DIY Doctor

lengthening the ring main cable?

Postby kmc00 » Wed Jan 16, 2008 6:23 pm


I'm in the process of replacing a wall mounted double socket with a recesed unit. However, as I'm also moving the position of the socket unit I no longer have sufficient length of cable from the ring circuit (2.5mm2 Twin-core-and-earth cable).

I could lift further floor boards and replace the entire length of cable. However, it would seem easier to simply add an additional lenth of cable. My question is how do I do this. I was presented with a confusing array of products in the local DIY store (junction boxes etc?) and need some advice.

Many thanks in advance for any help.

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Simply Build It

Postby ericmark » Thu Jan 17, 2008 1:17 am

1. One must be able to work out the run of a cable so if you replaced the socket with a blanking plate and extended in the future someone can still see that plate and hopefully would not drill etc where the wires run.
2. Any junction box must be accessible. Putting any box under the floor boards may be considered as not accessible.
So you must consider what you are doing. To replace the cable will cause no problems but to include a junction may cause a problem with inspectors if it is hidden, inaccessible, or overloaded. I would most likely fit a junction box, but to tell you to do that without being on site and seeing exactly what is going on I could easy be giving wrong advice.
Do ensure it is a ring. I have come across a 4mm² radial where someone mistakenly though it was a ring and used 2.5mm² cable instead of 4mm² to extend it. All electrical work should be tested once complete. I don’t think what you are talking about will need Part P but you may wish to go to projects and building regulations and use link to part p regulations and check for yourself.
If you alter a circuit you must ensure you have not exceeded the parameters in doing so. We would normally test earth loop impedance before and after to ensure it was one not right on the edge to start with, and two still OK when complete. We would have to do it so we could fill out a minor works certificate, many electrician don’t bother and get away with it. Only if something goes wrong is there a problem. I say that in case you have building inspectors coming to the house.

Postby HomeBrewPower » Thu Jan 17, 2008 2:20 am


You have a couple of options

1. Isolate the ring mains circuit & test with a BS7671 tester & proving unit
2. Take off the surface mounted socket & backbox
3. You should now have 2 x T&E cables exposed from the wall

A) If the wall is Stoothing (Hollow) then it will be easier for you
B) If it is brick then you will need to chop (CHASE) out the new socket outlet position and fit a steel 2 gang backbox

Assuming the wall is solid brick then you will need to chase out a recess approx 100mm deep and 150 x 150mm long and wide directly behing the area where the existing cables come out of the wall. BE CAREFUL not to chop into the existing cables!!

Then chase a channel approx 50mm deep and 60mm wide from the existing cable location to the new recessed outlet position. ONLY CHASE VERTICAL & HORIZONTALLY to get to the new position NEVER chase diagonally from A to B.

If you want to know more then please reply to this post so I know to continue the 2 options of extending the ring mains sub circuit....
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