Extending wiring to socket

Postby landed » Sat Jul 16, 2011 10:52 am

We have taken out a partition in modern uk house and there were 2 sockets either side. The cable to them is not long enough to allow the sockets to go to either side now, so is it possible to use some product and an extra piece of wire to extend them both.
Or do we trim them and finish the circuit with a junction box to stop the ends being live ?

Thanks for ideas , safety is the prime important thing.
We cannot easily trace the wire back as it disappears into the ceiling joist through a hole.
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Simply Build It

Postby ericmark » Sat Jul 16, 2011 3:11 pm

Likely you must join the cables together or it may overload one of the cables as the system is designed to share the load between both cables.

There are many ways to join cables. What ever is used must require a tool to access the cables. So insulation tape would not comply. Likely easy method is maintenance free junction box.
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Postby landed » Sun Jul 17, 2011 11:36 am


It sounds about right for me. Thanks for your reply. Are there any limitations to length one can extend household wires. I am guessing the socket comes from a wire from the main 'home junction'.
I was also concerned about coiling up the excess length (heat energy or antena effect wireless interfearance etc), so I am guess if I want to remove the socket then it is better to trim the wire and end it with a junction box. I assume I can end it with a junction box or should a junction box have wires entering AND leaving it.

Just curious on the previous points but the original issue seems to be solved. Thanks eric and great forum.
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Postby ericmark » Sun Jul 17, 2011 12:51 pm

Working out exactly what you have is a problem so I may have this wrong but I would expect there to be two cables going into junction box to complete the ring.

Yes there is a limit to cable length. A final ring to BS7671:2008 would be limited to maximum of 106 meters of cable. Before 2008 it was less. There are a number of things which determine the length which include volt drop (Limited to 5%) and impedance (1.44 ohms with standard final ring).

When you alter any electrical circuit you should complete either a minor works certificate or an installation certificate and these require you to test things like earth loop impedance.

I know most DIY people do not test their work but this does not make it right.

The rules can get complex. Even electricians argue about what they mean the 3 meter maximum (434.2.1) being one often brought up. There are three ways to supply a socket.
1) Final ring circuit normally fused to 32A
2) Radial circuit normally less than 30A
3) Spur fused at termination rather than source normally 13A

What it seems you are doing is turning a final ring into two radial circuits which would likely mean the 32A MCB would need changing for a 20A MCB. Unless something is done either ensuring still a ring or reduced overload size then there is a risk of fire.

Once a final ring is split there is also an impedance problem and it may result in the MCB even at 20A being too big. As the total for radials is not half of the cable used for ring but far less.

I really do think you need to bite the bullet on this one and get some one who can do the testing required. Even to hire the test equipment for a week (normal min hire time) costs around £75 so likely cheaper to get some one to do it for you.
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Postby callcom » Tue Aug 30, 2011 11:32 am

Is it now clear that the maximum cable length for a 'ring main' (2.5mm / 30A MCB) is 106m? There seems to have been some dispute about this in the forums although some of those posts may pre-date current regs.

I have a DB c1998 without an RCD but with MCBs. I'm concerned if I exceed the cable length the potential voltage drop could mean increasing the time it takes for an MCB to trip, thereby reducing the protection of the circuit.

I can't get an accurate estimate of the cable length using resistance readings, although I think it might be around 45-50m. If so, and if max cable length IS 106m then I am ok (floor sqM <100)

Would appreciate any views!
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