# Meter Tails

S

#### steviez

Can anyone tell me why you put 25mm tails from meter to CU when most of the time there is only 16mm tails from service head to meter?

If as you say in one of your previous posts you're in college then you should be able to work it out for yourself.

Think about what the purpose of the main fuse/mcb is?
What is the required relationship between the two?

If as you say in one of your previous posts you're in college then you should be able to work it out for yourself.

Think about what the purpose of the main fuse/mcb is?
What is the required relationship between the two?

I am in my first year and have not been there long! we have not even covered anything like this yet! was only asking to learn.

Think about what the purpose of the main fuse/mcb is?
What is the required relationship between the two?
I can't see how that will help to answer the OP's question. Whatever answer he gets to that question will be the same for the cables from cutout to meter as for those from meter to CU.

The answer to the OP's actual question (as to why cables are so often of different CSA on the two sides of the meter) is clearly 'bureaucratic' (e.g. differing sets of regs on either side of the meter) and absolutely nothing to do with engineering! The laws of physics are clearly not different on the two sides of the meter!

Kind Regards, John.

Think about what the purpose of the main fuse/mcb is?
What is the required relationship between the two?
I can't see how that will help to answer the OP's question. Whatever answer he gets to that question will be the same for the cables from cutout to meter as for those from meter to CU.

The answer to the OP's actual question (as to why cables are so often of different CSA on the two sides of the meter) is clearly 'bureaucratic' (e.g. differing sets of regs on either side of the meter) and absolutely nothing to do with engineering!

Kind Regards, John.

Most likely it is because an electrician has fitted a new CU and 'uprated' the tails but is not allowed to alter the DNO part of the installation.

Whether the uprating is necessary is another matter.

Most likely it is because an electrician has fitted a new CU and 'uprated' the tails but is not allowed to alter the DNO part of the installation. Whether the uprating is necessary is another matter.
Quite. If, as is often the case, the 16mm² (or imperial equivalent!) DNOs cables have been there for years (or decades) and have not yet melted, in engineering terms the need for the 'uprating' is clearly very much in question.

In terms of the usual CCC figures, 16mm² ought to be OK with an 80A cutout fuse, but not with a 100A one (but it seems that some DNOs happily use it for both).

Kind Regards, John.

Think about what the purpose of the main fuse/mcb is?
What is the required relationship between the two?
I can't see how that will help to answer the OP's question. Whatever answer he gets to that question will be the same for the cables from cutout to meter as for those from meter to CU.

The answer to the OP's actual question (as to why cables are so often of different CSA on the two sides of the meter) is clearly 'bureaucratic' (e.g. differing sets of regs on either side of the meter) and absolutely nothing to do with engineering! The laws of physics are clearly not different on the two sides of the meter!
Kind Regards, John.
Open you eyes John, look at his previous posts viz
How to extend cables to cu.
RCBO's to fit Contactum board.
Outside sockets
PIR sensors in his house
Sourcing Schieder RCBO's for a Merlin Board.
Do you see a pattern here? Or are you so wrapped up in the technicalities of the regulations that you fail to see what is in front of your own eyes.
If this guy is in college studying to be an electrician - he said he was in college 27th August 2011 (bit unusual since term doesn't start until September) then considering the questions I posed should help him understand the importance of correctly sizing cables in relation to the the fuse.
This might go some way to get him to answer his own question. Like the size of the DNO's cable may be because its protected by a 60Amp Fuse.
So this is not a bureaucratic issue as you state and one you cannot answer without all the facts.

Open you eyes John, look at his previous posts viz ... Do you see a pattern here?
I already spend far too much time with this forum than I should, and I certainly can't afford myself the luxury of hunting back in history. I take posts at face value, which is what I did here. I'm not saying that you're necessarily wrong in anything you're saying about the OP, merely that I do not do detective work before repsonding to what appear to be straighforward questions (unless the nature of the question causes me concerns).

