twin and earth tails

[mickles]

Hi everyone, i recently had a look at a first fix which has been abandoned and now needs completing. this is a shop premises, with the meter located in basement. shop premises are ground floor and there is a firstfloor flat (no work to do in there). The shop consumer unit is connected to the meter by approx 10 metre length of probably 16mm twin and earth. there is also a 16mm earthing conductor.
looking through the regs, i am under the impresion that tails need to be 25mm. from a loading point of view, 16mm would be ok
interestingly the upstairs flat is also connected to the meter via twin and earth, and i have seen this done in a few flats i have had cause to visit for faults etc.....is it legal? and if so, under what authority/ regulation

 

[telectrix]

fine as long as the tails are protected with the correct size fuse ( 60A is my preference for 16mm )

 

[mickles]

thanks for that, i was intending to fit a 100amp double pole isolator with 60 amp fuse as the tails are longer tham 3 metres, just wasn't sure on the legality of anything less than 25mm. matter of interest, i can find reference in the osg which quotes tails must be 25mm, but haven't found anything in the ed17. supppose it must be in there. do you need to quote 16mm as a departure in the eicr?

 

[telectrix]

no departure as they are fused at 60A. 25mm tails are required when coming directly from supplier's 100A fuse.

 

[mickles]

thanks again

 

[kingeri]

The OSG seems to have introduce this feeling that tails always have to be 25mm. I always install 25mm on new installations now, but didn't start doing so until I'd used up all my 16mm!

 

[Engineer54]

The OSG seems to have introduce this feeling that tails always have to be 25mm. I always install 25mm on new installations now, but didn't start doing so until I'd used up all my 16mm!

Nope, ....Your all looking at a illustration diagram, and because it's showing 25mm on the tails, now THINK that all tails from a domestic cut-out has to be 25mm!! Nowhere within the OSG, or anywhere else, states or implies that 25mm is the minimum size tails!! You size your tails to the service cut-out installed fuse not to what an illustrative diagram or what's printed on a fuse cut-out cover!!!

 

[Geoffsd]

To be fair, 2.2.3 does state a minimum of 25mm. but then goes on to reference 1.3 where this is not the case.
In my opinion this is irresponsible.

I think the OSG is a poor publication and following 7671 is a far better method.
I did not buy the latest version until the scheme assessor would not pass me without it. £20 wasted.

In the OP's case Table 4D5 applies just as it does to the rest of the installation.

 

[mickles]

osg page2 para2.2.3 "minimum cable size should be 25mm" that is what was causing my concern

 

[kingeri]

Nope, ....Your all looking at a illustration diagram, and because it's showing 25mm on the tails, now THINK that all tails from a domestic cut-out has to be 25mm!! Nowhere within the OSG, or anywhere else, states or implies that 25mm is the minimum size tails!! You size your tails to the service cut-out installed fuse not to what an illustrative diagram or what's printed on a fuse cut-out cover!!!

I didn't say that 25mm tails have to be used.....or that that is what I believe, I referred only to the general feeling that this is the case these days.

 

[Engineer54]

Well if it does stupidly comes out with a statement like that, gives me yet another reason not to give these OSG's too much credence. But then i don't and never have needed a guide, to tell me what size tails to use...

 

[kingeri]

Well if it does stupidly comes out with a statement like that, gives me yet another reason not to give these OSG's too much credence. But then i don't and never have needed a guide, to tell me what size tails to use...

It's the same guide that gives tables to allow circuits to be installed without calculation.

 

[Engineer54]

It's the same guide that gives tables to allow circuits to be installed without calculation.

Well i bet that's an absolute god's send, for those 17 day whizz boy's!! lol!!

 

[sjhall]

25 mm is a must.......





If you can't think for yourself and apply knowledge learned

 

[Octopus]

25 mm is a must.......

Why ???

 

[D Skelton]

Read it again Murdoch

 

[Guest55]

i find the whole " which size tails do we use ?" debate a bit catch 22.
unless you know for sure what size fuse is in the dno cut-out its hard to make that decision correctly.
and you cant do that without cutting seals , removing carrier , etc , etc.
;-)
i'm definately in the pull fuse camp lol.

 

[Engineer54]

i find the whole " which size tails do we use ?" debate a bit catch 22.
unless you know for sure what size fuse is in the dno cut-out its hard to make that decision correctly.
and you cant do that without cutting seals , removing carrier , etc , etc.
;-)
i'm definately in the pull fuse camp lol.

It won't be long before the not-so-sure's will be whacking in 35mm tails ...Just to be on the safe side!! lol!!

 

[GLENNSPARK]

To be fair, 2.2.3 does state a minimum of 25mm. but then goes on to reference 1.3 where this is not the case.
In my opinion this is irresponsible.

I think the OSG is a poor publication and following 7671 is a far better method.
I did not buy the latest version until the scheme assessor would not pass me without it. £20 wasted.

In the OP's case Table 4D5 applies just as it does to the rest of the installation.

deliberate mistake methinks....
gives em an excuse to `amend it`...lol..

 

[Deleted member 26818]

You can use whatever CSA tails you want.
The rating of the DNO fuse should not be of any concern, as it is there purely to provide protection for the DNO equipment.
If your tails are 3m or less in length, protection for the tails is provided by the protective devices in your CU.
If you wish to use tails that are longer than 3m, you are required by BS7671 to provide overcurrent protection, at either the point in reduction of the CCC of the conductors, or within 3m of where that reduction occurs.
As the DNO equipment is not yours, the start of the 3m occurs at the consumer terminal in the meter.
The 25mm² and 16mm² CSA advised by the OSG, will cover all installations up to 100A, one size fits all.
No calculation required no thought required, just do as it says in the book.
A bit like painting by numbers.