When I rewired my house, I put in a few spurs to feed low power things like table lamps. I probably didn't need to, but all these spurs are single sockets.
Now I want to turn one of them into a double socket to feed a WMC and a TD, so I'm not happy about having a single 2.5mm cable feeding them.
I have two options: either break into the ring and extend it (using a JB behind the existing double socket that the spur comes from), or simply double up the cables to the spur.
The latter seems more sensible, but there is something at the back of my mind that says it is wrong.
I'll ignore your comments about learning more about wiring. I've re-wired several properties, all have improved on the wiring by so called professionals.
So why would you say that doubling up the cables to a spur (rather than extending the ring using a JB) is a bad idea?
Remember that according to the regs I am not obliged to use more than
Yes, spurs can feed one single or one double socket, the assumption being that the load on a double socket won't in practice exceed 20 A.
Well it should be fine unless the cable's completely surrounded by thermal insulation. All the Table 4D5A ratings of 2.5 T&E equal or exceed 20 A. If you think the load will exceed 5 kW for long periods then you should use two single sockets (in the ring or on separate spurs) and not a double.
Inserting the new socket into the ring is the better option, since it will ensure better distribution of current in the two legs of the ring (unless the spur point is exactly at the ring's mid-point).
The regs require you to ensure that the sustained load current in any part of a ring under the actual intended conditions of use won't exceed the as-installed cable rating, said rating not to be less than 20 A.
On Mon, 23 Jan 2006 16:04:26 +0000 someone who may be Nigel Molesworth wrote this:-
You are going to put a junction box behind a socket outlet; presumably by removing the box it is currently fitted to, making a (deeper) hole, installing the junction box, re-fitting the box and then fitting the socket to that? Bearing in mind the requirements for being able to inspect joints, how do you intend to make the connections within the junction box?
I suggest that you do not, until you have learnt some more. At the moment ISTM that you are at the stage where a little knowledge is dangerous.
1) conductors in parallel are not appreciated by the IEE. Since you do not appear to understand why, I suggest that you find out.
2) it is unnecessary. Unless you intend the duty cycle I suggested and which you snipped (or there is a particular combination of insulation and/or ambient temperature) the existing cable is probably capable of meeting the duty cycle [1].
3) if the existing cable is incapable of meeting the duty cycle the next size up should be used.
4) such an arrangement is non-standard and will confuse anyone coming across it later. You do understand the bits about considering future maintenance/new work in the Wiring Regulations?
5) getting four conductors into the terminals of most sockets is going to be interesting. If you do get them in then, other than in a deep double box, you are unlikely to be able to fit the socket back on the wall. Apart from anything else, this is unlikely to meet the requirement for good workmanship.
Incorrect. That is a minimum cable size. The actual size depends on the precise installation.
[1] while heating, a washing machine and tumble dryer may draw 13A. However, they do not do so continuously. The worst case would be starting both going at the same time, so they both draw 13A for say
20min (likely to be less), after which the heating element in drier may cycle on and off but that in the washing machine is unlikely to come on again until it is loaded again.
You would think so, wouldn't you - this is what was at the back of my mind. But if you think about it, it makes little difference, the current will still be balanced from the spur take-off.
On Mon, 23 Jan 2006 17:49:45 +0000 someone who may be Nigel Molesworth wrote this:-
Only in this posting. In previous postings you were proposing using two 2.5mm conductors in parallel.
I have yet to insult you and see no reason to do so in the future. I have pointed out that the questions you have asked imply a rather lower level of knowledge than you appear to think you have.
On Mon, 23 Jan 2006 21:17:14 GMT someone who may be Chip wrote this:-
That is a particular case of conductors in parallel which has been thought through and, provided it is tested properly, provides an appropriate balance between risk and cost.
I have always assumed that 2.5mm wiring would be OK for a double socket as long as the wiring isn't embedded in insulation. However by saying he is wrong without giving the reason why is not helpful to the OP nor the group.
No I'm not trying to be insulting but trying to be helpful so that your replies are seen as a positive contribution in the OP's and group's eyes.
That's not true. BS 7671 has some specific requirements for conductors in parallel, from which the 'classic ring' is exempted.
In summary, the requirements are these (my précis):
473-01-06: where protected by a single device there shall be no branch circuits, switches or isolators in any of the individual parallel paths;
473-01-07: where protected by a single device and current sharing is equal, the value of Iz for the purposes of overload current protection (reg. 433-02-xx) is the sum of the individual Iz's. (Iz is the as-installed current rating, and current sharing is deemed satisfied if 523-02-01 is satisfied.)
473-01-08: where currents are unequal, the design current and overload protection of each conductor shall be considered individually;
473-02-05: if a single device is used for s/c fault current protection it must be effective for a fault at any position on one of the parallel cables (and various alternative methods are given if this can't be achieved);
533-02-01: measures shall be taken to ensure equal current sharing, the requirement being satisfied if the cables are of the same material, construction and CSA, and approximately the same length with no branch circuits (and special precautions for cables greater than 50/70 mm^2).
It is quite obvious that paralleling-up two ~equal lengths of 2.5 T&E for the purpose being discussed does not violate any of these rules.
On Tue, 24 Jan 2006 06:57:14 +0000,it is alleged that David Hansen spake thusly in uk.d-i-y:
True, I was playing devil's advocate.
This thread has however got me thinking. What the OP is proposing is certainly extremely unconventional, almost like a ring with 'joined bits', intersecting rings, etc.
But... the lengths paralleled would be almost identical in length, thus the usual reasons for not using the arrangement would not apply.
Personally, like many others in this thread, I'd say 2.5 can handle it unless buried in insulation, but would doubling the cables be *wrong*?
On Tue, 24 Jan 2006 10:28:58 GMT someone who may be Chip wrote this:-
The question I answered was whether it would be a bad idea? My answer to this question is, yes.
The Wiring Regulations are, rightly, not as prescriptive as some would like and some think they are, but do make the point that departures from them need to be justified and recorded by someone who knows what they are doing.
On Tue, 24 Jan 2006 09:30:26 -0000 someone who may be "Fred" wrote this:-
Almost anything can be construed as an insult.
There are three things to consider:
1) some people ask genuine questions, others do not. Failing to reply to questions/points about duty cycle, how the connections are to be made and so on, then suddenly changing from two 2.5mm conductors in parallel to single 4mm or 6mm conductors, allows people to draw their own conclusions about this.
2) a little knowledge is often a dangerous thing.
3) if a question is genuine, spoon-feeding does is not helpful for individuals or groups. What is helpful is learning how to learn. The questions I have asked in this thread are an example of one way to encourage learning.
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