Meter Tails sizing.

[Greenie9657]

Good Morning all,

Hope you all had a good Christmas.

Right, im getting a REC 2 isolator fitted next week and been told the tails will need upgrading. At the minute I have 16mm which I know is good upto 87 Amps so im guessing my main cut out must be 100A and the upgraded tails will be 25mm.

From what I was lead to believe, from the sparkys de-brief, was he is going to disconnect the CU and put those into the REC2 but then fit 25mm tails from the isolator back into the CU. Now I cant see what the point is but I never questioned it. Surely upgrading tails means it goes back to the cut out, or at least the meter as the name METER TAILS suggests.

Regards

 

[Taylortwocities]

The DNO is responsible for the supply to the meter/isolator and they will use their own rules (Electricity Safety, Quality and Continuity Regulations, etc) to decide how big the infrastructure needs to be up to that point.

The ELECTRICIAN is bound by a different set of rules (BS7671) that dictates the size of the tails from the DNO's demarkation point to the consumer unit.
So if BS7671 says the ELECTRICIAN must provide meter tails that are 25mm² then that is what he must do.

This may not sound logical, but that's the way things are.

PS, You may find that when the DNO fit the isolator that they upgrade the tails from the meter. Or give the guy a coffee and ask him to

 

[Greenie9657]

Thanks taylor,

Yeah it doesn't make much sense to me. but I guess as the meter is sealed then he could only upgraded the tails from the CU to the isolator anyway.

 

[EFLImpudence]

Or - he could be one of the modern breed who through ignorance wastes copper and believes that 25mm² "must" be used regardless of the supply fuse.

 

[Greenie9657]

Or - he could be one of the modern breed who through ignorance wastes copper and believes that 25mm² "must" be used regardless of the supply fuse.

Im not sure if he wants to waste copper but I can see your point. The price isn't much more, looking at the cost of the tails. Its only a meter or so that ill need to fork out for. I just couldn't see the point when everything back from the new isolator will be 16mm2!

 

[EFLImpudence]

No, but there are those who believe that tails must be replaced with 25mm² regardless.

It does use 50% more copper and there is a shortage.

 

[JohnW2]

No, but there are those who believe that tails must be replaced with 25mm² regardless.

It is pretty crazy, particularly when there is upstream 16mm². In addition to those who just don't (or can't) think, I suppose that there might be some who feel that, since the consumer has no control over what/when the DNO may do, if the cutout has the ability to take a 100A fuse, then (even if it is known to currently have a 60A or 80A fuse) one should assume that a 100A fuse might be put into it at some point in the future. However, that sort of anticipation of possible future events is, IMO, a pretty lame argument! Another practical issue is, of course, that electricians will not normally have any way of knowing what size of fuse is in a cutout - so, again, maybe feel that they have to assume that it might be 100A.

Kind Regards, John

 

[EFLImpudence]

That may be true in some instances but, as I say, there are those who just believe that tails must/should be increased to 25mm² - when a CU is replaced etc.

Not helped, of course, by it being stated for no reason in the wonderful OSG.

 

[RF Lighting]

I only carry one size of meter tails on my van as standard....

 

[JohnW2]

That may be true in some instances but, as I say, there are those who just believe that tails must/should be increased to 25mm² - when a CU is replaced etc. Not helped, of course, by it being stated for no reason in the wonderful OSG.

Yes, all agreed - I was merely pointing out that there might be some logical thought behind it in at least a few cases. However, I've certainly been told, by people who should know better, that there is some sort of 'blanket requirement' in the regs for 25mm² tails.

As for the OSG, we're obviously due to get a new one of those pretty soon (30th January), as well as the new regs (and this one fairly important in view of the changes to all the Zs tables etc.) - so it will be interesting to see what it has to say about all sorts of things!

Kind Regards, John

 

[JohnW2]

I only carry one size of meter tails on my van as standard....

Dare we ask ....?

Kind Regards, John

 

[westie101]

if the cutout has the ability to take a 100A fuse, then (even if it is known to currently have a 60A or 80A fuse) one should assume that a 100A fuse might be put into it at some point in the future.

Which shows an unsurpirising lack of knowledge as to how DNOs work and a insult (??) to their procedures and staff.

Any service installed prior to the late 80's could generally be 16mm2 anyway. but i guess too much trouble to ask.

Though we have now started to downrate fuses to 80A if we spot 16mm2 tails whilst doing work.

 

[JohnW2]

if the cutout has the ability to take a 100A fuse, then (even if it is known to currently have a 60A or 80A fuse) one should assume that a 100A fuse might be put into it at some point in the future.

Which shows an unsurpirising lack of knowledge as to how DNOs work and a insult (??) to their procedures and staff.

It's not an insult - merely a suggestion as to the view that some people might take that one should not rely on things that are beyond one's control - no matter what practices and procedures are meant to be adopted by those over whom one has no control.

Though we have now started to downrate fuses to 80A if we spot 16mm2 tails whilst doing work.

Are you talking about 'tails' on 'your' side and/or the consumer's side? Whatever, does this imply that you are now working to CCCs similar to the BS7671 ones?

Kind Regards, John

 

[westie101]

Are you talking about 'tails' on 'your' side and/or the consumer's side? Whatever, does this imply that you are now working to CCCs similar to the BS7671 ones?

Mainly on the customer's side, if it was on ours we'd change the tails.
It seems this is a recent decision that I don't know the whys and wherefores behind. But given that an 80A fuse would be happy at over 100A continuously it seems a bit nonsensical

 

[JohnW2]

Are you talking about 'tails' on 'your' side and/or the consumer's side? Whatever, does this imply that you are now working to CCCs similar to the BS7671 ones?

Mainly on the customer's side, if it was on ours we'd change the tails. It seems this is a recent decision that I don't know the whys and wherefores behind.

As I implied, it sounds as if your part of the industry has decided to work to CCCs similar to the BS7671 ones.

But given that an 80A fuse would be happy at over 100A continuously it seems a bit nonsensical

I wouldn't call it nonsensical. It's analogous to saying that since a B32 MCB would be 'happy' (wouldn't operate) with over 36A continuously, it is acceptable to run a 36A load continuously through one of them. There are, of course, generous 'margins' in all the figures we deal with, but one is not meant to 'eat into them' by design.

Kind Regards, John