the conductors for the Hob through a current detection relay, this coupled with a delay off relay is then used to supply power to the cooker hood fan and light.
That way the hood fan starts each time the hob is used and stays on for a little while afterwards.
Michael Chare
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I have an oven + hob connected to a single 30A fuse. In 18 years I have never blown the fuse. - even cooking Xmas dinner. (Having said that maybe now I should look for the fuse wire.)
Michael Chare
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