Jointing mains cable with crimps

[Dave Plowman (News)]

for ring main cable.

[Alan]

I dunno - would probably be ideal for splitting a mains cable to feed 2 combi boilers...

[John_ZIZinvalid]

Either are good, in industry crimps are used to save time.

You can break up the chock-block connectors and push them into the corners, tape them up with insulation tape if it makes you feel better. Remember this is a ring circuit, therefore their should not be any connection between the CPC, Phase or neutrals between each leg.

This is better explained in the IEE On-Site guide.

J

[Stefek Zaba]

Come again, chuck? Are you trying to say 'keep it as a ring, don't end up with a multi-socket spur, you want a 'goes in' and a 'comes back' leg to your new sockets and they should be separate in the weirdo backbox' - in which case, yes you're right, and Andy L prolly knows already. If not, could you explain what you did have in mind?

This extending-for-new-sockets topic came up a couple of weeks ago. One of the nicest solutions was to use a dual box, to mount two singles side by side, each with one of the old ring cables and a leg of the new bit of ring. No chocbloc, room to breathe, and general Goodness all round...

Stefek

[BigWallop]

But all this is hidden behind a cabinet or cupboard, which sounds built-in to the kitchen scheme.

[Tony Williams]

Bare copper is a reactive metal, subject to long term oxidation, or verdigris, or other chemical attack. To make a reliable crimp joint onto bare copper needs something that first of all cuts through any oxidation that may be there, and then makes a gas-tight connection.

This needs a heavy gauge crimp, with sharp serrations down the bore, used with the correct tool.

Add to that pre-insulated crimps in a light gauge with smooth bore, or at best rounded crinkles down the bore.

[Andy Luckman (AJL Electronics)]

I was very careful NOT to mention the K word. It could be in a bedroom. :-)

[John_ZIZinvalid]

Yes. Keep within the ring where possible, and don't cross connect between both legs, this is better explained in the IEE On-Site guide with the aid of diagrams.

I don't have a copy of the IEE Regs here with me, so cannot state the exact IEE regulation rule, but there is a rule which states the amount of spurs you can have on ring, and that spured sockets should not exceed the amount of sockets which make up the ring.

You are also limited to the amount of sockets on a spur, and these should not exceed one double/single, if you want more then you need to install a fused spur.

Your solution of using two single sockets side by side is a good way of solving the problem, rather than using connectors and one double/single socket.

J

[Andrew Gabriel]

That's a recommendation in the On-Site Guide only. It's not part of the Wiring Regulations.

I suspect it is because there are rarely if any spurs on a newly installed ring, and if you get the the point where you've doubled the number of points on a ring circuit with spurs, you really should be looking at rewiring it to meet your current needs, as the original design is clearly no longer appropriate to current usage.

[David Hearn]

How then are screw terminals acceptable? Normally these are just the means of fixing the wire into the terminal - not the main conductor. A screw point is not terribly big, so I wouldn't expect it to remove a significant amount of oxidation when screwed up - especially if two wires are present (eg. ring main in back of socket, where the screws tend to be smaller than, say a junction box). Much of the main contact is by the surrounding metal in the terminal - which doesn't have any means of removing oxidation etc. Then there's the secondary problem of the screws coming lose over time and the fact that a round wire doesn't perfectly fill a square hole, so not all the copper wire is in contact with the terminal (made worse when running a spur from a ring where you have 3 wires in a termainal).

I agree that in most case, you'll probably find that a standard crimp doesn't cover the oxidation issue - but I don't believe many other common fixings (other than soldering) make much, if any, attempt at doing his either. At least with crimps you have a better mechanical joint than screws, practically all the external surface of the wire is in contact with the crimp - and I'd doubt the electrical joint was significantly worse than a screw terminal. Based on that, a crimp joint seems better, or am I missing anything?

D

[usenet]

I don't see how the type of wire affects how well the crimp works really. The copper in stranded wire can creep (or not) to exactly the same extent as the copper in non-stranded wire.

My crimps came from my local Electrical Wholesaler so presumably are designed for use with mains cable, the cataloge certainly implies that is their intended use.

[antgel]

electrics, the shower (9.5kW) fed with 2.5mm to name just one!

Ok. I've read enough of this thread to feel nervous. I have crimped some 1.5T&E with some blue crimps and a ratchet crimping tool. If these crimps are not correct for the job, can someone point me to some that are? I have never heard of mains-rated crimps.

Antony

[Dave Plowman (News)]

Think red is the right size for 1.5mm.

But any crimped joint doesn't really consider the voltage. It's the current that matters. The insulation must be up to the voltage, though.

[Andrew Mawson]

I was taught that a properly made crimp is actually a gas tight joint at the points of contact, so if the wires were acceptably free of oxide when crimped, they should remain so.(This is apparently true also of a 'wire wrapped' joint previously used in telecoms and early computers, where a wire is tightly wrapped round a square post to make a connection.) Crimps used in low voltage applications where high humidity is expected (external joint boxes for telephones for instance) are filled with a vaseline like anti-oxidant water repellant.

AWEM

[Andrew Gabriel]

Copper Oxide (unlike Aluminium Oxide) is a conductor though. So providing there's only a thin layer and the copper is still reasonably copper coloured and not dark red or black, there's normally no problem. The deformation of the surface of the copper in any type of terminal will break through it anyway (again, unlike Aluminium Oxide which is extremely hard).

[Andy Dingley]

It's because they're not very big. Small area leads to a high pressure and good gas-tight joints. However this also means poor mechanical strength. A screw terminal with a separate slipper pad under the screw terminal gives a better mechanical connection, but the pressure is low and they're not gas tight. Take your pick.

[David Hearn]

electrics, the shower (9.5kW) fed with 2.5mm to name just one!

Aren't blue crimps meant to be used on 2.5mm T&E. For 1.5mm T&E you're meant to use red crimps.

[David Hearn]

But whilst a small area will increase the pressure of the joint and improve the gas-tightness of it - it also means that the area of contact (and therefore electrical conduction) is smaller meaning it heats up.

I guess a lot of this is theoretical and academic. If it works in practice then its fine - which I guess they all do, else people wouldn't be allowed to use them.

D

[Andy Luckman (AJL Electronics)]

That would be a jolly wonderful idea. Depends on the clearance to the back of the cupboard though.

[Andy Luckman (AJL Electronics)]

Yes, that's fully understood, to keep the ring intact. I was going to parallel the earths though. I suppose not any good reason to, as long as one of them connects to the back box.

There will be no spurs. The whole point of doing it this way is to preserve the ring. If there was any sensible route, I would have taken it back to the CU, but this will be the next best option.

With the cost being similar, it seems a nice idea. I'm sure that some use could be found for the spare sockets once fitted. Just thought. I bet the tails will be too short to reach the right hand socket. :-(