Tails in cavity wall

catdog1121 · May 21, 2019

So I have a consumer unit to change, the consumer unit was in the garage but it has been converted to living accommodation years ago. The utility cupboard is on the outside wall, the consumer unit is obviously on the inside wall but near the ceiling.

The issue is when they changed the garage to living accommodation they created a second skin using breeze blocks. The tails from the utility cupboard were run in the cavity wall between the meter cupboard and the consumer unit, these tails need upgrading to 25mm plus 16mm earth.

I know just running new tails in the cavity will not provide enough protection, the cables would be less than 3m so from a DNO point of view thats fine. Its going to be hard enough to run the 25mm cables through and up the cavity wall let alone put any sort of copex to protect it, the alternative is 25mm 3 core SWA however then would need a fused isolator box in the meter box to terminate. (Can't find any sort of wiska box that would be able to make connection from tails to 25mm 3 core SWA).

Im sure I am not the first to come up against this issue so thought I would see what others thoughts were.

mattg4321 · May 22, 2019

Rough job that!

Luke238 · May 23, 2019

Luke238 said:
Is this unacceptable for meter tails >3m in a cavity wall/ceiling as per the regs ?.
Saw a client today who wanted to move some sockets. Installation about 3 years old but with no certificate or labels on the CU.
25mm meter tails ran in cavity of wall/ceiling for around 4m to the CU.
Tails ran from a double pole isolator.

View attachment 49492

so to protect the tails with an rcd may prove very inconvenient if there is a fault which would wipe out all of the board, but this has to be weighed up against the risk of tails in cavity/ceiling <50mm

Midwest · May 23, 2019

Why do the tails need RCD protection, if they are run in a cavity, or have I missed a pertinent post again?

Fire barrier; would it not be just the meter tails coming out of the cavity, that would require sealing?

Luke238 · May 23, 2019

The tails are around 5 metres long, fed through a wall cavity and ceiling. They have no mechanical protection and at points are buried at a depth less than 50mm.

Midwest · May 23, 2019

Luke238 said:
The tails are around 5 metres long, fed through a wall cavity and ceiling. They have no mechanical protection and at points are buried at a depth less than 50mm.

At the points where they are less than 50, can they not be capped with a suitable capping?

Luke238 · May 23, 2019

I could only see the start of the run from the DB back to the cut out which was less than 50mm.
I suppose the problem is that as I can not see the run as a whole Im not sure what is less than 50 and whats more than 50mm.
Albeit not ideal a RCD main switch would offer protection ?

Wilko · May 23, 2019

I’ve some of these in the shed to fix places with tails the between plasterboard and blocks

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[ElectriciansForums.net] Tails in cavity wall

Ian1981 · May 23, 2019

Wilko said:
I’ve some of these in the shed to fix places with tails the between plasterboard and blocks .

View attachment 49503

I’ve used it but you can drill through it with an sds quite easily so I’m in two minds whether it compliant for mechanical protection

davesparks · May 23, 2019

catdog1121 said:
Tails look really small, have not taken it apart but looked at next doors that had been cut off and I would say they are 10mm tails so really need upgrading. I thought the regs say they only need RCD protection if more than 3 meters,

So you looked at the tails next door to establish what is required in the house you are working on?

Midwest · May 23, 2019

I’ve re-read your OP. I think I would consider routing an swa cable from the meter box, terminating it in one of those bracket things or metal adaptable boxes, running cores straight out through suitable gland. In the meantime ask the supplier to install one of their isolators. Is the run 3 or 5 meters?

Wilko · May 23, 2019

Ian1981 said:
I’ve used it but you can drill through it with an sds quite easily so I’m in two minds whether it compliant for mechanical protection

Hi - agree it’s possible to drill through. I’m thinking that 522.6.204 (iv) doesn’t require the protection be drill proof, just prevent penetration by nails and screws. I’m relying on the British manufacturer who recommends it for this job. Maybe I’m too trusting?

In the end it would be better if the tails were not there in the first place.

catdog1121 · May 24, 2019

davesparks said:
So you looked at the tails next door to establish what is required in the house you are working on?

Next doors had the same size tails that had been replaced but cut at the wall so could see the size of the conductor.

SparkieMarky · May 25, 2019

I would agree that running tails in a cavity or any other cable is bad practice for a number of reasons including:

1) Possible unseen damage to cables pulled through the cavity over the hidden portion.

2) De-rating factors should insulation be present or added to the cavity at a later date.

3) The cavity (if it hasn't already been filled with crap already from the bricky) is a potential chimney. So should there be an issue with the installation and an electrical fire starts in the cavity then the fire integrity of the building could be compromised.

4) Even before things like Part P and the good old competent person schemes existed I believe that the old NICEIC Technical Handbook used to specifically frown on the practise of any cables being installed in cavities and it would have been considered poor workmanship if the practise was used.

So for what it's worth if the metre cabinet isn’t on the front elevation of the house and there is not neat and tidy way of routing the tails indoors, (taking account of requirements for suitable earthed mechanical protection etc. if buried) then I would go down the route of external SWA clipped direct on the exterior wall and through wall directly into the CU.

catdog1121 · May 25, 2019

The problem is even externally its in a narrow passage shared between two houses leading to gardens which again is not ideal. Added on to that DNO's don't like any customer equipment in their boxes. Its one of those between a rock and a hard place because of the alterations made. I think the best solution is to try and feed some kind of copex even if just pvc 32mm, at least it gives it some protection from future insulation or falling debris.

Remembering there is a utility box on the outside and consumer unit on the inside there is no real risk of anyone wanting to drill through the wall in that position.

Given its only a 3 bed semi and using 25mm tails de-rating will not be an issue.

dksanders · May 25, 2019

If the cut out is 100A then 25mm tails are mandatory irrespective of load (taking into account diversity).
No cables are allowed in a cavity other than meter tails 'which must pass directly through i.e. perpendicular to the wall'
So the choices are you either run up the outside then go straight through the wall, or through the wall then up the inside (both with suitable protection for the 'up' bit)
And if the total length will end up more than 3m, then you need to provide overload protection to that run.
Those are the rules, the customer has to be told to like it or lump it.