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Long meter tail upgrade, no nice solution comes to mind.

Russell Edwards

Hi All.

Thought I'd open this one up for discussion as I'm sure some of you can come up with a nice solution.

I've been given a friendly notice that i need to do something about the meter tails in my house by my DNO following the installation of a smart meter. They're the wrong gauge for the master fuse (16mm2 on a 100A fuse) and too long, about 4m. I'm having a job coming up with a solution I like. The current installation is original from when my house was built in 1996 and has gone unnoticed until now.

The meter is in a box on the front wall of the house and the tails exit this, run up the inside of the wall and over the ceiling of what was originally a bin cupboard in an uninsulated roof space before entering the distribution board directly though the hallway wall. The logical thing to do would be to protect the tails by replacing them with SWA but I have two problems with that. 1 that I have no way of terminating the SWA in the meter box without invading the DNO's space and 2 I'm not confident in brining in SWA from behind the board, as I'd effectively have to terminate it long and push some excess back into the roof space, and end up with the SWA termination in the wall screwed to the back of the board. So I can see this option needing me to bring the SWA in from above or below the board which would be ugly as sin in my hallway, doubly so as it'll have a bend radius easily measured in miles.

So I've considered using Steel Conduit instead, as that should be as tough as SWA. However unless I've missed something the largest I can get is 32mm OD which has an ID of circa 27mm. As I'm expecting some quite high temperatures in the roof space during the summer 25mm2 (rated to 101A if I'm reading Appendix 4 properly) could well end up under rated in the summer, so I'd be best to use 35mm2. This has a max OD of 13.5mm according to Doncaster cables meaning it'll be tight getting 2 down it.

I considered using XLPE Insulated 25mm as that can run hotter but of cause there's the 70 degC conductor limit of the board so it's not a help.

Bringing my back to SWA again.

I don't want to put joints in above the bin cupboard as there's no way of inspecting them.

So any other ideas of what I could do to protect the cables sufficiently to ignore the limits?

Thanks in advance.

Parents

  • This looks like an interesting project.

    What is the Peak demand for the property?

    100A Switch Fuse 1P+N Complete with 100A,80A & 63A Fuses are available.  If your peak demand was say 50amps the a KVM(100A Switch Fuse)  with a 63A Fuse would help.  100A Switch Fuse will allow you to properly terminate the SWA. I assume a 3 core SWA 25mmSquared CSA.  If XLPE - Thermosetting Pvc Insulation then that give current rating of 124 Amps which allows for derating.   Next step to RCD or Not RCD the 4 meter of SWA burried in the fabric.  BS7671 says yes it should be.  So a S type or time delay RCD with the KVM.  This would then bring into play a single point of failure for possible nuisance tripping of the RCD.  Change the 30mA RCD to a 100mA or even 300mA at the KVM.  Then at the CU/DB 30mA RCBOs.  With all the above considered get an SPB type 2 or 2/3 added to the CU/DB.

  • in reply to Sergio Fernandez

    Sergio - Next step to RCD or Not RCD the 4 meter of SWA burried in the fabric.  BS7671 says yes it should be.

    Sorry what regulation number are you referring to? Doesn't a swa have a metallic sheath? 

    Regards,

    CPC

     

  • in reply to CPC

    No need to rcd sub-mains in SWA or steel conduit or similar unless TT supply...
    M.

  • in reply to mapj1

    I Agree Thumbsup tone1 

  • in reply to CPC

    Hi CPC

    It might be a rule by the ENA (Energy Networks Association).  Some DNO/DSO say 2.5 meter some say 3 meter max for tails and then additional protection is required. 

  • in reply to Sergio Fernandez

    Some DNO/DSO say 2.5 meter some say 3 meter max for tails and then additional protection is required. 

    I think in that context it's additional overcurrent protection that they're referring to (not that it necessarily makes any physical difference, e.g. putting another 100A fuse downstream of their 100A fuse) but it transfers responsibility to the consumer (or rather their electrician) to ensure loop impedances etc are correctly co-ordinated).

       - Andy.

Reply
  • in reply to Sergio Fernandez

    Some DNO/DSO say 2.5 meter some say 3 meter max for tails and then additional protection is required. 

    I think in that context it's additional overcurrent protection that they're referring to (not that it necessarily makes any physical difference, e.g. putting another 100A fuse downstream of their 100A fuse) but it transfers responsibility to the consumer (or rather their electrician) to ensure loop impedances etc are correctly co-ordinated).

       - Andy.

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  • in reply to AJJewsbury

    And I took it to mean additional protection of any form which would lower the likelihood of a fault. So in my case, as you can tell, I'd chosen protection against damage. The use of a Fused Switch didn't occur as a solution as it' doesn't do much more than the master fuse, as has been pointed out.

    Thanks for the input all. Much appreciated.

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