Meter Tails in cavity wall

[Midwest]

I am going to do a CU change for part of my scheme provider assessment, so I want to get this right. Currently the old CU (Wylex bs3036 rewirable fuses) is fed with 16mm tails & 10mm earthing conductor. The CU is positioned against the ceiling on an outside wall in the kitchen and the meter cutout is on the other side of that wall. It is TNC-S supply with 100amp bs1361 fuse. The customer has elected for RCBO's in the new CU.

I'm going to replace the tails to 25mm & 16mm EC. However the old tails are run in the cavity, for about one meter. Now my understanding is that the tails do not need rcd protection, because they are more than 50mm from the surface, they will not be excessively strained because of they short length. I'm not changing whats already there with anything that could be conceived worse. But what about protection from cavity insulation, not currently installed, dampness migration and anything else I might have missed?

Another slight issue, is from the cutout the current tails leave via the bottom of the cutout and run to the left of the cutout and then up to the CU. Routing the new tails will be difficult. Is it acceptable to make a hole in the top of the cutout box to fish the new cables (terminating via the original hole at the bottom), as long as this is properly resealed afterwards?

 

[pistolpete]

Is it definitely 100amp fuse as its only16mm tails at present and distributor connected tails to meter would suggest only 60amp fuse in the carryer and therefore not need upgrading

 

[Midwest]

Fuse carrier said 100amp.

 

[markc123]

99% of the time the fuse carrier lies.

 

[pistolpete]

Yea doesn't mean it is 100a brought this up with Nic last yr and said if 16mm tails connected by distributor no need to change however if new install I would provide 25mm tails 16mm for main earthing conductor

 

[itselectric]

I would persevere in getting the tails out the bottom if you can. Try moving them, if they do then get somebody you can trust to feed the tails in and tie onto existing earth wire to draw them up. I say this because you wont be able to guarantee a seal if you go out the top and if any water gets into the cavity ou could have problems.

 

[telectrix]

99% of the time the fuse carrier lies.

obviously not supplied by ronseal.

 

[Darkwood]

Re' the cavity your tails will require suitable mechanical protection that can be needed for falling debris etc, where situes's are that cvity insulation is present or block insulation then protecting from falling debris can be overlooked, , cables shouldn't really be running along the cavity of an external wall regardless of depth as its hard to give the mechanical protection required.
It depends on how sharp your scheme provider onsite assessor is really.

 

[Midwest]

Hmmmm, not conclusive then. I think I'm going to have to replace the tails anyway due to their length and the size/position of the new CU. I can't see much alternative, I can't run them on the surface in the kitchen, the cooker/hob is adjacent. I could run them surface (in trunking or swa) on the outside wall, but thats in a very prominate position & can't see the customer liking that. As regards fishing them in the cavity around the cutout! Another point, the earthing conductor, main bonding are terminated in the DNO carrier. I think if I do replace the tails, I will install a MET. Not sure how the cover to the earth of the DNO is removed? Or is this a job for the DNO?

 

[Darkwood]

Its by all means should be sealed and yes its DNO property.... how you procede with upgrading earth is your call im not going to suggest anything on that matter as it wouldn't be in the forums interest to suggest otherwise..

 

[itselectric]

Can you not disconnect from the consumer end and use the earth as a draw wire to meter cabinet, pull a draw wire through then draw your new tails in to CU .

 

[Rockingit]

Check what's actually in the fuse carrier (not going to spell out how), that will give you the basics of what size you need. If it's 16mm then why bother changing it? If it needs to be changed then use the old tails to drag the new ones through.

 

[Engineer54]

If it's not a 100A fuse in the DNO cut-out, then you don't need 25mm tails, it's that simple!!
Personally i wouldn't have a problem running sheathed (commonly known as Double Insulated) tails in the cavity for such a short distance.
What i wouldn't do, is start messing around with the cut-out cable entries, that's DNO work and for good reason. After reading your OP again, i can't quite make out if your talking about the external cabinet or the cut-out itself!! If external cabinet , i can't see a problem, providing you can maintain IP rating!!

