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

 

[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.

 

[Engineer54]

Though probably in the majority of cases where domestic installations are concerned it wouldn't particularly matter much what size the tails are (16/25mm), ...It could make quite a substantial difference in some commercial/ industrial situations as well as in the odd domestic. The tails and/or mains/sub-main cables, will not only be providing power for that installation (or section of installation) but also acting as a limiting/restricting factor for the fault currents that can flow in that cable/installation. So in effect, over sizing these cables, can then become a problem. The closer to the distribution Transformer, the more a one size fit's all and bigger means better is not the route to take!! lol!!

These OSG's seemed to be aimed around the domestic/light commercial arena rather than towards the rest of the industry...

 

[GLENNSPARK]

Though probably in the majority of cases where domestic installations are concerned it wouldn't particularly matter much what size the tails are (16/25mm), ...It could make quite a substantial difference in some commercial/ industrial situations as well as in the odd domestic. The tails and/or mains/sub-main cables, will not only be providing power for that installation (or section of installation) but also acting as a limiting/restricting factor for the fault currents that can flow in that cable/installation. So in effect, over sizing these cables, can then become a problem. The closer to the distribution Transformer, the more a one size fit's all and bigger means better is not the route to take!! lol!!

These OSG's seemed to be aimed around the domestic/light commercial arena rather than towards the rest of the industry...

well its like owt isn`t it eng....
what happened to good old fashioned common sense eh?....like askin the client and using your own intuition to decide the best course of action regards tails.....
look, how many....say 2 bed houses are gonna get anyware near 60A for a start?....

 

[Engineer54]

well its like owt isn`t it eng....
what happened to good old fashioned common sense eh?....like askin the client and using your own intuition to decide the best course of action regards tails.....
look, how many....say 2 bed houses are gonna get anyware near 60A for a start?....

Think it goes a lot deeper than that, ..far too many here, haven't had the training to even make a commonsense judgement, let alone going down the calculation route. Many rely totally on these one size cover all statements and pre-calculated type circuits etc, in OSG's and the like!!

 

[mickles]

i never thought a simple request for clarification would cause so much controversy, thanks to all those contributors who have responded positively, greatly appreciated. for those suggesting 17 day wonder, no common sense, etc, i am a time served electrician qualifying after a 5 year apprenticeship and an onc in electrical engineering back in 1976, simply wanted clarification on a statement in an official( if not legally binding) document that differs from a lot of actual installation practice.

 

[HandySparks]

If your tails are 3m or less in length, protection for the tails is provided by the protective devices in your CU.

You've said this on another thread as well. Do you really believe this? In many normal installations, the sum of the ratings of the final circuit overcurrent protective devices is significantly more than the rating of the DNOs fuse and the tails, so how can they be protecting the tails?

And if the rating of the supplier's fuse is not relevant, why is it required to be noted on an EICR or EIC?

 

[Deleted member 26818]

You've said this on another thread as well. Do you really believe this? In many normal installations, the sum of the ratings of the final circuit overcurrent protective devices is significantly more than the rating of the DNOs fuse and the tails, so how can they be protecting the tails?

And if the rating of the supplier's fuse is not relevant, why is it required to be noted on an EICR or EIC?

The CPDs in the CU, provide protection in the same manner that a 13A plug top fuse provides protection for the cables of an unfused spur from an RFC.

Here's a simple question for you:
If the DNO's allow their fuses to provide protection for the tails, why do they stipulate a maximum length of 3m?
Do the DNO's fuses not work if the cables are longer than 3m?

Yes we are required to record the rating of the DNO's fuse, the rating informs us of the amperage of the supply that is being provided by the DNO.
We are also required to record the frequency of the supply, what relevance do you think such information has?

 

[HandySparks]

The CPDs in the CU, provide protection in the same manner that a 13A plug top fuse provides protection for the cables of an unfused spur from an RFC.

The difference is that on an unfused spur from a RFC, you're limited to one double socket, so the maximum sum of fuse ratings is 26A. On a CU, there's no such limitation, and as it's becoming more common to install multiple socket and lighting circuits, the sum of ratings is frequently much more than 100A. (In my particular case more than 200A.)

If the DNO's allow their fuses to provide protection for the tails, why do they stipulate a maximum length of 3m?

If the DNO didn't allow their fuses to provide protection for the tails why wouldn't they mandate an additional fuse on the output side of the meter rather than specifying a 3m limit on the length of the tails?

I don't know why there's a 3m maximum tail length before fusing, but maybe it's because anything over that length is becomes unreasonably difficult to inspect before connection?

 

[Deleted member 26818]

Please, difficult to inspect.
Does that mean that tails are not allowed to be concealed in walls?
I think tails concealled in walls would be harder to inspect than tails that are 3.1m in length.
The DNO's are required by law to ensure that their equipment when situated on a consumer's property is fit for purpose, installed so as far as is reasonably practicable to prevent danger and provided with protection by either a fuse or circuit breaker situated as close as is reasonably practicable to the consumer supply terminals.
The only grounds that a DNO has for refusing a connection to their supply, are on safety, or that the installation does not comply with BS7671 or ESQR 2002.
If a length greater than 3m was allowed by BS7671, the DNO's would have great difficulty proving that any length greater than 3m is dangerous.
However, they don't have that problem, as the maximum length as specified by BS7671 is 3m.

 

[HandySparks]

About the 3m tail length without additional fusing, like I said "I don't know...".

However, you still haven't explained how the OCPDs in a typical CU can adequately provide overload protection to the tails.

 

[Deleted member 26818]

CPDs in a typical CU protect circuit conductors that are rated much lower than the tails.
If they can protect the much lower rated circuit conductors, then they will have no problem protecting the higher rated tails.
You appear to be under the impression that the majority of installations have demands in excess of their supplies.
If that were the case, the DNO's would be inundated with calls to replace their blown fuses.
Unless you know something that I don't, the DNO's are not inundated with calls to replace their fuses.
In fact such calls are quite a rarity.
If you want to add up all the ratings of the CPDs in a CU, and take that as the demand, knock yourself out.