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Second CU, where to mount and what cable to run?

Discussion in 'Electrics UK' started by unclebob1, 14 Sep 2020.

  1. unclebob1

    unclebob1

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    I'm looking to put a second consumer unit in my loft, to feed the loft (lights, a couple of computers, network switch - nothing heavy), and the first floor (lights and sockets) of the house - No real easy way to run twin and earth within the house, and the existing wiring is black/red and well past it so want to replace with new runs. The existing CU, meter, isolator, henley block are all under the stairs, with an outside wall so easy enough to take a new feed from the henley, out of the wall, up to the loft and then into the new second CU.

    1) The roof at the top of the wall i would run the wires up, is a HIP, so mounting the new CU on this will not be easy to install or maintain. The only flat vertical wall is on the other end of the loft space - approx 6m straight line, a shared brick wall. Should I put up a board on the HIP side, to mount the CU onto or run it all the way across the loft and mount onto the shared brick wall? Loft floor is covered in solid insulation so running the wires under the boarded floor is likely a no go - it would have to go up the joists and across the ceiling.

    2) 1st floor will be standard household stuff - hair dryers, hair straighteners, tv's, computers etc, along with lights - most likely LED's Possibly a heat recovery unit for air exchange. No electric shower, put possibly washing machine and tumble dryer. What size cable should I look to run (taking into account the run will be approx 8m from ground floor henley to CU if its in the HIP side, or approx 14m if on the wall. I'm thinking 10mm SWA with RCD under the stairs, and metal isolator in the loft, before going into the CU? - in the future I am looking to externally insulate the wall, but will ensure sufficient gap is left around the cable. (box approx 50mm square?)

    3) with the floor of the loft covered in solid insulation, can I use conduit to come up along the wall, and through the insulation? Once its in the loft, its it better to run against the floor joists on top of the insulation? there is approx 15mm gap between the floor insulation and the boards on top. Or is it better to get the wires out and then run on the roof joists to the next socket/CU?

    Your thoughts and criticism is appreciated!
     
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  3. Risteard

    Risteard

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    Why do you want it in the roof space?
     
  4. JohnD

    JohnD

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    What are the floors made of in your house?
     
  5. muffking

    muffking

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    You sound clued up, but this is not a DIY job. Should be done by an electrician and is notifiable.

    Without reading all the way through I'd say your options here could be 16mm SWA from the Henley to the new board running no more than 63A.
    Or work out the max demand for the loft and do a calc to work out cable size and come off a new breaker in the existing board. Say 10mm SWA of a 50A breaker if it checks out ok.
     
    Last edited: 14 Sep 2020
  6. unclebob1

    unclebob1

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    Thanks for the replies,

    @Risteard
    The alternative is it's in the living space on the first floor, which becomes problematic once cupboards go in etc


    @JohnD the ground floor is solid, first floor is timber, as is the loft floor. The issue is the cupboard under the stairs comes out either on the stairs, or under a tiled toilet floor, and a second room which is a bathroom, again with tiles, neither of which I am not ready to take up. So no direct route from where the existing cu is to where I need to take power. Taking it to the loft space allows me to drop down as needed in each room on first floor, lights on first floor and to same in the loft space.


    @muffking I'd rather take it straight off the Henley, existing CU is full! 63A at 230v is 14 kilo watts, way over what I would be using.

    I'm hoping to go the self cert route, I did my city and guilds in electrical installation years back, so electrics wise I'm good, just not so up-to-date on the current 18th edition regs (I did 17th edition some time in 2007-8). And calcs are long forgotten!
     
  7. JohnD

    JohnD

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    Where will the Main Switch for the installation be?

    What is the fuse rating for the cable running to your second CU?
     
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  8. unclebob1

    unclebob1

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    @JohnD I have an isolator already between the meter and the Henley block. For the second cu, I'd fit a second isolator after the Henley and an RCD+ mcb. The swa would then go out of the wall and to the second cu.
     
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  9. unclebob1

    unclebob1

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    If I go down the 16mm route, Can I use the armour as earth or is it better to get 3 core and use one of them as earth?
     
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  11. Taylortwocities

    Taylortwocities

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    Steel is not as conductive as copper, so you Would need to carry out the usual calculations using the adiabatic equation.
    It’s here if you aren’t sure
    https://mycableengineering.com/knowledge-base/the-adiabatic-equation

    this is very worrying. You (obviously to me) need to limit the current going to the SWA.
    But why an isolator plus an RCD plus an MCB??
    Your second CU will have it’s own RCD, or use RCBOs and you do not need or want two RCDs in series!

    The SWA does not need RCD protection (which is a DP isolator btw). And getting discrimination between MCBs is difficult- consult the tables and you’ll see the challenge.

    All you need is an appropriate SWITCHFUSE after the Henley.
     
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  12. mikeey84

    mikeey84

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    Do you have all the calibrated testing gear required?
     
  13. unclebob1

    unclebob1

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    @mikeey84 I'm looking to borrow the equipment/be supervised by an a electrician my father uses for annual testing at rental properties
     
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  14. ericmark

    ericmark

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    My whole three story house is supplied with a 60 amp fuse, so it seems likely a problem if using over 63 amp, the old loadmaster did a 70 amp MCB, but for a modern system hard to find over 63 amp, your going into moulded breaker where you can easy get an 80 amp, but think you will have a rethink when you see the price.

    I was involved in the wiring of an old mill, where we were not sure if 100 amp would be enough, so fitted three consumer units and brought them back to a fused isolator so if found too much for a 100A DNO fuse we could use split or three phase, but the 100A never ruptured, unless you have an electric car charging point most house holds can run on 60A.
     
  15. EFLImpudence

    EFLImpudence

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    It's a loft.

    Unless you want electric heating, 20A will suffice.
     
  16. unclebob1

    unclebob1

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    @ericmark 63Amp, may be more then needed, but I guess having slightly more available on the first floor is better then not having enough? Something like this:

    https://www.tlc-direct.co.uk/Products/CGFS100.html

    Using just the 63A fuse?

    @EFLImpudence loft and first floor - granted the heavy users are oven/cooker etc which are on the ground floor so not a problem, I got a wife and two daughters - in the future no doubt there will be 3 hair straighteners, or hair dryers running at a time etc.

    Ideally i'll like to be able to push solar back down if possible but that's way off in the distant future...

    So would I be ok with the fused switch above with 63A fuse, and then using 16mm SWA(three core)?
     
  17. EFLImpudence

    EFLImpudence

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    What you are proposing is way over the top and would do for a whole house.
    10mm² and 60A fuse is often used for whole flats.

    If it weren't for the installation problems then you could connect the loft sockets to the first floor sockets and lights to the light circuit.
    So, all you need is another socket circuit, 20A, and light circuit, 6A.
     
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