 

[Midwest]

Not sure how you do multi quotes, so here's the next best thing;

Sealed DNO head: the current EC & MPC terminate in the supply head, this part is covered with a plastic lid (Not where the L&N supply/load enter & terminate). I have a picture when I can remember how to post it, i.e. not proposing to cut their seal, but can't see how the lid is removed to the DNO earth terminal?

Using old tails as draw wire: I don't think the old tails will be long enough anyway, so can consider this method, although the tails enter the external cabinet on the right side/bottom of the cabinet, and the tails run in the cavity to the left of the cabinet, so that will be difficult. I might need to remove a couple of external bricks to fish cables.

External Cabinet: I meant external cabinet not cut-out (apologies for confusion), so I meant cutting a temporay hole in the top, just to facilitate fishing the tails in the cavity. Not sure how to reseal that back up, so perhaps not an option.

 

[Midwest]

Incidentally, I emailed my DNO for advice on the matter;i.e. meter tails in cavities, ‘You willneed to put this question to the customers electric billing company. The meter is owned by the supplier not thedistribution company, this includes the meter tails’.
I thought the meter tails were owned by the customer,unless I confused them with the cabling from the cut-out to the meter.

 

[Rampantchilli]

If its says 100a on the carrier then you must assume that's what's there, as you can't take the carrier apart to check without cutting the seals,
"we'll Mr assessor, I didn't want to change the tails so I cut the seals and checked the size of the fuse inside, and how dare they put an 80a in, I've put a note on my certificate and written to Western Power demanding an apology" lol

as for your earth, cut the existing wire and use the length from cut-out to CU to terminate into your new MET , as you can't access the connections for the DNO earth connection, as its in the cut-out and you need to cut the seals to access.

and can we no longer call the meter box a meter box?, has that now been harmonised into an external cabinet, cut the new hole where YOU need it, apply your fishing skills and when finished reseal the hole with fire stop, but I question if they need to be sealed as they are not particularly sealed when fitted.

 

[Darkwood]

100amp on the service carrier is the max' rating fuse it could carry, it is not an indication of the fuse within- this should be done by enquiry but as we all know this takes forever so assume its 100amp and upgrade to 25mm tails.

I see this carrier rating getting confused all the time with what fuse size should be fitted... many a control panel with oversized fuses in the carriers because the on-site 'maintenance' dosn't understand the basics.

 

[Midwest]

Yes sorry about that, external cabinet it is, can't think where I might have picked up such bad terminology from.........Meterboxs Direct Online Shop | Electrical Meter Box Doors & National Grid: Domestic meter box spares and repairs further help and guidance etc etc, whereas external cabinet........Cabinet Secretary for Culture and External Affairs - Wikipedia, the free encyclopedia

Anyhows, appreciate all the advice. I know that replacing tails is the subject of countless posts. I will need to change these (insufficient length) and I will install 25mm or even 35mm, to take into account thermal insulation (not currently installed)! Therefore I will have to replace the current 10mm EC.

From what I have been able to research, installation of cables in cavity walls is undesirable as applicable requirements of BS7671 cannot be met: damage to cable during installation, cable lacking support, mutual damage to cables from thermal insulation, cables in thermal insulation (reducing ccc), unknown presence of flora or forna & bridging of the cavity by moisture (Building Regs). Apparently though (I can't find the actual section), NHBC Sec. 8.1, section 2 standards state 'no cables, other than meter tails are to be installed in cavities.

So therefore my thoughts are that as long as I can comply with BS 7671, install tails in cavity, protecting them in flexible conduit or similar (protection against damage, mutual damage, strain etc it should be okay. I've checked a few DNO sites, and most demonstrate the installation of tails in the cavity, only one specifically stated they cannot. What da ya think?

 

[Darkwood]

Its all down to interpretation you have just listed many area's where cavity installed cables would be a bad idea direct, the environmental sheath (outer) provides minimal mechanical protection from falling debris which often falls from the full height of the cavity from loose debris at the top, i have also encountered this damage as well as damaged cables done on install.... if you can reduce the risk of things falling on cable then sorted ie... remove a few bricks and create a ceiling of expanding foam above the cable run..... but here's the thing many dont bother most just run it anyway but it dosn't mean its right to do so.

 

[staggie65]

If the cavity has any metalwork in it it should be protected by RCD (other than screws/nails)