meter tails

[sunsno]

Been asked to change CU for someone. They have an old board with fuse wire and just want something newer. All the circuits test up OK my only query is the meter tails. They are obviously an old imperial size, somewhere between 16 & 25 mm. I don't go that far back but I'm sure some of you do!
So do I need to replace them with 25mm? I have a feeling it says somewhere that you should. In fact the 16th OSG does say that tails should be 25mm but is that referring to a new installation? I'm not adding any extra circuits or load and bearing in mind the cables on the suppliers side of the meter are the same size as the existing tails it would seem a bit pointless.

 

[oharaf]

When changing a CU, the Tails must be 25mm, the main bonding must be 16mm and supplementary bonding 10mm.

Must meet the requirements of the 17th Ed.

The entire property must be given a full test and inspection using suitable, calibrated test equipment.

You will be personally responsible for every circuit in that property whether you installed it or not.

This is notafiable work

Have a nice day

 

[sunsno]

Thank you for your reply oharaf


Well as I hinted in my post I did suspect they had to be 25mm, I was more trying to establish what the point was (other than because it says so...) if the supply company's cables are smaller?

The main bonding is 16mm and the sup 10mm, now I have changed them.

As I said the circuits have been tested, using my calibrated meters.

And if I go ahead and change the board it will get notified through the part P scheme I am a member of.

Hopefully one day I will be as perfect as you...

 

[mikeyd]

Oharaf:

the Tails must be 25mm, the main bonding must be 16mm and supplementary bonding 10mm.

You recently told me Here to go away and read up about the differences between main and supplimentary bonding. Could I suggest you do the same.

 

[sunsno]

Mikeyd, you are correct. What I suspect we both meant was Main earth 16, Main bonding 10mm.

 

[mikeyd]

Hi Sunso. Apologies for having taken your thread off topic. Oharaf was a bit rude to me in a previous post when I suggested he was wrong. I was just concerned someone might read his threads and believe what he says.

Best of luck with the board change.

 

[sunsno]

No prob Mikeyd.
But to get back on track, what is the point of installing fatter tails than the incoming supply's?

 

[securespark]

The distributor often has undersized tails/cables on their supply.

Often, in day past, they would use 16mm² or the imperial equivalent because the fuse was often sub-100A, sometimes as low as 40A. But these days you often find they have been upgraded to 100A but the tails between the cut-out and meter have been forgotten.

I saw an old paper-wrapped cable which can't have been more then the imperial equivalent of 10mm² connected with a 100A main fuse. The DNO guy muttered something about the low underground temperature keeping the cable cool!!

Anything beyond your meter tails (going back to the supply) is their responsibility anyway. Just make sure your bits are to the regs & you'll be OK.

 

[nozspark]

When changing a CU, the Tails must be 25mm, the main bonding must be 16mm

No they don't!! They have to have a higher current carrying capacity than the main fuse, so if you have a 60A main fuse then you can use 16mm tails and 10mm main earth!!

However fitting 25mm tails is good practice

 

[sunsno]

Thanks securespark & nozspark for two pertinent yet conflicting views. The particular installation in question has a 60A fuse, so according to one view 16mm conforms, albeit not best practice.
What page of the Regs' are you looking at here so I can check again myself?

 

[Spark123]

Section 54?
What is your supply type?

 

[sunsno]

Thanks, having a read now. Its PME

 

[securespark]

As I said, you can fit smaller tails if the main fuse is <100A. However, some are not marked up as <100A so you won't know. Best to play safe & fit the full monty then if you have a smaller fuse and need to upgrade for any reason, you're prepared.

 

[sparkyspike]

Last week I ran a 95mm sub main from a 300A busbar to a new 250A panel board. The main incomer was 130mm. But after speaking to YEDL, they told me that it was only 130mm above ground. Below ground, it changes to 95mm all the way back to the substation! Yet they are giving me 300 Amps. When I asked why, they said not to worry about it. And I don't.

 

[Click-Sure]

16mm2 tails used to be able to carry 100A (in ideal conditions, I suppose) now they can't, just grin and bear what you're supposed to do.