Meter Tails to CU

[andyh]

Hi all,

In the middle of a new build and starting to plan the electrics. The outside meter box has already been installed which is a flush unit which protrudes into the cavity wall. The meter has been fitted and connected with an 80A fused switch to allow connection of tails to CU inside. Here is what is proposed:

1. 25mm2 meter tails, from fused switch, run through back of meter box to internal wall. 16mm2 earth cable also run through back of cabinet to internal wall - supply is PME.
2. Meter tails run vertically up internal wall and and across horizontally into kitchen, then dropped vertically again into CU. Earth cable taking same route. Route of tails will be channelled into internal insulated block and then galvanised metal capping on top.
3. Total length of tails from fused switch to CU is 5m.

My query is regarding the small distance from the back of the meter box to the internal wall. From lots of reading it would appear that cables should not be run through a cavity wall. However with an outside meter box and internal CU, you have to run the tails through the cavity, albeit for only a few inches. For this small distance should the tails be in conduit or is it acceptable to run them without? Also should the metal capping be earthed?

As a matter of interest, if cables can no longer be run through cavity walls, how does one supply outside lighting on an exterior wall if you can't route through the cavity?

Thanks for the advice.

Andy

 

[johnnyb]

u dont run d/ins cables in a cavity or any other but u can bridge the cavity for outside lights.
there may be an issue of 5mtr run mate, to long for d/ins i stand corrected but i think 3mtrs max.

 

[Des 56]

Your cable needs to follow safe zones
Rcd protection for the tails unless mechanically protected or deeper tham 50mm

 

[marksparky]

Tails can be 5mts as long as there is means of isolation in ex cupboard as far as i kno, if ur using metal capping that is sufficient as long as it IS earthed there is no need for rcd protection.

 

[andyh]

Thanks for the replies guys.

As I understand it the tails length is only restricted if they're connected directly from the meter. That's why there's a fused switch (sometimes referred to as KMF switch) which means no restriction on meter tail length. If they were connected to an isolator or directly to the meter then, yes, the restriction of 3m applies.

I also mentioned having the tails mechanically protected with galvanised capping. Surely this means I don't need RCD protection on the tails as well? Tails will also follow safe zones with them running vertically from point of entry and within 150mm of ceiling, then vertically to CU.

 

[marksparky]

yeah that i mean earthed cappin ok so no need for rcd, and ur switched fuse is sufficient for lenght of tails

 

[slocm3105]

Metal galvanised capping is not sufficient mechanical protection for the tails to not require rcd protection. Im pretty sure it has to be 3mm thick steel/galvanised trunking or conduit. Now I think that the only way to overcome this is to fit a 30ma time delayed rcd in the meter cupboard. Im not 100% on this but i think that this could be one of the ways that you can overcome this regulation when faced this problem.

 

[Des 56]

Capping will not stop penetration by a nail and is not sufficient for mechanical proteciton
If the tails are taken up the wall,the consumer unit needs to be directly above to highlight the safe zone unless its within 150 mm of a corner

 

[marksparky]

hadnt heard of min thickness myself, i thought that as long as it was earthed this was sufficient can anyone quote me the reg on this? watabout a draw in box on wall back to back with ex cupboard with a blank plate and tails run through it and steel conduit up the wall

 

[andyh]

Well that complicates things quite a bit.

I hadn't heard of the minimum thickness of protection before. If adequate protection was provided, would that satisfy the regs?

The CU is directly below the tails on the kitchen side but the run up the wall on the other side is obviously not marked. I can't put the CU on this wall which is the reason for the extended tails.

Anyone point me in the direction a supplier of suitable mechanical protection which will meet the requirements?

Alternatively what if a channel deep enough so the tails are 50mm below the surface of the internal wall? Would this be acceptable?

 

[slocm3105]

Alternatively what if a channel deep enough so the tails are 50mm below the surface of the internal wall? Would this be acceptable?

Of course. and make sure you run it it safe zones .

 

[andyh]

The problem will be on the side the tails enter the house as there would be no visible cue that a service was there and hence no safe zone vertically - or am I missing something? I guess I could be a recessed box on this side, run the tails through it and put a blanking plate which would be visible on the wall.

 

[marksparky]

just wat i said lol! only i call it a draw in box

 

[Deleted member 19807]

Chaneling more then 50mm deep could be a breach of building regs as we are limited to 1/3rd of thickness of wall for vertical runs.

30mA time delay rcd's won't comply even if you can find one as the trip time will be too long.

I can't find the bit about protection from nails and screws but its in there somewhere, i advise 3mm steel.

 

[andyh]

A little late on advice at this stage!!!!

However 5mm steel was used in the end and all passed by BC.