Meter Tail installation that exceed 3 meters length passing through the building fabric?

[MDB]

Hi, Please help............

Up until now I have been lucky enough to install CU's almost directly side by side to the main incoming supply/meter. I have just been asked to wire a new build from scratch and my lack of experience in this situation has raised a question...

The plans show the Consumer unit positioned approximately 10 meters away from the meterbox. (The main incoming TNC-S supply and meter box is on the outside front wall of the building and the CU is proposed to go under the stairs).

I have always worked on the rule that if the meter tails are no longer than 3 meters and clipped direct to a surface wall then there is no need for any additional protection or a 100amp main switch between the two, but this job does not fall in this category.

My questions are how would you best advise me to run the meter tails from the meter to the CU?

Do I need to fit a 100amp DP main switch next to the meter?

Is the 100amp DP main switch fitted for maintenance purpose or does it hold any other values? (ie a better choice rather than henleys.)

Do I need to fit the 100 amp DP switch between the meter and CU at all?

What would you advise is the best way to run the supply tails to the CU?

Should I run the supply tails through the cavity of the wall because there will be no RCD protection, and therefore they are buried more than 50mm? If so where should they run internally of the installation (ie when they pass behind the plaster board)? If I am to run them through a partition (buried less than 50mm) then from my understanding they will either need to be RCD protected (which would cause a discrimination issue between an RCD main switch at the origin and at the CU) or mechanically protected eg 25mm SWA (which I have never come across and seems over the top) to comply with 17th edition.

Sorry to sound incompetent, but I have checked the regs and can't find an answer. This is my 1st new build so im unsure. It is an install that I am not familiar with and would rather ask the question now and avoid problems in the future.

Your advice would be very much appreciated...................

 

[MKS]

Switched fuse is required for protection, not DP isolator, can come and do when I have morning off lol Joking aside, some basic info, that you should really know

---------- Post Auto-Merged at 18:33 ---------- Previous post was made at 18:29 ----------

Use SWA, gland in Aadaptable box, henley off, tails out, jobs a good un

---------- Post Auto-Merged at 18:34 ---------- Previous post was made at 18:33 ----------

Use SWA, gland in adaptable box, henley off, tails out, jobs a good un

 

[MDB]

Sorry but to clarify Another 100amp fuse (switched) by the side of the main 100amp distributors supply at the origin?

 

[MKS]

From DNO to switched fuse, run cable of required length to box, gland off, tail out to consumer unit, sorted!

 

[M107]

Meter tails in to - 80amp switch fuse - 25mm swa out of switch fuse & run to cu location - use metal clad cu gland swa to cu.

 

[MKS]

Or use metal clad

 

[unphased]

There is a regulation in BS7671 that may be the root of the 3m nonsense spouted about tails.

433.2.2 The device protecting a conductor against overload may be installed along the run of that conductor if the part of the run between the point where a change occurs (in cross-sectional area, method of installation, type of cable or conductor, or in environmental conditions) and the position of the protective device has neither branch circuits nor outlets for connection of current-using equipment and fulfils at least one of the following conditions:

i It is protected against fault current in accordance with the requirements stated in in Section 434.

ii Its length does not exceed 3m, it is installed in such a manner as to reduce the risk of fault to a minimum.....

I believe this could be the origin of the misinformed requirement. If you had tails at 25mm2 from the origin, and wanted to serve a fuseboard more than 3m away, provided you kept the tails at 25mm2 all the way and met the other requirements then there is no need to install another protective device (switched fuse) along the run. However, if you wanted to save some dosh and, say, have the first 3m in 25mm2 and install the remaining in 16mm2 if it was an exceptionally long run, then it could be done using that reg. This is an example of how hearsay and misinformation gets entrenched in this industry.

 

[Steve3948]

So run in cavity, so when they decided to have cavity wall insulation and the contractor starts drilling his holes for the fill, suddenly he hits a Live 6181Y, it's wet on the ground, he's holding the chuck and he has to rely on the service head fuse for protection, sorry but NO,

 

[Sukhoi]

If meter box is flush type (sunk into outside wall) electricity supply companies prefer for tails/cable to supply C.U. to be installed through bottom corner of meter box (even though their instructions might instruct top entry). This is incase rain water enters the cavity and penetrates into the top of the meter box & hence into meter, cut-out etc. If any gap in hole left where tails/cable pass, fill with intumescent foam.