If this guy is in college studying to be an electrician ... then considering the questions I posed should help him understand the importance of correctly sizing cables in relation to the the fuse.

Or are you so wrapped up in the technicalities of the regulations that you fail to see what is in front of your own eyes.
As above, all that was in front of my eyes was what appeared to be a valid question and a not-very-helpful response.

But, goodness, does what you've just written represent the impression you have of me? Speaking totally in terms of personal interest (and I accept I'm a fairly atypical case!), I'd be far happier if there were no wiring regulations and no Part P, merely 'guidelines' etc.. I've been doing electrical work 'to my own satisfaction' for decades without having to think about such formalities too much, but we are moving into a progressively more regulated world, and I can't totally ignore that.

I can read and digest a set a regulations as well as anyone else (and I certainly have extensively read those parts of BS7671, OSG and some GNs which are of 'relevance' to me, as well as associated books) - but the reason I came here was to try to gain some insight into how electricians interpeted and applied those regulations in practice. Perhaps naively, I had expected to find consensus amongst electricians far more often than has proved to be the case, and I certainly didn't expect to be seeing, let alone participating in, many robust arguments about interpretation of the regulations.

So, in truth, I am anything but 'wrapped up in the technicalities of the regulations', even if that might appear to be the case. Maybe one thing which distorts the picture is that there are a lot of things which I think but would not feel appropriate to say in a DIY forum - e.g. in relation to aspects of the regulations I would (certainly if left to my own devices) happily ignore, twist or violate (and some instances in which I would want higher levels of 'safety' than is strictly required by the regs), and how I personally would deal (or not) with part P. It may therefore appear that I am far more concerned/obsessed about compliance with technicalities of the regs than I really am

However, as I have said a few times, I suspect we may be approaching the point at which the forum has little more to offer me (many of the discussions are already becoming very repetitive), at least on a regular basis, so I'll probbaly fairly soon have to review my 'time prioritisation'!

Kind Regards, John.

Perhaps also the question should be reversed, why is it that when the DNO service is a 2c.0225 PILC (16mm2 equivalent, or an older 16mm2 concentric) why do installers over specify the size of the tails and install 25mm2.

Continuous rating of a .0225 is 120A, we ascribe a rating of (if I recall) over 100A to 16mm2.

Perhaps also the question should be reversed, why is it that when the DNO service is a 2c.0225 PILC (16mm2 equivalent, or an older 16mm2 concentric) why do installers over specify the size of the tails and install 25mm2. Continuous rating of a .0225 is 120A, we ascribe a rating of (if I recall) over 100A to 16mm2.
That is precisely the 'bureaucratic' (different regs) issue to which I was referring. Your regs may 'ascribe' those CCCs, but BS7671 thinks that the CCC of a 16mm² single is only 87A - hence inadequate for a 100A fuse (but OK for an 80A one).

Kind Regards, John

Somewhere I have an old BICC cable rating book, when I'm back at work (off for a week or so after an operation) I'll have a look.

There is no doubt in my mind that the BS7671 figures are possibly a bit conservative

Duplicate deleted

Somewhere I have an old BICC cable rating book, when I'm back at work (off for a week or so after an operation) I'll have a look.
Thanks. I hope you are recovering rapidly!

There is no doubt in my mind that the BS7671 figures are possibly a bit conservative
Yes, I'm sure that's true. I think the interest is probably in how conservative. Empirically, my DNO-side 16mm² cables do not even get notceably warm to the touch at 50A-60A, so they clearly are not going to melt or explode at 100A!

Kind Regards, John.

Well if we take the fusing factor of a 100A cut-out fuse as 1.47, it is apparent that tails will need to carry at least 147A for a period without failing. The same will apply to any cable as we don't want cables failing before fuses/MCB's operate on load.

I realise this crosses into another thread, but it is that difference (margin) that the DNO's exploit to establish higher ratings (though sometimes time limited). Whereas at "one size fits all" approach with BS7671 limits possibilities and design freedom

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