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Old 1st Jun 2018, 8:57 pm   #61
Red to black
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Default Re: 13A Twin sockets.

Biting my lip here a bit, I did once have a quick glance at the standard mentioned and they are supposed (DS outlets) to be type tested to 20A as mentioned by others.

2.5mm twin and earth when clipped direct, ref method C, is good for 27A, so theoretically a ring final circuit is good for 54A max, it is not quite this simple as the ring can be unbalanced, ie. the longest leg can have a greater voltage drop than the shortest leg.
Although the IET don't count a RFC (ring final circuit) as a "parallel circuit, and guidance is given to avoid designs running an RFC unbalanced for any prolonged period.
Indeed anything fixed load exceeding 2 kW should be on it's own circuit.
This is also why an unfused spur (which may be at the consumer unit itself) on a single 2.5mm cable is permitted
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Old 1st Jun 2018, 8:57 pm   #62
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Default Re: 13A Twin sockets.

"You can buy a copy for (are you sitting down?) £254"!

Ah yes, the great BSI rip-off! These regs affect us all, are produced at taxpayers' expense, and as a matter of public safety and civic responsibility. They should be freely available in the public domain so that we can all see what is expected for the systems in our homes. Of course, the main cost burden falls on professional electricians who need them to do their work.

I think they might be available in some public libraries, but that's not good enough.
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Old 1st Jun 2018, 8:59 pm   #63
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Default Re: 13A Twin sockets.

It is worth observing that very few domestic appliances other than kettles use more than 10 amps these days.
Most washing machines and many tumble dryers are now 10 amps maximum as are most portable heaters.
It is hard to find a true 3KW portable heater these days that is fitted with a 13 amp plug.
I suspect that this reduction in loading is due to manufacturers trying to avoid warranty claims for melted 13 amp plugs.

Many fast boiling kettles use a full 13 amps, but this is seldom a problem in view of the very short duty cycle.

So the chances of using more than 20 amps from a twin outlet is small, and the chance of it being sustained is smaller still.

I have a modern true 3KW heater, but it was sold "for industrial use" WITHOUT a fitted plug. The warranty specifically excludes any plug fitted by the customer.
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Old 1st Jun 2018, 9:04 pm   #64
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Default Re: 13A Twin sockets.

@ Red to Black - "This is also why an unfused spur (which may be at the consumer unit itself) on a single 2.5mm cable is permitted".

But an unfused 2.5mm spur with a twin 13A socket on it could have up to 26A through it before a fuse (13A in plug) blows. If that spur's 2.5mm feed is buried in plaster (as often will be) then it'll be overloaded
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Old 1st Jun 2018, 9:07 pm   #65
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Default Re: 13A Twin sockets.

@ Boulevardier
No it won't, buried in plaster is still ref method C @27a, don't forget cables are VERY conservatively rated, there are quite hefty safety margins applied
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Old 1st Jun 2018, 9:36 pm   #66
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Default Re: 13A Twin sockets.

@ Red to Black - You're right in the "plaster" scenario that I mentioned, but only for type C installation. Current rating of 2.5mm T&E ranges from 13.5A to 27A, depending on environment, so the 27A is best case. In modern houses, where lots of insulation is used, 20A is a much safer figure, and even that may be optimistic -

http://www.lightwiring.co.uk/lightin...rrent-ratings/

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Old 1st Jun 2018, 9:41 pm   #67
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Default Re: 13A Twin sockets.

Just as a quick clarification here, as I just said A normal RFC in 2.5mm cable is theoretically good for 54A max, actually a bit less in practice, due to volt drop in longest/shortest leg, hence being due to different resistances of the different lengths, which is also incidentally why the IET don't class RFCs as true parallel circuits.
The IET already down graded the "theoretical" max of 54A" to 30/32A which is a large safety margin in itself.
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Old 1st Jun 2018, 9:47 pm   #68
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Default Re: 13A Twin sockets.

@ Boulevardier
Yes I am fully aware of the different reference methods, however that is up to the designer and the person signing off said works to say they are compliant with BS7671 ergo the install is safe for use.

If something is not compliant from the outset then that is hardly the fault of the framework we are supposed to work within, or the standard we are supposed to be working to, is it ?
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Old 1st Jun 2018, 10:06 pm   #69
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Default Re: 13A Twin sockets.

I thought 2.5mm cable was 20amp and a ring not quite doubled that.

At least I erred on the safe side.
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Old 1st Jun 2018, 10:18 pm   #70
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Default Re: 13A Twin sockets.

