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My guess is that the IET are simply 10 years out of date, plus someone somewhere calculate the specification, then a sub committee adds 10% to that, and the main committee thinks - let's be in the safe side, and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

 

[GLENNSPARK]

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.

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.....

 

[Edd]

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.

 

[GLENNSPARK]

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.

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....

 

[Edd]

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"

 

[1shortcircuit]

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.....

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.

 

[Tuttle]

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?

 

[GLENNSPARK]

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....

 

[1shortcircuit]

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....

I preferred the look of the brand new 25mm tails in the CU I had fitted but wait... Nice new sparkly seals too... Now how does that work?

 

[GLENNSPARK]

I preferred the look of the brand new 25mm tails in the CU I had fitted but wait... Nice new sparkly seals too... Now how does that work?

well if your refering to what i think you are short...then thanks for the reminder mate..lol....

 

[flukey]

but the only thing that puts me off this approach is DNO seals on the meter connections. How do most sparks deal with that?

do the maths and leave the 16mm in if ok.

if dno want to come along at a later date and up to 100A fuse thats their look out just make sure you comply with your work and leave them meter seals alone!!

 

[flukey]

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.

i take it your quoting for surface mount in trunking then?? perhaps others are chasing in properly??

 

[Tuttle]

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....

Yeah, I suppose, as long as he is not of the opinion that tails should always be upgraded to 25mm.

For meter tails the OSG says: "the minimum cable size should be 25mm. The distributor may specify the maximum length and the minimum cross-sectional area". It doesn't say anything about sizing the tails according to the maximum demand. I'm not saying that you're wrong, I'm just saying that I think that I would have a hard time justifying why I ignored what it said in the OSG if it came up in an assessment.

 

[Tuttle]

The other thought I had was if the distributors tails between cut-out and meter are 16mm then perhaps I could assume that their minimum csa couldn't be any larger than 16mm.

 

[GLENNSPARK]

The other thought I had was if the distributors tails between cut-out and meter are 16mm then perhaps I could assume that their minimum csa couldn't be any larger than 16mm.

distributers tails are DNO property..so you wouldn`t worry about those anyway......unless they were damaged..then you pick up the phone.....

 

[jase]

distributers tails are DNO property..so you wouldn`t worry about those anyway......unless they were damaged..then you pick up the phone.....

and which tails are not their property then? the ones coming from the meter to the fuseboard? the meter you are not spose to touch cos that is also their property and it has a seal on it? so i essence we really shouldnt worry about changing any tails cos they are all sealed up and the DNO's :confused5:

 

[topquark]

and which tails are not their property then? the ones coming from the meter to the fuseboard? the meter you are not spose to touch cos that is also their property and it has a seal on it? so i essence we really shouldnt worry about changing any tails cos they are all sealed up and the DNO's :confused5:

Where the tails run from meter to CU/DB is not normally sealed, just a removable cover plate.

 

[flukey]

Where the tails run from meter to CU/DB is not normally sealed, just a removable cover plate.

with a seal on it lol

if meter is sealed and tails need changing this is where mr henley comes in

 

[Tuttle]

distributers tails are DNO property..so you wouldn`t worry about those anyway......unless they were damaged..then you pick up the phone.....

Ok, I know that, what I was trying to say was perhaps I could assume that the distributor's requirement for the csa of the tails on the consumers side of the meter would not exceed the csa of their own tails on their side of the meter, I just didn't explain myself very well. I'm guessing that the main fuse is probably only a 60A or 80A but I haven't checked yet.

Why do I feel like I am digging myself into a hole

 

[topquark]

with a seal on it lol

if meter is sealed and tails need changing this is where mr henley comes in

Not when I find them, not sure why that is.

 

[Tuttle]

When British Gas did some work around my house they sealed the Henley blocks as well!

For safety reasons I think as they are on the consumers side of the meter ... you'd have to be pretty daft to try and nick electric from there

 

[Geordie Spark]

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.

I rewired my house recently - in fact, truth to tell, it's still ongoing - but I fitted 25mm tails from the new 15 way main switch + RCBO'd board to my isolator.

From the isolator back to the meter, & from the meter back to the cut out, the tails are the "old coloured" 16mm. Then the bloke came to change the meter not asked for by me, but sent by DNO and I handed him a nice new pair of tails & asked him to fit them in place of the old 16mm pair.

