Need to wire in an additional socket - advice

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Currently using extension cables to bring power to one corner of my computer shop work (in a spare bedroom) which is not ideal for many reasons.

Now assuming it is not an end of spur (which it shouldn't be as it is in a middle room but I shall not assume) I could simply take a spur of that.

However the existing socket only has around 3 amps of load on it, I use it for a laptop so the easiest way would be to simply put the new extension on a plug but it is not ideal.

So my questions are:-
1) How do I calculate the correct cable thickness? I assume there must be some formula which takes cable length and thickness of the conductors. The cable will put into plastic trunking.

2) Is wiring it up as extension lead with flex completely against the rules providing the correct thickness flex is used?

The new socket will typically have around 10 amps load at the most, but how many sockets could I have of one spur? I was think of installing two doubles, to give me a 26amp capacity (though in reality it would only be ten amps).

3) Do any new sockets need RCD protection?

4) How do I test the existing socket is not already a spur?

In terms of test equipment I have an oscilloscope/multimeter and a very basic socket tester, but I have no proper test equipment.

Thanks in advance.
 
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So my questions are:-
1) How do I calculate the correct cable thickness? I assume there must be some formula which takes cable length and thickness of the conductors. The cable will put into plastic trunking.
I always stick with the same conductor size that is already supplying the circuit, if you were to install a fused spur the you would be able to to reduce the cable CSA.
2) Is wiring it up as extension lead with flex completely against the rules providing the correct thickness flex is used?
If you mean hard wiring the flex to a socket outlet and running the flex in trunking or clipping cable direct and securely fitting the bank of socket outlets on the extension, no not completely against the rules, but not the recommend way.

The new socket will typically have around 10 amps load at the most, but how many sockets could I have of one spur? I was think of installing two doubles, to give me a 26amp capacity (though in reality it would only be ten amps).
If you installed 2 doubles then the best method would be to either extend the circuit correctly or use a 13A FCU (fused connection unit) this will allow more than one double outlet to be added to this part of the circuit.
3) Do any new sockets need RCD protection?
If RCD protection is not already at the board under some circumstances yes.
If you intend to run your addition in trunking, RCD protect will not be required, if cable was buried in wall less than 50mm it would.
4) How do I test the existing socket is not already a spur?
There are both visual tests and instrumental test.
I would first check to see what type of circuit was at the board whether it's a ring or radial. The fuse rating and number of conductors parked in the fuse/MCB can often give this information, but it's not an absolute. Also check at the outlet you are taking the feed from for cable numbers.
Using a continuity test, would also help. If the circuit was a ring and the socket was not a spur you would expect it to give phase and earth/CPC (R1+R2) readings, that are within 0.05 ohms as the reading at the board.
In terms of test equipment I have an oscilloscope/multimeter and a very basic socket tester, but I have no proper test equipment.
Your multimeter would help with continuity and isolation testing, the socket tester also with polarity and connection of conductors.
 
If I was to wire into a 13a fused connection unit then would I still need to all the testing to ensure its not a ring final etc? The fuse box is a 60amp 8 way Wylex unit with wired fuses so no RCD protection at all, but the cable will go in plastic trunking attached to the wall itself, so it will be obvious it contains a cable.
 
Even if the cable was not in trunking but clipped directly to the wall, it would not need RCD protection as you can see there is a cable there.
That is why if it was buried in the wall at a distance of less than 50mm, it would need RCD protection as the cable can not be seen and it is a distance within the wall that could still be easily mechanically damaged by fixing, such as screws/nails.

You can not add a FCU to an already spurred socket, so yes you must first find out if the socket is suitable for taking a FCU from.


If you cut and paste this:
17th Edition, BS 7671:2008 Corrigendum (July 2008)
In to google search engine you will find a downloadable PDF that will explain RFC, Radials and methods of spurring.


Under

17th Edition, BS 7671:2008 Corrigendum (July 2008)
electrical.theiet.org/wiring.../bs7671-2008-corrigendum-jul08.cfm
 
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A bit drunk now so apologies, but the ring main is on a 30amp fuse, there is four sockets upstairs as followed:-

1) Master bedroom - Two sockets in different places
2) Middle room (the room concerned) 1 double socket (was one single but I converted to a double many years back)
3) Back room 1 double (again converted to double).

In the middle room I seemed to remember three different cables going into the socket, but it was a lot time ago, would need to open it up to confirm.

My electrical knowledge is also mostly DC, so I didn't quite understand the testing methods you described.

To get round this issue, would it be frounded open to just use a 13 amp plug and plug that into the little used existing socket? I.e wiring a fixed extension lead?
 
If the lead and the bank of sockets attached to the lead are fixed securely and the flex of the extension is not run under lino, carpet etc..
It can be done, it's not really what I would recommend but providing the cable and socket box are secure, it should not cause any hazards.
 
Thanks, it is what I may well do, as I am not quite certain how to test the socket for being end of spur, and the fact the existing socket only currently powers a lamp and a laptop.

I was thinking of buying some 2.5mm flex and the length would be around 8 metres. That should be perfectly fine for a maximum load of 13 amps, although in reality the load will be a lot less than that.

I will use one double socket, and the cable will be neatly put inside trunking it will go round the door frame and won't be under a carpet etc.
 
If you intend to run your addition in trunking, RCD protect will not be required, if cable was buried in wall less than 50mm it would.

Even if the cable was not in trunking but clipped directly to the wall, it would not need RCD protection as you can see there is a cable there.

That's great news! - so we don't have to 'RCD' protect socket outlets any more? :)
 
joydivision said:
I was thinking of buying some 2.5mm flex and the length would be around 8 metres. That should be perfectly fine for a maximum load of 13 amps, although in reality the load will be a lot less than that.

There's no need to use such heavy flex for an extension that'll be protected by the 13 amp plug fuse. You might have a problem getting it into the plug anyway. :!: :!: :!: Just use standard 13 amp flex. :) :) :)

and also said:
--I seemed to remember three different cables going into the socket --

If that's true, it's already got a spur on it. You can't add another one. :( :( :(

Edit: I mean you can't wire another spur into the back of the socket. There's nothing to stop you plugging in your extension lead.
 
Yeah I realised when I was doing the calculations after posting that I over specced the flex some what :D. I am planning to use 1.5mm flex.

I would need to open the socket and have a look, it was many years ago I was last in there so I may well be getting mixed up, either way I will just use a 13 amp plug to be sure.
 

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