replacing a CU and part upgrading meter tails

[mgx]

We have a 60A wylex CU with meter tails being T&E (16mm probably) length 7m out of our flat through a utility room and into a common meter room, all circuits on 60a fuses and isolators. I am (subject to my LBA approval upgrading the CU as part of a part-rewire to a Hager 17th ed, which is 100A rated. In the future we may try and get our supply upgraded to 100A, but I am not going to do that now. However, while stuff is ripped apart I want to take the opportunity to upgrade the meter tails so they can be take 100A in future. But I don't want to draw all the way back to the meter room right now, only to part of the way to the utility room (a whole lot of different flats meter tails feed through the utility room as well). Is it okay to terminate the current 60A t&e in a junction box in the utility room, and connect it to 100A rated wiring (I think double insulated 25mms should be fine) through to my CU? So in future, a supply upgrade to 100A is easier on the wiring bit?

 

[Steve]

Your LABC may not pass it if you have two sizes of cable supplying the CU. As well as this, if you're relying on the earth in the 16mm T+E, its probably undersized.

And any length of meter tail over a metre (IMO) should not be singles. You should install armoured cable or concentric cable (with seperate earth) back to the meter room. Or else its singles in conduit.

dont worry about your supply fuse. You wont blow it, unless you install 2 electric showers and try to use them, and even then you'd have to be in them for about 2 days before the fuse will blow.

 

[mgx]

Need to think about that - are you suggesting that even if I leave the existing 16mm T&E and just upgrade the CU, there might be an issue due to the 16mm earth being undersized? In other words, upgrade the CU I have to upgrade the meter wiring. I can only see 25mm cable in singles on TLC direct, unless I need to look elsewhere. I had planned for it to run in conduit. The current cable is not even in conduit - it runs through a false ceiling (not even clipped, just resting on the battons), drops out a wall, and runs with ~10 other cables down to the floor and through an underfloor void into the meter room. Wiring would have been done ~1972, so this probably not to current standard. Perhaps better if I don't touch any of that and just do the CU.

 

[ericmark]

Using something like a Henley Block you could join cables but one has to be careful as it has been made plane that the burried in walls rule also includes feeds to meters and the RCD option is not open so looking at 3mm steel protection.

The Electrical Safety Council have made some best practice guides and the Forum as they call it where there are four lists of questions and answers.

NDQ4
Do ‘meter tails’ concealed in walls or partitions need to be protected in accordance with Regulations 522.6.6 and/or 522.6.8?


NDA4
Yes. Meter tails concealed in a wall or partition at a depth of less than 50 mm from a surface must be protected in accordance with Regulation 522.6.6. Also, irrespective of the depth from a surface, meter tails concealed in a wall or partition having internal metallic parts (except nails and screws, etc) are subject to the requirements of Regulation 522.6.8.

However, additional protection for meter tails by means of an RCD is not an acceptable option in respect of Regulation 522.6.7 (which in consequence rules out reliance on 522.6.6(v), routing in the ‘safe zones’ alone), or in respect of Regulation 522.6.8(v). Also, for TT systems, the only option remaining is to provide suitable mechanical protection (that is, to comply with Regulations 522.6.8(iv) and/or 522.6.6(iv) as appropriate).

This may consern you!

I seem to remember reading somewhere that there was also a limit to the lenghts of singles used as tails between meter head and consumer unit 3 meters seems to come to mind although I can't remember where I read it so could be wrong! However unless in trunking I would never consider running singles for more than a few feet.

I would be looking at a SWA feed and to termate one would need some form of enclosure so would not be very practicial to route only half way.

Working through LABC you I would think will have to give a detailed proposal and then they would pick up anything they did not like. But I know there are moves being made at the moment over the problem where the only way to isolate the tails is to remove the suppliers fuse. Seems the permisions given in the past for contractors to remove fuses has been with-drawn and only method as it stands is for the supplier to install an isolator and you to connect to that. Or you install an isolator and the supplier connects to that. But as things stand unless you get supplier to install an isolator you are likely to be without power at some point.

Not sure how Council works but if to rule book once power is removed you will not be permitted to re-instate power until council has tested so again be aware you may be without power for some time. It will depend on your LABC as to what they will allow.

 

[Steve]

By changing the CU, you're changing every circuit on the installation. It therefore must comply with the 17th edition - all of it. This includes the meter tails.

You cannot supply your new CU with undersized tails - the LABC wont pass it. For a start, you cannot use the suppliers fuse to protect your cable.

You need a switchfuse after the meter (i think you may have one), and if the meter tails are buried less than 5cm (ie. laid on a ceiling or buried in plaster) the switchfuse must also be an RCD. You dont want that!

So we replace the current setup with metal conduit and singles (though SWA or concentric would probably be cheaper / easier) and protect with a switchfuse.

 

[mgx]

Thanks ericmark, good advice - I may be best getting a spark to do this part of the work. Currently, due to ripping apart part of the property, the CU has only two active circuits (one 32A and one 6A), so I could get someone to upgrade and test those, then after that, do my own LABC approved work on the internal circuits. btw. there are a bank of switchfuses in my side of the meters fortunately, so that will make the work a bit easier. and I also found 25mm swa, cheap at only £7/m .

 

[sparkybird]

If you are in a block of flats I doubt if you'll be able to get your main fuse upgraded to 100A as 90% sure you'll have a 3 phase supply with individual flats supplied off that via the 60A switch fuses.

As someone else said, 60A will be fine unless you're planning run a load of big appliances.... so maybe not much point in upgrading to 25mm.

You'll need to work out fault current and min size of main earth using the adiabatic equation to see if the 6mm earth in your t and e complies - might do if TNS but unlikely (in my experience) if TNCS/PME. Location of MET for earth bonding is also an issue as you can't use the 6mm I think..

SB

 

[mgx]

Okay, but even if I don't upgrade my supply to 100A, it appears that merely by changing my CU (which I have to do, since the existing CU is inadequate for my needs) I may be forced to "fix" the meter cabling anyway.

The rated load of my two ovens (3.2kw + 3.7kw) plus induction hob (7.2kw) is already at 60A. Hopefully we will remember not to have a a huge dinner party and do the washing, the dishwasher and a spot of vacuuming at the same time

I reckon I will get a spark to upgrade and certify the CU and if he doesn't say anything about the meter supply cable, neither will I!

 

[sparkybird]

Yes, I agree, let him/her worry about it - that's what we're paid to do!!

You'll be fine for 60A - they take ages to blow on overload... And don't forget you have 'diversity' allowing for the fact that you won't have everything thing on a full blast at the same time

Not unless you've got ALOT of friends!!

Good luck