Pulling cable through with existing one / Clipping cable in loft

Two quickies...
1. I am shortly to rewire my upstairs lighting, (the wiring is in the loft). When new loft insulation is shortly added the cable will be buried under the insulation, but I am not clear whether the regs require that the cable needs fixing to the joists, or can just be laid loose along the ceiling, (as the current wiring is). [NB. We have already discussed the cable derating implications in another thread].
2. In replacing the wiring to the light switches, I intend to use the existing cable to pull through the new stuff, (it is in oval conduit). Has anyone got any tips for how to mechanically connect the two cables, obviously with the smallest possible cross section to enable the joint to pass through the conduit. Is insulating tape generally strong enough ??
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Mike Hall wrote on 11/01/2004 :-

The usual way would be to strip several inches of the copper bare to allow a loop to be formed in the copper in each of the wires to be joined. That keeps the bulk down and is sometimes covered with a layer of tape. Much depends on how much room is to spare in the conduit and how freely the cable moves inside it.
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On 11 Jan 2004 02:01:55 -0800, mike snipped-for-privacy@peppertree-broadcast.co.uk (Mike Hall) wrote:

If space in the conduit is limited (as it is bound to be) one option would be to pull thru a length of curtain wire, and also a piece of string (so that you've got the string and curtain wire in the conduit), or perhaps pull the old cable out and pull some string thru first, then pull thru the curtain wire AND further string.
Then you can put a hook in the end of the curtain wire to attach to the new cable, and start pulling - without using the string.
The purpose of the string is that if the curtain wire becomes detached from the new cable you can pull the curtain wire thru again on the string.
Once you've got the new cable thru pull the string out because it isn't needed any longer.
Make sure you use string which is strong though - none of this stuff that spiders would reject for building their webs - if you can break it by pulling between your hands it isn't strong enough.
You would also be well advised to dust the new cable with talcum powder as it enters the conduit - this will greatly assist its passage thru the conduit. However on completion it would probably be a good idea to put the vacuum cleaner on the end of the conduit and give it a good blow/suck to remove the "loose" talc - though it isn't likely to be a big problem fine dust can be a fire hazard.
PoP
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As will pushing *and* pulling the cable simultaneously. Two people needed...
Thomas Prufer
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Or some sort of grease: silicone, vaseline, KY jelly, butter etc
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the screwfix silicone spray for gutters and plumbing is good on all sorts of plastic including pvc
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Strip back the outer sheath about 6 inches or so and cut off the earth. Bare about 3 inches of the conductors. Form a loop by wrapping the bare conductor over the insulation *in opposite directions* then do the same with the new wire but of course thread one of the conductors through the loop on the other first. Now pull tight and crunch the whole lot up as smooth as possible with pliers, and cover with PVC tape. This will make a mechanical joint nearly as strong as the cable itself - I've never had one pull apart.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Do you mean wrap the red core over its insulation then the black core over its insulation?
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No - red over black and the reverse. You're forming a 'woven' loop.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Thanks to everyone for some top tips - how about the clipping cable in the loft question.
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Don't clip it where it will be stood on or something laid on top of it. Try to keep cable runs around the edges if you plan not to drill through joists and other timbers. If you do have to cross over a joist where you plan to store items, then give the cable some robust protection so it doesn't get squashed. That's about it.
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Mike Hall wrote:

What I do is as follows.
Strip off insulation outer on both.
Cut back all except one condictor and strip that.
fold over that and teh similar one on teh new wire, to make 'U's
Interlock those and twist together hard.
Cover the while joint in insulation tape.
This is about as small as it gets.
Some people use the old to draw a doubled string through, cut that, and use one half to attach to the new wire., and leave the other in place 'just in case'
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the
conduit).
enable
generally
And if the conduit, or hole is badly restricted, solder the twisted ends, not much chance of that coming undone.
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Gavin Gillespie wrote:

Not neeeded. I have tested a twisted joint to destruction. The cable failed elsewhere :-)

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Soldering weakens the copper due to heat. A properly made twisted joint won't come undone before the cable breaks. I've used 'my' method hundreds of times - it was taught to me by an old sparks.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Mike Hall wrote: <snip>

Recently had the whole house rewired and the sparks semed to loop and twist one end of the conductors of one wire. , thread all the conductors of the other wire through the initial loops twist the individual loops together, then cover the lot in insulating tape they didn't use talc., baby oil or other lubricants.
Pic just in case my description is a bit garbled
http://www.cheesesoup.pwp.blueyonder.co.uk/images/w.jpg
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Mike Hall wrote:

Don't clip wiring if it can be avoided! Just lay it with enough slack, it's better to run it through then over the insulation.

As said before, hook the wires together then twist (so they're joined by "eyes") - additionally, I have found fewer problems when pulling wiring *up* conduits, rather than down. If you have the misfortune to have a cable joint break so you have nothing left to pull through with, you can suck a piece of strong string through the conduit with a vacuum cleaner nozzle.
J.B.
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