Normally electricity supply company will supply double pole isolator (usually M E M make).

 

[Steve3948]

If meter box is flush type (sunk into outside wall) electricity supply companies prefer for tails/cable to supply C.U. to be installed through bottom corner of meter box (even though their instructions might instruct top entry). This is incase rain water enters the cavity and penetrates into the top of the meter box & hence into meter, cut-out etc. If any gap in hole left where tails/cable pass, fill with intumescent foam. Normally electricity supply company will supply double pole isolator (usually M E M make).

And your point is ?

 

[Sukhoi]

Cannot run cable into cavities, only directly through cavities, eg: from outside directly to inside of buildings. Conservatories etc. when customer does not want trunking/conduit, cables are run along the top of cavity (supported, below sill) then ran vertically into the outlet.

My point is good practice & common sense!! Any good meter installer will tell you how they prefer the box installed, so what is your point Steve

 

[NozSpark]

I always understood that the service fuse was there to protect the supply cable only and that the DNO only allow their fuse to protect the customers tails if they are <3M in length.

 

[Steve3948]

The op is asking about tails not the meter box, cables should not be in a cavity full stop not even supported under a sill.

That's why we have designated cable run zones on internal fabric of building.

---------- Post Auto-Merged at 21:20 ---------- Previous post was made at 21:05 ----------

I always understood that the service fuse was there to protect the supply cable only and that the DNO only allow their fuse to protect the customers tails if they are <3M in length.

And if >3M in trunking and or additional protection and means of isolation required.

 

[Sukhoi]

If internal fabric of building is decorative brick, no plaster or any other finish on wall, that is only way to install said circuit for hidden cables, even cleared it with NICIEC area manager (which he came out & had a look), it was about 8 years ago, I had to run it through conduit for protection even though strictly speaking it was run in safe zone (within 6 inch of top of wall and had to increase size of cable because of thermal effect of insulation in wall.

 

[Steve3948]

See this thread

http://www.talk.electricianforum.co.uk/question-answer-board/10007-consumer-unit-further-than-3-metres-metering-position.html

 

[MDB]

Some really interesting comments, thanks for all advice... Cavity wall suggestion of mine was stupid, just trying to interpret the regs as best I can, without breaching building control. (I don't get the 50mm depth, if you chase any deeper you weaken the block work and breach building control)cables behind dot and dab are less than 50mm and nobody batters off on new builds that I've seen. I don't feel comfortable running tails in safe zones with no rcd protection either.., guess it will have to be swa. Any other suggestions would be appreciated thanks.

 

[Steve3948]

Depending on load, 16mm

 

[batty]

Don't you love the people who design these houses no thoughts for the person who has to install the electrics in it.

 

[unphased]

So run in cavity, so when they decided to have cavity wall insulation and the contractor starts drilling his holes for the fill, suddenly he hits a Live 6181Y, it's wet on the ground, he's holding the chuck and he has to rely on the service head fuse for protection, sorry but NO,

Erm...yes. It has nothing whatsoever to do with running cables in cavities.

---------- Post Auto-Merged at 07:56 ---------- Previous post was made at 07:44 ----------

I always understood that the service fuse was there to protect the supply cable only and that the DNO only allow their fuse to protect the customers tails if they are <3M in length.

Noz. I understood that the supply fuse (cut-out) is sized according to the size of the tails to the meter. The cut-out limits the total load on the supply, of course. Hence the diffferent rating of cut-out fuses encountered. The meter tails are protected by the cut-out. They do of course indirectly ensure excessive load is not placed on the street supply. What is the purpose of a rule that imposes a requirement to introduce a switched fuse at 3m? What justification would the DNO have for this?

Regards

 

[MKS]

It's the DNO they usually don't need to justify anything! Usually it is their way or the highway