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General: Wiring a loft conversion

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  1.  
    • CommentAuthormarsaday
    • CommentTimeFeb 5th 2010
     
    Couple of questions about wiring. i will be using a sparky, but am also going to be doing a fair bit of donkey work myself.

    Can i run my lighting cable in the rafter? It is 100mm and will have 75mm of celotex in there. then there is a 25mm breather gap and then you have the bare slate. So is it ok to run the cable in this 25mm breather channel, basically up against the slate?

    next i plan on running the power cable over the current ceiling joists. Above this will be laid a new 8" joisted floor. insulation will go between these joists. Is there anything i need to think of in terms of protecting the cables? The insulation will be on top of the new ring main and so will this pose a problem? Alternatively i will lay the new ring main on top of the insulation, but i deally i would like it to go down below the new floor so out of the way.
    • CommentAuthorCWatters
    • CommentTimeFeb 5th 2010 edited
     
    Best run through everything you plan to do with your sparky to save mistakes. I'm not an electrician but...

    The maximium load a wire can carry changes if it's burried in insulation. May not be a problem but depends on the load. I believe they must be burried a minimium depth (50mm?) from the surface of the plasterboard (to stop nail damage).

    There are safe area where cables can and can't be run in walls. They should go vertically up or down to sockets never at funny angles. I believe there are also safe area in the corners of rooms and ??

    Technically you don't need an electrician if you are making a Building Control Application. The official rules say the BCO is meant to test everything you DIY install and isn't meant to make you pay for an electrician to get it tested. It's all meant to be covered by the fee you paid for Building Control Approval - John Prescott said so in writing to planning departments way back... but it hasn't stopped many BCO insisting though.

    Has he said you need a new breaker in the consumer unit or if you can tap off an existing ring?

    Don't forget wire for the smoke alarm?
    • CommentAuthormarsaday
    • CommentTimeFeb 5th 2010
     
    It is a new ring for power and lights. it was put in when i rewired 2 yrs ago with the sparky. smoke alarm is all sorted as well. so all ready and just needs to be connected in the CU.

    i have had a good idea! if i remove the 100mm insulation between the ceiling joists i can then lay the cabling on top of the ceiling. Above this will be a new 8" floor filled with 200mm insulation. so the wiring will be sat below all of this.

    i will be using the sparky to help me and will be doing lots with him.
    • CommentAuthorjamesingram
    • CommentTimeFeb 5th 2010 edited
     
    As cwatters said , it up to your electrian , he'll be the one who has to sign it off . i think the description is work must be carried out by a 'skilled or instructed person' .
    Make sure the pvc cable is protected from the PUR board , recent research quoted showed problems with cable degrading if in contact ( EPS and pvc cable problems have been known for a while )
    Cables should be only routed in the prescribed zones unless mechanicially protectd and at a depth >50mm and avoid being covered by insulation where practicial or I.t. down rated with reference to BS7671:2008
    No reason I'm aware of why you can run them in the area suggested on the rafters.
    Why would you remove insulation and then pay to put more back , I think regs require 30minute ceiling ? so skimmmed 12.5 with 100mm rockwool on chicken wire should be fine. run your cables on the sides of the new joists >50mm below the floor .
    • CommentAuthormarsaday
    • CommentTimeFeb 5th 2010
     
    i have uprated the old 100mm insulation with 200mm. so will use this between the new joists. the old 100mm will be removed where i run cables. so not loosing any money buying unnecessary insulation.

    as you say the sparky will advise properly, but i am doing some of my own research as i like to know what is what.
    • CommentAuthoraa44
    • CommentTimeFeb 5th 2010
     
    Most circuits in houses (including new builds) don't satisfy the 50mm requirement and the way round it is to put the circuit on the RCD side of the consumer unit so that, if you do put a nail through the cable, the RCD will protect you.
    • CommentAuthorbatkinson
    • CommentTimeFeb 8th 2010
     
    I think the 17 th edition (wiring regs) requires mechanical protection to cables where you dont have 50mm cover- a RCD alone is not sufficient.
    The reason why aa44 suggests that many new builds dont comply with the standards is one of the main reasons wiring was brought into building regulations but unfortunatly the CLG then came up with the competant persons schemes!!!!!!!!!!
  8.  
    "I think the 17 th edition (wiring regs) requires mechanical protection to cables where you dont have 50mm cover- a RCD alone is not sufficient."
    i pretty sure thats not the case. Importantly cables must be run in permitted zones
    check 522.6

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