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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
  • 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
    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
    Good point. I will be having a countertop mounted cupboard unit so probably the bottom of that just above worktop level should work for accessibility.
  • 0
    Z, that is a practical consideration when the conduit is more filled. Given time and care 4 90 degree bends is perfectly possible, particularly if one uses a pulling lubricant. In this case I can see no reason why a BESA box should not exist between sections, this is simply a matter of good installation practice. Adding a submain is an idea, but more expensive. Finding an electrician who can install steel conduit well may be more difficult. As to expansion, does this not happen to SWA too, and duee to its "soft" nature the screeding will have much more difficulty to stay in one piece with the concrete.
  • 0
    davezawadi (David Stone):

    Z, that is a practical consideration when the conduit is more filled. Given time and care 4 90 degree bends is perfectly possible, particularly if one uses a pulling lubricant. In this case I can see no reason why a BESA box should not exist between sections, this is simply a matter of good installation practice. Adding a submain is an idea, but more expensive. Finding an electrician who can install steel conduit well may be more difficult. As to expansion, does this not happen to SWA too, and duee to its "soft" nature the screeding will have much more difficulty to stay in one piece with the concrete.


    I was just thinking that if installed in cool weather and then the conduit is subjected to higher temperatures due to a hot summer, or cables running warm if the conduit can't expand laterally it may rise up like a serpent and become proud of the floor, or push up the screed. Plastic will expand more than steel conduit.


    If BESA boxes are used in the floor won't the cover screw heads stick up under the floor covering?


    Z.


     


  • 0
    A quick Google gives the coefficient of expansion of concrete as 13-14, mild steel as 10.8 - 12.5 and PVC as 54 - 110
  • 0
    parts per million per degree C
  • 0
    And now for something completely different....

    IET Forums - steel conduit expansion sleeve (theiet.org)


    Z.
  • 0
    Wouldn't something like cornice trunking be easier? (e.g. https://www.mkelectric.com/Documents/English/EN%20MK%20Catalogue/Ega%20Cornice.pdf) and drop down to the units - either in trunking in an inconspicuous corner or chased in. Or just chase in in the top of wall zone - you've got 150mm there so should be able to get quite a number of cables in.


    I'm dubious of cables embedded in concrete floors - too shallow and they're at risk of damage from floor covering fixings (nails of carpet gripper strips or screws from threshold strips) too deep and there are structural risks (even ground concrete floors could be beam & block or part of a raft foundation)


       - Andy.
  • Former Community Member
    0 Former Community Member
    Cornice trunking was what I had imagined would be used for sockets and lighting. 


    The problem with doing this for all cables is the flat is essentially split in two by a hallway and (tiled) bathroom that run down the middle, with two bedrooms on side where CU is currently and kitchen and front room on the other side. So essentially I need to get a lot of cabling to the other side of the flat, far more than would probably fit in coving, and also coving would be best avoided in bathroom and hallway because the doors have glass panels above them to the ceiling which means the coving needs to terminate at each doorway. So my idea was to have lighting and socket circuit for one side of flat, and then another lighting and socket circuit on the other side from the cable/cables that I can route to that side of building. 


    This is what was leading me to thinking about taking the cables through the floors. Chasing around the tops of rooms is doable and is my last resort, but best avoided in this instance I think as the route that needs chasing out is always cut off by the glass door tops except for the back wall of the bathroom which is tiled and would require ripping the tiles down. 


    You’re right about the depth being an issue for carpet grips etc, which is why I posted the question in the first place to see what best practice is for this sort of situation. While I’m living here there won’t be any carpets or thresholds as the floor is going to be one unified wood floor throughout but obviously could be a problem if I ever move. Although obviously if there were cables in the floors I’d leave a diagram showing exactly where they are to next occupier. 


    Thanks for all the replies, they’ve given me a lot of pause for thought, particularly regarding expansion, sub mains etc which I hadn’t even considered. 


    Next step I’m going to try and plan the whole thing out with an electrician, but this has given me a lot of ideas to discuss as to what might be possible.

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