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Running conduit in very shallow screed

Former Community Member
Former Community Member
Hi,


I am preparing the kitchen in my flat for a rewire and need to run cables for 2 x ovens plus dishwasher, washing machine, fridge freezer etc. 


The flat has solid ceilings and   floors and CU is on opposite side of the flat from the kitchen so running cable is a challenge. When we rewire the rest of the electrics, the sockets and lights are likely to go either behind coving or chased into plaster, but this is not really feasible for the kitchen cables. 


I’m thinking my best option is to go through the floors. I recently dug a massive chase for a gas pipe (which in retrospect should have been made bigger and I could have run the cables through that). In doing so I discovered that the screed is only 30mm thick and delaminates from the underlying concrete really easily with a good whack from an SDS. The concrete is obviously MUCH harder work to get through (a few days work with a heavy duty saw plus annoying the neighbours and filling the flat with dust). So what I want to do is cut the screed with a grinder to 30mm deep to make a channel and run the cable in that. 


I’ve got a couple of ideas of how to do this but not sure what is permitted - is it ok to run cable at such a shallow depth? - or what would be best. 


Option 1 - SWA cable laid straight in the channel and mortared over. Obviously will never be coming out so difficult to replace without pulling the floors up but the easiest to install. 


Option 2 - 20mm high impact pvc conduit with 3 x singles in each run. I’m assuming that because there will be 4 x 90degree bends in the route that these cables will also be impossible to remove once installed but will at least be protected by the conduit from damage while the mortar is applied/sets. I’m also assuming that even at a very shallow depth this conduit will be ok to walk over once set in mortar with flooring over the top. 


Option 3 - something completely different. 


Any advice would be greatly appreciated. I do want to make clear I’m not actually planning on doing the wiring work, I just want to get cables in situ for an electrician to be able to work with at a future date. I just want to do this bit ahead to avoid digging up the floor again at a later date. 


Thanks,


Patrick
  • 0
    TurkishBreakfast:

    I hadn’t considered a separate CU in the kitchen, that’s probably a good idea presuming that it can be fitted inside a cupboard i.e cut the back out of a tall oven housing unit and fit to wall inside top of cupboard. Or rather do that in reverse as units not currently in place. . 


    Bear in mind that the CU must be accessible - 132.12 and 513.


  • Former Community Member
    0 Former Community Member
    Thanks again for everyone’s replies. 


    I hadn’t considered a separate CU in the kitchen, that’s probably a good idea presuming that it can be fitted inside a cupboard i.e cut the back out of a tall oven housing unit and fit to wall inside top of cupboard. Or rather do that in reverse as units not currently in place. 


    With regard to conduit, I think I’ll try and avoid that altogether as I wasn’t aware it had to be all in place before cables pulled. My original plan was to construct it around the cables before fixing into place as a whole unit. The number of bends makes me think it’s pretty unlikely I’m going to get the cables pulled if conduit laid in advance. 


    So single SWA cable buried to additional CU probably the best option for me personally. I will discuss with electrician as soon as I can get someone in. 


    Thanks agaim for all the replies.
  • 0
    davezawadi (David Stone):

    Advice for any more grooving of floors and walls. Hire a chasing machine for a day and a suitable vacuum cleaner. It then makes little mess, and virtually no dust in the air. So much better. I have a Makita one, really excellent with my Henry! A few mm into the floor will make no difference to the strength, this part is in compression and well strong enough. It is the underside, in tension, which could be damaged, avoid the reinforcement, which will be near the underside! 20mm steel conduit will do the job, whilst plastic will work, I always prefer steel buried in concrete. 10mm of strongish mortar over the top will finish the job. You say 4 bends, this is OK if you only have a small number of cables (small fill factor). Cable current rating installation method B from table 4D1A. Overall this is much easier than SWA, and tidier too, because SWA doesn't sit nicely in place whilst you fill in and is probably bigger in diameter!


    We used to limit 90 degree bends to 2 no. max in any one single run between access points. Has this been relaxed? Also the regs. require the conduit system to be fully installed between access points before cables are pulled in. 522.8.2


    Has anyone considered the expansion and contraction of metal or plastic conduit due to temperature changes. This may disturb shallow screeding.


    Z.


