Why oh why 25mm meter tails?

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Why oh why 25mm meter tails?

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My guess is that the IET are simply 10 years out of date, plus someone somewhere calculate the specification, then a sub committee adds 10% to that, and the main committee thinks - let's be in the safe side, and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

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.
 
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sparc said:
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)
Click to expand...

I was under the impression that the 25mm was "mandated" for new builds and rewires.
 
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JulianC said:
Interesting article on Page 18 here:

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

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.
 
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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!
 
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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
 
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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.
 
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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.
 
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Rockingit said:
Likewise, and I wouldn't dream of using anything other than 25mm+ just to not get the grief and the argument,
Click to expand...

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.
 
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imago said:
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.
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well any work we do (my boss n i) that requires additional protection by means of RCD....we just fit a garage board...fed either from a spare way in the existing c/u.......or henley...either way you dont need to be fitting a complete new c/u as firstly you will need to inspect and then test the whole install again...( a can of worms)....and secondly only work you do needs to comply with the current edition of BS7671.....
 
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I havent read all the posts!! but when I did my 16th edition and test and connect course years ago I was told that even though you may have a 60/80 Amp head at the time, the DNO may upgrade their service to 100 amp, therefore its prudent to fit 25mm tails.
 
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Edd said:
I havent read all the posts!! but when I did my 16th edition and test and connect course years ago I was told that even though you may have a 60/80 Amp head at the time, the DNO may upgrade their service to 100 amp, therefore its prudent to fit 25mm tails.
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and if you do your diversity calcs for the CU?.....100% of the highest load then 40% of the rest combined?..probably wont be 100A....and dont forget you can clamp it n all....bet you dont get anyware near 80A never mind 100A....
 
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Totaly agree Glennspark, if you do get near 100A in a domestic setting I would hate to see what the quaterly bill was!!! and 25mm just dont bend as easy as 16mm and im sure the design of most the domestic stuff aint designed for 25mm, all the main switches seem to destort and argue.. but nothing a hammer cant fix "I will get that tail in":hammer:
 
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GLENNSPARK said:
well any work we do (my boss n i) that requires additional protection by means of RCD....we just fit a garage board...fed either from a spare way in the existing c/u.......or henley...either way you dont need to be fitting a complete new c/u as firstly you will need to inspect and then test the whole install again...( a can of worms)....and secondly only work you do needs to comply with the current edition of BS7671.....
Click to expand...

I recently had to do work on a shower circuit and was going to do a two way wylex with main switch and 40A RCBO. Instead though, I discussed with the client and agreed to fit a 5 way board. My reasoning was that at any point in the future they could have an individual circuit tested and dropped in AND protected with a 30mA RCBO. If there was any fault finding to be done then there would be no interuption to their installation.
 
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I've wondered about this as well. I've got a CU change coming up in a house with 16mm tails. I am tempted to just fit 25mm tails from the meter to the CU because it's a no brainer, it's not that much hassle and is probably the safest option from a part P assessment point of view, even if it is not really necessary from an electrical point of view, but the only thing that puts me off this approach is DNO seals on the meter connections. How do most sparks deal with that?
 
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well you can always replace tails but how about showing your assessor how you do diversity calcs...for current using equipment/connected loads as well as C/Us.....that would be good wouldn`t it....;)
 
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