Taking 25mm tails over 3 metres

[kris jones]

Hi all,

This week while wiring a very old farmhouse for heaters I noticed the incoming cu tails were very long, around 20 metres from the fuse and meter which is located in an adjacent farm building to the house.25mm tails (double insulated) leave the meter and go into a kmf switch (sorry I don’t know the fuse size but I’m guessing 80 as the single phase 1361 is 100 but possibly also 100 in switch) then 25mm tails (double ins) leave the kmf, go outside at an unreachable height and ‘dangle’ between buildings for about 8-9 metres (to the relevant house) where they go into the wall and under the floorboards to the house cu. from the cu a 16mm earth to spike. At a guess the length is 15-20 metres.
The first thing that popped into my head was the 3 metre rule regarding tails without a cpc. Obviously this farm is many many years old and perhaps back then this was ok?

I could replace the lot with swa and then have a cpc etc but not knowing the answer I called the NIC who firstly suggested the kmf was good as it then meant that any overload fault wouldn’t rely on the dno provided 1361 fuse only as the only means of protection, the height of cables was good for obvious reasons and then the rcd covered cu (will be replaced as ancient) on a TT meant that all
In all it was looking pretty much ok but.........
I should be going fuse to kmf then kmf to 100ma time delayed rcd (with is own rod) to give the correct protection if running only L and N tails over that length.

pre empting a q’s, I do need the 100a really as a number of storage heaters and panels going in and although diversity wise im sure it’s fine , the house is huge! Otherwise I’d fuse down and run a new cable. I have considered splitting the feed my question isn’t around how to do this. It’s more of a regs things so I can be sure of myself and get options.

thank you all for taking the time to read.

k

 

[telectrix]

the 3m "rule" is to stop a fault on tails > 3m from blowing the DNO fuse. this is achieved by fitting a KMF with a fuse rated < DNOs. so that bit's OK. further investigation is reqiue=red as to the loadin and the routing of tails. difficult to judge on a keybbored.

 

[kris jones]

the 3m "rule" is to stop a fault on tails > 3m from blowing the DNO fuse. this is achieved by fitting a KMF with a fuse rated < DNOs. so that bit's OK. further investigation is reqiue=red as to the loadin and the routing of tails. difficult to judge on a keybbored.

thank you, great straight to the point answer unlike my post!. I had no idea this was the case but it makes sense. Thanks again

 

[davesparks]

At a guess the length is 15-20 metres.
The first thing that popped into my head was the 3 metre rule regarding tails without a cpc.

I could replace the lot with swa and then have a cpc etc but not knowing the answe


I should be going fuse to kmf then kmf to 100ma time delayed rcd (with is own rod) to give the correct protection if running only L and N tails over that length.

There is no 3m rule for tails without a CPC, there is however a 3m rule for meter tails. But as you have meter tails less than 3m there is no issue with that.

If you are suggesting running SWA overhead then I would suggest you don't, there are less heavy cables far more suited to this kind of installation method.

That's nonsense, you need to have a CPC run to all points on the circuit, an RCD does not remove this requirement. It doesn't matter how long or short the circuit is, if using ADS the cpc must be run to every point.

 

[kris jones]

There is no 3m rule for tails without a CPC, there is however a 3m rule for meter tails. But as you have meter tails less than 3m there is no issue with that.

If you are suggesting running SWA overhead then I would suggest you don't, there are less heavy cables far more suited to this kind of installation method.

That's nonsense, you need to have a CPC run to all points on the circuit, an RCD does not remove this requirement. It doesn't matter how long or short the circuit is, if using ADS the cpc must be run to every point.

Thank you, it’s the way I’ve written it. We’re on the same page, that’s what I meant with regard to the tails. I agree regarding the cpc which also confused me. I wouldnt have ran swa like that it was more of point to build the story of what I’ve come against. Cheers