     


  • 0
    Advice for any more grooving of floors and walls. Hire a chasing machine for a day and a suitable vacuum cleaner. It then makes little mess, and virtually no dust in the air. So much better. I have a Makita one, really excellent with my Henry! A few mm into the floor will make no difference to the strength, this part is in compression and well strong enough. It is the underside, in tension, which could be damaged, avoid the reinforcement, which will be near the underside! 20mm steel conduit will do the job, whilst plastic will work, I always prefer steel buried in concrete. 10mm of strongish mortar over the top will finish the job. You say 4 bends, this is OK if you only have a small number of cables (small fill factor). Cable current rating installation method B from table 4D1A. Overall this is much easier than SWA, and tidier too, because SWA doesn't sit nicely in place whilst you fill in and is probably bigger in diameter!
  • 0
    Sparkymark:

    Just a thought, have you got space in the kitchen where  you could put a small consumer unit?

    Then you could just run one suitably sized swa to the kitchen and new circuits could be connected to the new consumer unit.


    I was thinking exactly the same whilst reading down the thread.  Terminating several SWA cables might be problematic with some domestic CU's.  A second C.U makes it easier for future additions as well.



    Gary


  • 0
    Just a thought, have you got space in the kitchen where  you could put a small consumer unit?

    Then you could just run one suitably sized swa to the kitchen and new circuits could be connected to the new consumer unit.
  • 0
    "Plus I like to get an idea of what I’m getting myself into and if it sounds like a major job (eg metal conduit which I hate cutting threads on) then I’ll just leave it alone. But if I can just dig out and bury some SWA then this is easily doable so I’m happy to do it myself." 


    Do they now bury plastic conduit in floor screed?

    If your screed is 30mm, you might consider galv. metal floor trunking. There is 25mm by 75 or 100mm that would suffice. thats if there are any available electricians who have worked with metal containments in your area.

    Legh 



  • Former Community Member
    0 Former Community Member
    Thanks for the reply. 


    I’ve had a read through the relevant sections, I can’t see anything that would prevent the use of SWA buried direct and screeded over as long as I get the radius on the bends correct. 


    Protection from water and corrosion would be covered by the properties of the sheathing on the SWA cable? And if not presumably plastic sleeving of some sort would do? (“Suitable protection for application during erection may include protective tapes, paints or grease.”) Impact should be covered by the steel armouring. 


    Regarding intended use, I can’t imagine that walking on a floor would damage SWA once buried in screed: “ A wiring system buried in a floor shall be sufficiently protected to prevent damage caused by the intended use of the floor.” But my reasoning for thinking conduit might be better was that the impact of any movement in the slab or filling mortar could potentially cause damage? 


    To be honest I would prefer to avoid using conduit so it seems that SWA buried is the best option. I just wasn’t sure if there was a regulation that specified conduit was a necessity in an installation in screed. 


    I will be getting an electrician in to discuss before I start, I just wanted to get an idea of the regs before having someone come in, because what I don’t want is an electrician to tell me something like “bury some T&E and screed over it” and then I do it and lay the floors and then someone else won’t sign it off if I end up using a different contractor. Plus I like to get an idea of what I’m getting myself into and if it sounds like a major job (eg metal conduit which I hate cutting threads on) then I’ll just leave it alone. But if I can just dig out and bury some SWA then this is easily doable so I’m happy to do it myself. 


    So thank you all for you advice, I appreciate you taking the time to help me out. 


    Cheers,


    Patrick.
  • 0
    TurkishBreakfast:

    I’m thinking my best option is to go through the floors.


    I do hope that this flat is on the ground floor. (ETA: answer provided whilst I was typing.)


    Not really a wiring question!


    As Mike says, make sure that the sub-floor is not structural if you go into it. I would add that you do not want to breach any damp-proof membrane.


    Back to BS 7671 ...


    Section 522 - external influences - is relevant. You need to avoid or protect against water (522.3), corrosive substances (522.5), impact (522.6) and any other intended use (522.8.7). It might be possible to achieve that with a shallow chase, possibly covered by no more than floorboards (natural or engineered).


    If you want to use conduit, it must be installed before the cables are drawn in (522.8.2).


    Whatever option you choose, it would be wise to discuss with your chosen electrician beforehand. It might be a bit upsetting if somebody came later on and declined to accept what had been done.


  • Former Community Member
    0 Former Community Member
    Thanks for the replies. 


    The concrete is unlikely to be structural, it’s a ground floor flat. 


    Regarding running the cables behind the cabinets, this is what I intend to do, it’s the routing it from the consumer unit to the kitchen which is the issue as it has to go through bedroom and hallway. 


    The metal trunking option is not going to be easy as there are probably variations in the exact thickness of the screed over the proposed route so getting it to sit level with the existing screed will be a nightmare. 


    I’ll probably just lay down a few runs of SWA then as that seems to be the easiest as long as it will be fine in such a thin layer of screed.


    Thanks to both for your advice.


    Patrick

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