UK Thoughts on Meter Tails Install?

[Weezy]

I’m rewiring a massive house and there’s a feed to a c.u in the old black rubber cable that needs replacing.

I know that Meter tails follow the same guidelines as other domestic wiring, but i’m trying to avoid putting a rcd on supply end of the tails. The run is about 20meters long and going under the floor is impractical due to different floor heights etc.

I was thinking out of a switched fused isolator, up the wall inside in pvc trunking (if i can convince the client) then through the loft (on top of fibreglass) and down the wall in pvc trunking into cu. All ran in regular 25mm single sheathed tails.

I did consider swa however the loft is very tight in some places and a couple of angles it needs to bent at would make installing swa very difficult if not impossible.

Can anyone see any problems with this or point me to any info in the ‘book’.

Thanks.

 

[telectrix]

if you are doing a full rewire, would it be an idea to relocate the CU close to the meter?

 

[pc1966]

You need the 30mA RCD if the cables are at risk of screw/nail penetration (that is defined as < 50mm from any surface) and there is no earthed metal shield that can force disconnection in that case (like SWA armour).

Putting a 30mA up-front on a new build is an awful thing to do as it will be tripping like a hippy convention.

So your options are from:

• SWA and making sure it can disconnect in no more than 5 seconds on supply OCPD
• Tails in earthed metal duct
• Tails in a route that is protected from penetration, so either more than 50mm deep, or with something like 3mm steel plate to stop a typical nail/screw

 

[Dartlec]

A switch fuse is necessary because of the length, and as you say you want to avoid an RCD up front.

As long as the tails are surface mounted, (which can be in trunking or conduit), and are not buried or concealed in a wall (<50mm) at any point, then you don't need RCD protection for them, so your solution sounds OK to me.

You will need to consider grouping and sizing for any trunking and possibly derate the tails appropriately, as well as consider voltage drop etc.

 

[pc1966]

That is a good point, visible cables don't need additional RCD protection as it is assumed nobody is stupid enough to nail ones they can see.

Yes, you can stop laughing at the back!

 

[HappyHippyDad]

I’m rewiring a massive house and there’s a feed to a c.u in the old black rubber cable that needs replacing.

I know that Meter tails follow the same guidelines as other domestic wiring, but i’m trying to avoid putting a rcd on supply end of the tails. The run is about 20meters long and going under the floor is impractical due to different floor heights etc.

I was thinking out of a switched fused isolator, up the wall inside in pvc trunking (if i can convince the client) then through the loft (on top of fibreglass) and down the wall in pvc trunking into cu. All ran in regular 25mm single sheathed tails.

I did consider swa however the loft is very tight in some places and a couple of angles it needs to bent at would make installing swa very difficult if not impossible.

Can anyone see any problems with this or point me to any info in the ‘book’.

Thanks.

What is the earthing arrangement Weezy? If it's TT, you're going to need the appropriate RCD upfront anyway.
When I have done this before i have used steel plates in the wall to protect the tails. However, if the client is OK with surface trunking then I can't see a problem.

 

[ChrisElectrical88]

can you not rent a metarbo and stick them 75mm in the wall.

teletrix has the right idea just shifting the CU.

 

[pc1966]

What is the earthing arrangement Weezy? If it's TT, you're going to need the appropriate RCD upfront anyway.

The difference is for TT that can be a 100mA or 300mA delay RCD and unlikely to trip, buf if there is a cable penetration risk it has to be a 30mA 'instant' RCD for shock protection.

And that will trip for no obvious reason in any modern home with any typical levels of leaky electronics.

 

[Weezy]

if you are doing a full rewire, would it be an idea to relocate the CU close to the meter?

This place is so big we’re actually putting two cu’s in, in different locations, otherwise the cable cost would be insane, not to mention high zs readings.

 

[Weezy]

That is a good point, visible cables don't need additional RCD protection as it is assumed nobody is stupid enough to nail ones they can see.

Yes, you can stop laughing at the back!

The builder said to me “if you put trunking in the corner we can cover it with some fancy wood and make it look part or the building”
…hmm, that doesn’t sound very visible to me ? Wasn’t to keen on that idea.

 

[timhoward]

The builder said to me “if you put trunking in the corner we can cover it with some fancy wood and make it look part or the building”
…hmm, that doesn’t sound very visible to me ? Wasn’t to keen on that idea.

Maybe steel trunking and let him cover that?

 

[James]

In steel trunking, you could use singles instead of double insulated tails, might help to keep the size down.

 

[Lucien Nunes]

Although that might make the trunking bigger than it needs to be even for sheathed singles, and enforces a proper complete run of trunking from end to end. Using sheathed cables allows a certain amount of flexibility in the awkward angles and crevices that it sounds like the cables need to negotiate. The trunking can be used to hold them in position without having to be 100% continuous. OTOH you might get better CCC with singles if it matters.

On a side note I think 'meter tails' is a misnomer here. Surely meter tails are specifically the cables from head to meter and meter to isolator. This is a distribution circuit, part of the installation and no different to any other circuit, even if it is installed using a type of cable most often seen used as meter tails.

And on a vintage note, if the old TRS rubber distribution circuit is being pulled out, I would be very keen to get hold of the old cable. It is proving very hard to find usable samples of heavier TRS cables that can be formed into shape in a display. Smaller cables we have good lengths that are still supple but anything 7/.044 and up is a problem. I will happily exchange it for beers if any good runs can be saved.