Split Concentric as a submain

[frontier219]

Hi All,

Having a little discussion with my old man over a job. The job is a domestic property, the CU is around 7 metres in cable length from the meter. There is a switch fuse around 4m from the meter, in a cellar. However, the tails are run very shoddily in an old bit of water pipe and then hanging across the ceiling. We have suggested ripping the whole lot out and running a new submain in, with an isolator at the mains. We are deciding what cable to use. I have suggested split concentric, as it is smaller and easier to run than SWA. However, my dad seems to think split concentric cable should not be used in this way, as he 'heard something about it a few years back'. We can't find anything definitive on the subject so I wondered if anyone on here could give us a bit more of a clue?

Cheers,

Dan

 

[Leesparkykent]

What's the installation method going to be?

 

[frontier219]

Clipped direct to the joists in the cellar, then in trunking up to the CU. No RCD required as it is all surface.

 

[Leesparkykent]

What amp is the switch fuse going to be?

 

[frontier219]

DNO fuse is 80A, so a 63A Switch Fuse.

 

[Leesparkykent]

What about 16mm T+E with a separate 10mm earth.

 

[Farmelectrics]

cant see an issue with any of them down to you your the designer

 

[frontier219]

Not a bad shout, but I personally think concentric looks neater. I'm also looking at using it in a Nursing home I am quoting for. Having done a bit more research, I think my dad may have been getting confused between concentric and split concentric. However, I'm not that familiar with the stuff so I'd like to make sure it can be used. I'm also currently assuming that for current carrying capacity, table 4D2A in the BGB would be used. Can anyone confirm this?

 

[Charlie_]

What he probably heard was in respect to using it as a cable outside the safe zones.
Under the 16th regs it was a recommended cable but this is now frowned upon as its a 50 50 chance on whether or not you would hit the CPC or the neutral through a concealed wall..

 

[spark 68]

DNO fuse is 80A, so a 63A Switch Fuse.

Why are you limiting your available supply to 63A ?

Your SF needs to be within 3m of the incomer/meter, this is to satisfy the DNO by not relying on their service fuse, you can use an 80A SF to fulfil this requirement, a 63A SF won't discriminate with the DNOs 80A fuse anyway.

 

[frontier219]

Ahhh, I see, I was trained under the 17th. So presumably he has either been misinformed or has got the wrong end of the stick. I have always assumed it would be treated similarly to T+E, but from what you say it could have been used in the same way as a SWA pre 17th ed?

 

[frontier219]

Why are you limiting your available supply to 63A ?

Your SF needs to be within 3m of the incomer/meter, this is to satisfy the DNO by not relying on their service fuse, you can use an 80A SF to fulfil this requirement, a 63A SF won't discriminate with the DNOs 80A fuse anyway.

I have to admit I wasn't aware of this, I may as well use an 80A fuse. I had assumed that discrimination would still be an issue. Thanks!

 

[Leesparkykent]

I have to admit I wasn't aware of this, I may as well use an 80A fuse. I had assumed that discrimination would still be an issue. Thanks!

The Rule of Thumb is "An upstream fuse will discriminate with downstream fuse of half it's rating."

 

[frontier219]

Hmm. This isn't something I was ever taught. So if I have a 40A Submain DB, does this mean that I can only have 20A circuits fed from it?

 

[spark 68]

The Rule of Thumb is "An upstream fuse will discriminate with downstream fuse of half it's rating."

True, but we are not trying to discriminate here, it is only to satisfy the DNO, they don't like you relying on their service fuse past about 3m, I believe it is also so they can guarantee their Ze figure, they have to draw the line somewhere as to what they will accept, the 3m rule seems to be about being reasonably practical in this regard, after your SF is your problem so to speak .