@ Red and Black - "If something is not compliant from the outset then that is hardly the fault of the framework we are supposed to work within, or the standard we are supposed to be working to, is it ? "

Yes, totally agree with you. Interesting to ponder - if there were to be a major incident (huge fire and loss of life) that was shown to result from a "reasonable" domestic user overloading (say with two 3kW heaters) a twin socket that had been installed in good faith (ie not a kitchen, etc), who would be held legally responsible in a court case? Could the BSI be liable?
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Old 1st Jun 2018, 10:19 pm   #71
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Default Re: 13A Twin sockets.

Hi Frank,
A given cable is not rated at a given current, it all depends on how it is mounted, for example T&E (twin and earth) say 2.5mm is good for 27A when clipped direct (to brick work) even if covered with plaster(ref method C), however the same cable if buried in loft insulation and against a ceiling is only rated at half that (13.5A) as alluded to by Boulevardier above.
Flexes are generally in free air and can dissipate more heat so have a better rating than simple cross sectional area (csa) suggests.
there is no such animal as 20 A cable as it depends on how it is mounted.

As I said it is up to the designer to make sure that things are fit for purpose.
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Old 1st Jun 2018, 10:22 pm   #72
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Default Re: 13A Twin sockets.

Fair enough thanks for the info, not that I have done any wiring in the house but have put more insulation in the loft. The wiring there is only for lighting and the lighting circuit is fused at 5 amp, been in the house since 1970 when it was new.
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Old 1st Jun 2018, 10:29 pm   #73
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Default Re: 13A Twin sockets.

@ Boulevardier
When it comes down to this scenario, it comes basically "what was reasonably practicable" and "what would a reasonable person do"

Now if say you (or I) followed "reasonable" ie. conventional practice and did not deviate from "normal guidance" then it would most likely result in no action taken, however if you/me "skimped" and say did something stupid like wire a rfc for sockets in 1mm cable then the powers that be would come down on me/you like a ton of bricks and we would likely be in a jail cell
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Old 1st Jun 2018, 10:39 pm   #74
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Default Re: 13A Twin sockets.

@ Red and Black
Yes, that feels right. But in a case like that, where £millions compensation was at stake, the parties involved are unlikely to just write it off to "that's life". Insurance companies, contractors, local authorities, etc - are going to insist on blame (and compensation) being fixed on someone. That's why I wondered if the BSI could be found legally responsible. But probably no one can know that in advance.
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Old 1st Jun 2018, 10:46 pm   #75
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Default Re: 13A Twin sockets.

@ Boulevardier
Ha ha, that is about where I work right about now, you have no idea, I can't even talk about it either !!
I don't deviate from the drawings even if I know they are wrong !! the penalties are unreal!!!
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Old 1st Jun 2018, 11:33 pm   #76
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Default Re: 13A Twin sockets.

Quote:
Originally Posted by Boulevardier View Post
"You can buy a copy for (are you sitting down?) £254"!
You can have a 500+ page guide here to the 17th edition regulations for free here.

http://s1.downloadmienphi.net/file/d...92/1385308.pdf

If you have a slow connection then be prepared to bring a flask and some sandwiches LOL
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Old 1st Jun 2018, 11:44 pm   #77
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Talking Re: 13A Twin sockets.

Thanks for the link Charlie - didn't know it could be got -even the last edition
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Old 1st Jun 2018, 11:53 pm   #78
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Default Re: 13A Twin sockets.

That link is quite a bit out of date Charlie, there have been quite a few major changes to the 17th since 2008.

We are now up to 17th edition 2008:2015 amd 3, which means that since that guide was originally published there has been 3 fairly substantial revisions to the regs since then, I know because we had to buy the books! at least amd 1 and amd 3, amd 2 was never printed only available as a secure pdf from the IET, thanks anyway though.

Edit: 18th edition out next month
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Old 2nd Jun 2018, 12:12 am   #79
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Default Re: 13A Twin sockets.

Quote:
Originally Posted by Boulevardier View Post
Thanks for the link Charlie - didn't know it could be got -even the last edition
Page 211 of the booklet (but page 247 of the PDF) maybe of interest to you, it gives a chart of the various T&E cables (including the 2.5 mm) in use and their performance in lengths and some enclosures.
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Old 2nd Jun 2018, 12:17 am   #80
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Default Re: 13A Twin sockets.

Quote:
Originally Posted by Red to black View Post
That link is quite a bit out of date Charlie, there have been quite a few major changes to the 17th since 2008.

We are now up to 17th edition 2008:2015 amd 3, which means that since that guide was originally published there has been 3 fairly substantial revisions to the regs since then, I know because we had to buy the books! at least amd 1 and amd 3, amd 2 was never printed only available as a secure pdf from the IET, thanks anyway though.
Yes ok but it's free ..yes and while the regulations move on I thought there maybe some useful nougats in there concerning this thread.
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