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.

Midwest · May 21, 2019

Whenever I did a consumer unit replacement, the old tails were never long enough, so 25mm etc were always installed.

Octopus · May 21, 2019

OP - what is your estimate of maximum load? How is the property heated?

FWIW I would leave them well alone.

Deleted member 26818 · May 21, 2019

You may have to re-drill the holes at an angle, to allow the cable to pull through easier.

catdog1121 · May 21, 2019

Its a 3 bed semi so maximum load not going to be massive but they do want to add power to an outbuilding. 10mm tails really should be replaced imo. I know the OSG says 25mm but 16mm for most would be ok but 10mm is probably pushing it a bit too much.

I think it will be a case of do what I can but think just getting 25mm tails through the cavity will be enough of a challenge let alone thinking about anything else.. The regs are all very well but in the real world...

hasel5 · May 22, 2019

You may be surprised how easy the new tails pull in especially if you prep the hole each end and do a tapered joint

HappyHippyDad · May 22, 2019

catdog1121 said:
Its TN-C-S with 100amp cutout fuse (Although might only be a 60 or 80 amp fuse in the carrier but says 100amp so have to presume). 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, cable length will not be more than 3 meters, but thought there had to be some mechanical protections.

How about a large wiska box with a couple of henley blocks inside to make connection between tails and swa?

Saying that trying to get even 2 core 25mm SWA though the cavity could be a problem so may not have a lot of choice.

On a TNCS or TNS system 16mm tails are ok with a 100A cut out fuse providing the installation meets the following 2 criteria:

1. Max demand of the installation (taking into account diversity) should not exceed the CCC of the 16mm tails.

2. Regulation 434.5.2 has to be met (i.e protection of the tails against fault current).
This regulation will always be met on a TNCS system as long as Ze<0.35Ω which it should be.
It will be met on a TNS system is Ze<0.48Ω as at that value K²S²>I²t (just!)

happysteve · May 22, 2019

Could you not put some sort of trunking/conduit on the outside of the building, up from the services box? Or if you're going to use SWA, run that up the outside clipped direct then straight through the wall to the back of the DB.

HappyHippyDad · May 22, 2019

happysteve said:
Could you not put some sort of trunking/conduit on the outside of the building, up from the services box? Or if you're going to use SWA, run that up the outside clipped direct then straight through the wall to the back of the DB.

I agree with that. I have looked into using the copex before but always come up with a different solution as I just think it would be a right pain and expensive. I usually use 25mm conduit on the outside of the wall. 2 x 25mm and 1 x 16mm just squeeze in. For a longer run you may need 32mm conduit which would be a pain as its not as easy to buy all the bits that go with it.

DPG · May 22, 2019

I dislike meter tails in cavity walls. Any cables for that matter, but especially nice thick meter tails with no RCD protection - just ready for someone drilling through the wall for an outside light feed etc.

Luke238 · May 22, 2019

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.

[ElectriciansForums.net] Tails in cavity wall

DPG · May 22, 2019

No overcurrent present protection for tails longer than 3m.

chris gray · May 22, 2019

Could use split con

Wilko · May 22, 2019

Hi - I’m always happy to learn, but I don’t see a reg in BS7671 that says you can’t run cables in a building void, with greater than 50mm cover, properly done with no installation damage etc. I don’t do it, but there’s tons of homes with tails done that way.
If I was there to install outlets they’d be done by now

.

DPG · May 22, 2019

It was a regular thing to put tails in cavities yes. Not necessarily a good thing though.

Luke238 · May 22, 2019

This was the board the tails were fed into. No fire barrier were the cables enter the CU at the back either albeit installed recently. Looks like the IP rating is not too good either with all that white paint