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DIY Forum >> Electrical Questions >> Cooker circuit https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1242764162 Message started by Rookie on May 19th, 2009, 9:16pm |
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Title: Cooker circuit Post by Rookie on May 19th, 2009, 9:16pm Hi, I want to install a cooker circuit feeding two cookers one of 2.8kw and one of 4.6kw. I have done the initial sums and come up with a cable size of 6mm. However the cable is 15mts long from the consumer unit to the cooker switch, is bunched with two other cables for about 8mts and runs on its own inside an insulated stud wall of about six feet long. It is a modern consumer unit with mcb's. I have used the correction factors and come up with an answer of the circuit requiring a 25mm cable. I think I am going wrong somewhere in my maths or at least I hope I am. Could somebody please put me right with the details of coming to the correct answer. |
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Title: Re: Cooker circuit Post by Zambezi on May 20th, 2009, 7:53am Have you applied diversity to the cookers? (assuming they are being used in a domestic environment). |
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Title: Re: Cooker circuit Post by dingbat on May 20th, 2009, 9:53am As well as diversity you could also apply the principal of reg 433.3.1 (ii), if the cooker is an entirely resistive load, meaning that you would not have to base the cable rating on the protective device overload rating, as overload protection would not be relevant. (In this case, the full load of 20A for the largest cooker would safely be carried by a 6 sq mm cable installed to method #103) The fuse/cb would still afford fault protection if the Zs was low enough. (I'm not suggesting this as a normal solution, even though it is within the regs, because too few installers actually understand this approach and it doesn't provide allowance for future changes to load characteristics. In this case, however, I doubt you'd have a problem.) |
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Title: Re: Cooker circuit Post by Y3 on May 20th, 2009, 10:50pm dingbat wrote on May 20th, 2009, 9:53am:
I dont quite understand what you mean Ding, could you put it in my language please ;D Why isnt overload relevant on a purely resistive load but is on a load that isnt purely resistive? :-? |
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Title: Re: Cooker circuit Post by dingbat on May 21st, 2009, 7:16am Carl, a purely resistive load is, effectively, nothing more than a piece of conductor (unlike, say, a motor). The impedance of such a load is made up almost entirely of the simple resistance of that conductor. Such a load could fail in short circuit or earth fault, or go open circuit, but is incapable of creating an overload. So, in this case, the 20A (full) resistive load would easily be carried by a 6 sq mm cable at #103, with no risk of overload and therefore, no need to provide overload protection, but the circuit could be protected against fault currents by a 32A cb, simply by meeting the Zs limit for that breaker. (But, as I've already stated, this would be something of a last resort in most circumstances.) |
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Title: Re: Cooker circuit Post by Y3 on May 21st, 2009, 7:27pm dingbat wrote on May 21st, 2009, 7:16am:
The way you explain it has such elegance ;D joke! Thanx for that, I understand! The description that you give sounds similar to describing what a fuse is. |
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Title: Re: Cooker circuit Post by Rookie on May 21st, 2009, 9:08pm Thanks Guys. I have installed the circuit, livened it up and all testing ok including the Zs. I was going to use an rcbo but have saved a few quid by using a mcb. As an aside the board is a Hager and the few wholesalers that I went in to didn't stock Hager fuses. Luckily the board had a spare 32a mcb anyway. Could I have used other makes of fuses or is this not good practice? |
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Title: Re: Cooker circuit Post by Y3 on May 21st, 2009, 9:45pm Does the cooker switch have a socket incorporated within it? If so, then you should have used an RCBO for 30mA protection, as required by the regs for sockets rated less than 20a for use by unskilled people and portable equipment that can be used outside. |
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Title: Re: Cooker circuit Post by Lectrician on May 21st, 2009, 9:49pm Or indeed if the cable is buried in the buildings fabric in a tradional fashion. |
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Title: Re: Cooker circuit Post by Rookie on May 21st, 2009, 10:15pm No socket outlet incorporated. Am I still ok? I could fit an rcbo to cover any possibility that the switch might one day get changed to a switch with socket outlet. |
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Title: Re: Cooker circuit Post by Y3 on May 21st, 2009, 11:00pm As Lec says, above, if the circuit is buried within the fabric of the building less than 50mm and not protected by an earthed metallic covering then an RCBO will be needed. |
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Title: Re: Cooker circuit Post by dingbat on May 22nd, 2009, 7:18am wrote on May 21st, 2009, 9:45pm:
There's yet another of those anachronistic (long word there for Carl! ;D) practices. Why on earth do they still make and sell an item of switchgear that dates from The Ark? If this is such a good idea, why not manufacture shower switches with a built in fcu for, say, an extractor fan? Or why not build in a socket outlet on the front of a consumer unit? Or make a combined light switch and smoke alarm test button? ;D ;D ;D Any more bright ideas out there? |
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Title: Re: Cooker circuit Post by Zambezi on May 22nd, 2009, 9:01am I am off to the patent office.... |
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Title: Re: Cooker circuit Post by dingbat on May 22nd, 2009, 10:35am I've got it. A single phase DB, with a bit of dado trunking underneath, incorporating socket-outlets, telephone socket, aerial socket, doorbell receiver and sounder, intruder alarm panel and controller for whole-house sound and vision. Now, all we do is market this as a 'rewire in a box' and we'll make gazillions! ;D ;D ;D |
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Title: Re: Cooker circuit Post by Zambezi on May 22nd, 2009, 10:49am dingbat wrote on May 22nd, 2009, 10:35am:
I have just got back from the patent office, now I have to go again... |
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Title: Re: Cooker circuit Post by Y3 on May 22nd, 2009, 7:20pm dingbat wrote on May 22nd, 2009, 7:18am:
That is a long and unheard of word for me, for most people I think :o dingbat wrote on May 22nd, 2009, 7:18am:
They are good when the kitchen is tiny and there isnt much work top, say a 300 base unit with a bit of work top, where a cooker and fridge freezer is positioned either side of it. A cooker switch with integral socket comes in handy then, just so a kettle or toaster can make use of the 300mm wide work top. You know how small council kitchens can be, but on the other hand they can also be massive, ive seen a few! dingbat wrote on May 22nd, 2009, 7:18am:
Somebody stop him, ive never seen Ding get this excited? ;D |
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Title: Re: Cooker circuit Post by Lectrician on May 22nd, 2009, 8:29pm TBH I never fit the cooker isolators with a socket. I too see them as 'granny-ish'. |
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Title: Re: Cooker circuit Post by Y3 on May 22nd, 2009, 9:02pm Isolators without sockets have the benefit of not needing RCD protection if the circuit is installed according to that purpose, saves fitting an RCBO if one was needed and money. |
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Title: Re: Cooker circuit Post by Goodsparks on May 23rd, 2009, 10:57am Quote:
http://www.alibaba.com/product-free/11155089/Din_Rail_Schuko_Socket_Ac40_German.html |
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Title: Re: Cooker circuit Post by Lectrician on May 23rd, 2009, 12:16pm [gallery]Lectrician/1243077379.png[/gallery] |
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Title: Re: Cooker circuit Post by Y3 on May 23rd, 2009, 4:20pm I think Lec has been using Adobe Photoshop |
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Title: Re: Cooker circuit Post by Zambezi on May 24th, 2009, 10:42am I remember years ago (they may still be fitting them) that they used to fit all-in CUs into low cost housing (shanty town housing). It was a CU with a couple of sockets, light switch etc. All they did was fit the CU and then run a cable to the light, job done, move on to the next one. |
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