Joining 1.0, 1.5 and 2.5mm^2 cables

[RJH]

Does that just leave crimping for an inaccessible join on a 32A ring?

Cheers, Rob

[Robin]

Cable for a 32A ring doesn't have to be have capacity for more than 20A if the ring meets the special [wriggle-room] regulation for 2.5mm T&E. That is that the current in any part of the circuit is unlikely to exceed 20A for long periods.

[Fredxx]

1) Strip wires 2) Place wires in crimp of the correct size 3) Place crimper around crimp in the correct 'notch' 4) Squeeze handle of crimper as many times as required 5) Pull test pass/fail 6) If fail tighten crimpers and use new crimp if appropriate 7) Repeat and use new crimp if appropriate until success

Perhaps I'm tightening the crimpers too far? Either way I can generally pull out the wires, or at least one.

[Fredxx]

2.5mm^2 is allowed which has an unrated value of 26A. A double socket has a rating of 20A.

I feel an unbalance leg of 26/32A might be acceptable but 20/32A?

[Dave Plowman (News)]

The quality of the actual crimp thingies varies enormously. The thickness of the brass used. And impossible to tell with a pre insulated back to back.

[Dave Plowman (News)]

I would ask how often you need to access that junction box for maintenance etc. Mine are all accessible in terms of the regs, and my answer is never.

[Fredxx]

There is that, especially as I intend to screw any floorboards back into place.

But, by the time carpet is placed on top, while it may be deemed accessible it really is going to be join and forget.

[ARW]

Last forever IMHO.

[Dave Plowman (News)]

IIRC, a junction box under floor boards is deemed accessible. One plastered into a wall, not.

[Dave Plowman (News)]

Well, I wired this house some 40 odd years ago. Not has a problem with any terminal working loose. Perhaps because I tightened them correctly in the first place? I have worked on other installations where this wasn't so, though. And wondered if it was an electrician making work for the future?

[John Rumm]

Which suggests that something is not working as it should there. I find a well made crimp is far harder to pull apart than say a cable in a screw down terminal. The ratchet crimpers often allow for some adjustment to the tightness. Also make sure you are using the right type of *insulated* crimp in them.

Solder and heatshrink is ok. Start with a mechanical join first (i.e. twisted or wire wrapped) then solder.

Wago style terminals are also quick and easy - and probably easier to implement in many more difficult to reach circumstances.

[John Rumm]

Remember that the minimum installed cable capacity[1] required for a ring is only 20A

[1] i.e. once all de-rating factors are taken into account. So for example 2.5mm^2 T&E has a "clipped direct" capacity of 27A, but that reduces depending on how its actually installed. So enclosed in an insulated stud wall but in contact with one of the plasterboard surfaces for example would reduce its capacity to 21A. In a conduit in a insulated wall, reduces it to 20A.

See reference method 102 in the table here:

[John Rumm]

Much depends on the circuit IME... Lighting circuits never run anywhere close to the full rated capacity of the cable, and there is very little thermal expansion and contraction, and no heating effect other than at the terminals other than those on the lamp fittings themselves.

Circuits that cycle up to close to maximum load seem more susceptible to problems. Other factors like how well cables are supported and even the length of individual runs of cable can play a part.

[John Rumm]

I am sure you are doing it right, but just for the avoidance of doubt, the crimp needs to be positioned in the tool set back from the end such that the tool only compresses the part of it that contains the metal crimp terminal, and not the "spare" bit of insulated sleeve at the end.

The technique sounds fine.

The pull out resistance should be "firm" or better, but not necessarily impossible. A well made crimp will typically be harder to pull a wire from than from a screw terminal - but you can do it with enough force since copper is soft enough to deform with enough load. If they are pulling out easily, then it might suggest the crimps or the tool are not well enough matched or of adequate quality.

As to crimping too far - it seems unlikely unless flattening the terminal so much it actually splits along one side. The crimp should deform the wire a little - although short of cutting open a crimp that is harder to asses. (I might try that later)

[Mathew Newton]

The 773-173 3-core connector (note no others from the 773 range) are rated at 32A when used in maintenance free mode. Further info at

Incidentally, the testing procedures (in BS 5733) for maintenance free connectors and enclosures is fairly rigorous, including running them at rated current for 63 days whilst ensuring all performance parameters are retained along with less-than 10% temperature rise of the connector. Overload conditions at twice rated current for 1.5hrs are also tested, again with performance ratings having to be retained with a maximum mounting board temperature of 90c. It all adds up to instilling some confidence in approved maintenance free solutions in my view.

[Fredxx]

Thanks, I was aware of method 102 and believed it is OK for a 32A ring. And that 103 should use 4+mm^2 cable for a section in insulation. Something I would go out of my way to avoid.

I don't have my 17th edition book at hand but wasn't certain of the rating for any leg in a ring main. I didn't know if it had changed for the 18th Ed.

Thanks again.

[Fredxx]

Thanks, the idea of a joint in a wall doesn't appeal! Ironically one embedded in plaster is one least likely to cause a fire.

[Fredxx]

Apart from size, I presume there is a range of qualities. I'm sure the lot I got were from Screwfix.

That sounds a simple and attractive solution.

[FMurtz]

Also there is a variation in the quality of the actual crimp fitting, I have had a batch from an electronic shop as opposed to an electrical wholesaler that were soft or otherwise faulty.

[Dave Plowman (News)]

True. Probably why I don't have any JBs etc in high current circuits.

But do you have problems with the screw connections in your CU? Riser? Meter?