Why oh why 25mm meter tails?

[sparc]

Does anyone understand the logic of 'mandating' 25mm meter tails when upgrading a CU, when the sizing of other cables are left to the electrician's discretion?

I'm again having to explain to a little old lady with not a lot of money that she's need new meter tails with with tiny 6-way board, for her tiny 2-bedroom flat.

Massively cooking range to feed? Nope! 10.8Kw Powershower? Nooooh!

When copper is becoming an increasingly rare resource shouldn't we be fitting what's needed?

My guess is that the IET are simply 10 years out of date, plus someone somewhere calculate the specification, then a sub committee adds 10% to that, and the main committee thinks - let's be in the safe side, and adds another 10%.

Craziness gone mad!!!

(Rant for the day over - thank you for listening)

 

[telectrix]

if main fuse is 80A or less, you can use 16mm tails.

 

[Rockingit]

Trouble is Tel, that's not actually written anywhere that I know of, so peeps just looking at the pretty pictures in the OSG take it as granted.

 

[JulianC]

According to the ESC:

In an existing domestic installation, are 16 mm2 single-core pvc/pvc meter tails (having a current-carrying capacity of 87 A clipped direct) adequate if the rating of the distributor’s cut-out fuse is 100 A? This size of tails may be adequate provided the maximum demand of the installation, taking into account the load characteristics (diversity and load profile), does not exceed the current-carrying capacity of the tails, and also provided that the requirements of Regulation 434.5.2 for protection of the tails against fault current are met.
As a practical tip, the current-carrying capacity of the meter tails is likely to be adequate if they have been in service for several years and there are no signs of thermal damage at the terminations or to the cable sheath or insulation.
Regulation number(s)

• 311.1
• 433.3.1(ii)
• 434.5.2

 

[JulianC]

Interesting article on Page 18 here:

http://www.esc.org.uk/fileadmin/user_upload/documents/public/switched_on/SwitchedOn-Issue-15.pdf

 

[sparc]

Trouble is Tel, that's not actually written anywhere that I know of, so peeps just looking at the pretty pictures in the OSG take it as granted.

You're absolutely correct - I've always take the pretty picture in OSG as a summary of what is 'Gospel'. So it's very helpful to be educated otherwise.

 

[Octopus]

Does anyone understand the logic of 'mandating' 25mm meter tails when upgrading a CU, when the sizing of other cables are left to the electrician's discretion?

I'm again having to explain to a little old lady with not a lot of money that she's need new meter tails with with tiny 6-way board, for her tiny 2-bedroom flat.

Massively cooking range to feed? Nope! 10.8Kw Powershower? Nooooh!

When copper is becoming an increasingly rare resource shouldn't we be fitting what's needed?

My guess is that the IET are simply 10 years out of date, plus someone somewhere calculate the specification, then a sub committee adds 10% to that, and the main committee thinks - let's be in the safe side, and adds another 10%.

Craziness gone mad!!!

(Rant for the day over - thank you for listening)

I was under the impression that the 25mm was "mandated" for new builds and rewires.

 

[GLENNSPARK]

3 ways to do this...
1) 80A bullit....16mm tails
2) max demand at the c/u (after diversity)
3)direct measurement (clamp)
....well just my thoughts on it anyway.....

 

[sparc]

Interesting article on Page 18 here:

http://www.esc.org.uk/fileadmin/user_upload/documents/public/switched_on/SwitchedOn-Issue-15.pdf

Whether applying to inspectors or electricians with job of upgrading the CU, the question is whether there's really an 80A or 100A in the cut-out? So that old chestnut about not fiddling with supplier's side is raised. Personally, I've never been worried about open it up to isolate the installation - my safety trumps the supplier's ownership as far as I'm concerned.

 

[Octopus]

3 ways to do this...
1) 80A bullit....16mm tails
2) max demand at the c/u (after diversity)
3)direct measurement (clamp)
....well just my thoughts on it anyway.....

4. remove 100A DNO fuse and replace with 80A fuse

tin hat on

 

[SPARTYKUS]

4. remove 100A DNO fuse and replace with 80A fuse

tin hat on[/QUOTE]

FIX BAYONETS!!

 

[GLENNSPARK]

4. remove 100A DNO fuse and replace with 80A fuse

tin hat on

well theres method in the mag....sorry thats madness murdoch.....

 

[Rockingit]

If you read the full article, it makes a case that by using the adiabatic correctly for 16mm you can actually use it for a 100A DNO fuse (BS1361) due to the time/current characteristics versus adiabatic on the cable.

Guess ultimately it comes down to 1) knowing your trade properly 1.1) understanding what the regs/maths implied actually mean and 1.2) a good old fashioned dollop of experience.

And I guess that now counts an awful lot of people out!

 

[GLENNSPARK]

well theres method in the mag....sorry thats madness murdoch.....

Thanks Murdoch mate....i wasn`t sure how that statement of mine came across here.....

 

[Octopus]

Thanks Murdoch mate....i wasn`t sure how that statement of mine came across here.....

I've got thick skin so if you thought it offended it didn't.

 

[UNG]

The DNO's local to me have required 25mm² tails for a long time now the last time I can remember using 16mm² tails was around the early eighties

On all the sites I've seen where the meter has been changed recently the cut out to meter tails have been changed to 25mm²

May be the thinking is "what will do a lot will do a little" also as we all know there is no chance of the DNO's fuse blowing at any where close it's rated value so the larger tails will cope better with the potential overload beyond the fuses value

 

[Rockingit]

Likewise, and I wouldn't dream of using anything other than 25mm+ just to not get the grief and the argument, but it does make you think that there are an awful number of CU replacements going on where the tails are being replaced without any technical justification.

 

[stuart569]

And how much is a couple metre of 25mm tails?tenner?Hardly a great expense for a piece of mind and future proofing

 

[sparc]

That's exactly my complaint. The 'powers that be' simply add a little more contigency into everything because that's easier than updating the 19th century technology underpinning today's wiring regs. Current MCB and RCD 'technology' could be replaced with something vastly more effective and accurate, but the entire industry is geared to churn out the things for a couple a quid apiece, and no one wants to invest in developing and marketing alternatives. My other beef ( oh, dear, second rant of the day coming on) is that while cables have got thicker (and total loads have generally gone down) the casings for CU and isolators, etc have got thinner and cheaper. It won't be long before we left with a bunch of breakers wrapped up in a 'plastic bag', wafting around in the air on top of a couple of rigid meter tails.

 

[imago]

Likewise, and I wouldn't dream of using anything other than 25mm+ just to not get the grief and the argument,

That's it in a nutshell for me. With the proviso that as long as you have the knowledge (or the professionalism to want and gain the knowledge) as to why what you're fitting isn't strictly necessary then it's OK.

A similar situation IMHO to the "it must be RCD protected, and if it is all's well in the world". I've been to quote on a couple of jobs in the last few weeks where the customer has been told "It must have an RCD" or "You need a new CU" when neither is true. I just can't make up my mind if it's an excess of greed, or a shortage of knowledge driving them to tell customers such things.