Meter Tails to CU

[AndyNewBuild]

Hi all,

In the process of finalising electrical design for a new house. Unfortunately the meter box is in a position which requires the meter tails to be run internally for about 5m - this is the total distance from the meter to the CU. The meter box has an 80A fused switch fitted - DNO said they don't have problem with this. This will allow more than the normal maximum of 3m tails to be run to the CU from this switch.

The problem is with the internal run of meter tails where they come through from the meter box and fused switch. If I understand the 17th edition correctly then I have the following options:

1. Bury the meter tails in excess of 50mm into the internal blockwork and run them to the CU. I understand that they must also be run within the safe zones.
2. Mechanically protect the tails with steel conduit and run to the CU. This conduit would also require earthing. I understand that galvanised capping is NOT sufficient as it doesn't offer mechanical protection.

Issue with option 1 is obviously burying the cable and also creating a safe zone where the cable runs up from the point of entry. I had thought of putting a back box into the internal wall with a blanking plate thereby providing a visual indication for the vertical run towards the ceiling. Option 1 also entails a large degree of channelling which is time consuming.

Option 2 has the issue of still having to channel the conduit partly into the wall so as it won't be seen and earthing it. Would 25mm conduit be sufficient to get both live and neutral tails into easily?

Anyone think of any other options or does anyone think the interpretation of the regs is wrong? Spoken with several electricians and have had several different answers!

Thanks for the help.

 

[RF Lighting]

You are fitting a switch fuse at the meter to supply your CU anyway, so you can still go with either option you mention above, or you could use one of the cable types which is exempt from RCD protection such as SWA or (but probably not) pyro etc.

My preferred method would be steel conduit, but it is not easy to work with if you are not used to it, and can end up being very labour intensive. You might also struggle with 25mm tube to fit your tails and an earth in.

 

[GaryMo]

You have the option of just mechanical protection against the likes of drills, nails and screws.

A manufactured 3mm steel plate silimar to the profile of capping to cover the tails would suffice and wouldn't require earthing.

 

[AndyNewBuild]

Thanks for the quick reply.

Yes it's not ideal I know but have to work with it. Would it be acceptable to run the 16mm earth cable outside the conduit? I'm guessing not! Could it run along side the steel conduit with galvanised capping? Again I suspect not.

Thanks again for the reply.

 

[AndyNewBuild]

You have the option of just mechanical protection against the likes of drills, nails and screws.

A manufactured 3mm steel plate silimar to the profile of capping to cover the tails would suffice and wouldn't require earthing.

Thanks for that. I had indeed thought of that but wasn't sure whether this would satisfy the 17th Edition as the steel plate wouldn't offer protection from all angles - e.g. someone banging a nail or screw in at an obscure angle! If it would work, I would be happier doing that. I wasn't aware it didn't need earthed. This would make things much easier. Anyone know where I could obtain 3mm steel plate of a suitable width?

 

[GaryMo]

It would if the plate was of the same profile as capping, like a bridge.
If it's sufficiently wide enough then there's no way you'll hit the cable with a nail, screw or drillbit.

Definitely doesn't need earthing.

 

[AndyNewBuild]

It would if the plate was of the same profile as capping, like a bridge.

Indeed but finding that profile of steel plate could prove difficult.

 

[GaryMo]

It would if the plate was of the same profile as capping, like a bridge.

Indeed but finding that profile of steel plate could prove difficult.

Not at all. Ask a local fabricator - that's what I've done for a set of tails running internally vertically to a high level consumer unit which needed protecting.
Not got the plate yet though.

It's a garage conversion into a kitchen. The tails were run within PVC trunking.
The walls were being drylined so that was pretty much our only option to protect the tails.

 

[AndyNewBuild]

It would if the plate was of the same profile as capping, like a bridge.

Indeed but finding that profile of steel plate could prove difficult.

Not at all. Ask a local fabricator - that's what I've done for a set of tails running internally vertically to a high level consumer unit which needed protecting.
Not got the plate yet though.

It's a garage conversion into a kitchen. The tails were within PVC trunking.
The walls were being drylined to that was pretty much our only option to protect the tails.

Good plan. Will contact a few local companies and see how I get on. Thanks for the advice, much appreciated.

 

[Chri5]

A manufactured 3mm steel plate silimar to the profile of capping to cover the tails would suffice and wouldn't require earthing.

Yes it would require earthing. why do you think it wouldn't. It's a metal barrier and as such the same as galv conduit or trunking.


Talking of which why not use 2 x 2 galv and part recess it via chase ?

If your happy to consider a metal plate, then again why not simply use galv lidding ?

 

[GaryMo]

3mm plate offers greater protection against penetration and no it does not require earthing.

 

[Chri5]

3mm plate offers greater protection against penetration and no it does not require earthing.

Extranous conductive part- 544.2

 

[JohnD]

Must the cables be hidden in the wall?

You could surface-mount them or use surface trunking or maybe run them under the floor.

 

[GaryMo]

Haha Chris, thats made my day!

Is a random piece of metal in a wall really extraneous?

Think about it!

 

[NotHimAgain]

Its not an extraneous-conductive-part because it is not liable to introduce a potential. Nor is it an exposed-conductive-part as it is not part of any equipment.

However, the interesting question is would it be strong enough to prevent penetration by nails screws and the like .

Right I am just off to hang some pictures - where did I leave my Hilti gun - black cartridge anyone .