He refused, claiming it wasn't his job, so I now have what I think is the ludicrous situation of having 16mm "old coloured" tails from the cut out as far as the isolator, & then 25mm tails from the isolator to the CU.

The mind boggles !!!!

 

[Geordie Spark]

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....

Show off !!!!! :lol:

 

[jase]

When British Gas did some work around my house they sealed the Henley blocks as well!

For safety reasons I think as they are on the consumers side of the meter ... you'd have to be pretty daft to try and nick electric from there

saftey reasons? you would still have to pull the cutout even to mess around on that side of the meter, and the cutout would be sealed!!!!

 

[1shortcircuit]

saftey reasons? you would still have to pull the cutout even to mess around on that side of the meter, and the cutout would be sealed!!!!

I never leave home without one???

 

[Tuttle]

saftey reasons? you would still have to pull the cutout even to mess around on that side of the meter, and the cutout would be sealed!!!!

Yeah, I think they just wanted to stop curious people from removing the lid and killing themselves, I thought that it was a bit strange, nothing to do with them really.

 

[UNG]

Where the tails run from meter to CU/DB is not normally sealed, just a removable cover plate.

with a seal on it lol

if meter is sealed and tails need changing this is where mr henley comes in

Some meters have a split terminal cover on the consumers side that is not sealed and has an isolator behind the cover so you can work dead occasionally some DNO's fit an isolator after the meter which has a split cover arrangement allowing access to the consumers side terminals only

Ok, I know that, what I was trying to say was perhaps I could assume that the distributor's requirement for the csa of the tails on the consumers side of the meter would not exceed the csa of their own tails on their side of the meter, I just didn't explain myself very well. I'm guessing that the main fuse is probably only a 60A or 80A but I haven't checked yet.

Why do I feel like I am digging myself into a hole

Years ago when you got the electricity board to connect tails after a rewire they would expect you to install 16 or 25mm² tails but the tails from the cut out to the meter where usually well undersized

I rewired my house recently - in fact, truth to tell, it's still ongoing - but I fitted 25mm tails from the new 15 way main switch + RCBO'd board to my isolator.

From the isolator back to the meter, & from the meter back to the cut out, the tails are the "old coloured" 16mm. Then the bloke came to change the meter not asked for by me, but sent by DNO and I handed him a nice new pair of tails & asked him to fit them in place of the old 16mm pair.

He refused, claiming it wasn't his job, so I now have what I think is the ludicrous situation of having 16mm "old coloured" tails from the cut out as far as the isolator, & then 25mm tails from the isolator to the CU.

The mind boggles !!!!

Last few meter changes I have seen done they changed the tails on the DNO side to 25mm² in the new colours

Yeah, I think they just wanted to stop curious people from removing the lid and killing themselves, I thought that it was a bit strange, nothing to do with them really.

Years ago the old electricity boards always sealed the henley blocks

 

[flukey]

Some meters have a split terminal cover on the consumers side that is not sealed and has an isolator behind the cover so you can work dead occasionally some DNO's fit an isolator after the meter which has a split cover arrangement allowing access to the consumers side terminals only

i have yet to come across a meter with an unsealed cover but sounds like a bloody good idea.


if isolators were fitted as standard then this thread wouldnt exist.

truth is, not many dno fit them so the choice is either install your own with 25mm tails on consumers side or henley them

 

[sparc]

with a seal on it lol

if meter is sealed and tails need changing this is where mr henley comes in

If the meter is sealed and the tails need changing then how does a henley block help? If there's one already installed between meter and CU, certainly. If not, you're going to have to open the meter to release the tails.

 

[flukey]

If the meter is sealed and the tails need changing then how does a henley block help? If there's one already installed between meter and CU, certainly. If not, you're going to have to open the meter to release the tails.

put meter tails into henley or isolator then into cu :dizzy2:

im all for cutting cutout seals but meter seals is a big no no

 

[1shortcircuit]

put meter tails into henley or isolator then into cu :dizzy2:

im all for cutting cutout seals but meter seals is a big no no

1 seal = Meter tail teminations other seal = ??? There's a reason I don't know the answer to the second. The reason I know the first is only because I read it on a public forum somewhere :thumbsup