Tails in cavity wall

[catdog1121]

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.

 

[Dustydazzler]

I don’t think it’s an issue to utilise the ‘existing’ tails set up so long as the are the correct size csa
However
If like you , they need to replace the tails I don’t think pulling up in the cavity will be acceptable practice nowadays due to lack of rcd protection

 

[Octopus]

What is your reason to upgrade the tails?

 

[Wilko]

Hi - If I understand Regs 522.6.202 and 203 correctly, if the cables are more than 50mm deep from both sides and there are no metal parts in the wall then the cables do not require RCD protection. Happy to hear if I’m mistaken .

 

[Midwest]

The only issue I feel with dragging any cable in a cavity, is damage being done to the cable unseen. Before anyone says anything, the bit coming out the end, may have missed the cavity tie etc, but the bit inside may have not.

That said, I I've pulled meter tails through a cavity over a short distance (less than metre) in flexible conduit. If it goes in freely, should be okay

 

[telectrix]

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.

if tails are 16mm, i'd leave them alone (apart from connecting up to your new CU.). tin hat on.

 

[Dustydazzler]

if tails are 16mm, i'd leave them alone (apart from connecting up to your new CU.). tin hat on.

If it’s just a simple like for like board swap I tend to agree

(Jumps back into bunker)

 

[Wilko]

And if it’s a 60A main fuse the 16mm tails are fine no matter what load the Customer puts on it.

 

[Jon Bateson]

Yep leave em be. What’s the earth arrangement? And csa ?

 

[Charlie_]

Any idea what size the main fuse is?
Maximum demand?

 

[catdog1121]

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.

 

[catdog1121]

My guess is that the consumer unit was just the other side of the wall so was not an issue but when they did the conversion the cut all the twin and earth shorter and moved the cables up and put the consumer unit at the top so it did not get damaged... It was either done by a cowboy builder or a very long time ago as its an old wylex rewireable consumer unit.

 

[Jon Bateson]

I would just pull flexible tails in. If it’s back to back with board what’s gonna happen ?

 

[Ian1981]

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.

If you think that tails need rcd protection for being over 3 meters you better read again.
Why would you even think that?

 

[catdog1121]

Sorry yes over current protection is required was from reading an above post not RCD, this is a requirement from DNO if tails are longer than 3m.

My alternative is run the cables straight through the wall then use trunking up to consumer unit but dont think that would go down too well with customer.

You can get some 50mm double walled pvc copex which would protect the cables from installation damage at least

 

[Midwest]

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

 

[Octopus]

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

FWIW I would leave them well alone.

 

[Deleted member 26818]

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

 

[catdog1121]

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]

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]

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]

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]

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]

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]

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.

 

[DPG]

No overcurrent present protection for tails longer than 3m.

 

[chris gray]

Could use split con

 

[Wilko]

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]

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

 

[Luke238]

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

 

[mattg4321]

Rough job that!

 

[Luke238]

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]

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]

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]

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]

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]

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

 

[Ian1981]

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]

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]

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]

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]

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]

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]

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]

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.

 

[loz2754]

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.

View attachment 49504

Electrical safety first recommends minimum 3mm steel for this application

 

[Bobster]

If the cut out is 100A then 25mm tails are mandatory irrespective of load (taking into account diversity).

Care to share the regulation that states this?

 

[DPG]

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,

Not sure what you mean here - any cable can pass directly through a wall.

 

[dksanders]

Care to share the regulation that states this?

OSG 2.2.3.1 Consumer's Tails, 2nd paragraph. '...and the minimum cable size should be 25mm"

 

[dksanders]

Not sure what you mean here - any cable can pass directly through a wall.

Yes, but not run in a cavity. You're right, any cable can pass perpendicularly through the cavity, it's preferred that meter tails do so also, however they are the only cables that may deviate e.g. enter the cavity then go up slightly, then exit, recognising that we don't live in an ideal world.