I'm running 6 feet of electrical wire from the wall to kitchen island and going to do it this way:

  1. make a trench around 1 inch in depth
  2. put 1/2 inch flexible conduit there
  3. cover conduit with about 1/2 inch of concrete
  4. apply liquid membrane (preparation for tile)
  5. lay tile over it.

Is this solution is reliable? I know that trench perfectly should be deeper but I wouldn't like to cut my slab too deep and my guess is that if I lay large format tile over it then I should be good. Thoughts?

  • I think they sell/rent concrete groove cutters but if I was doing it with a carbide blade, I might cut a V groove to get the concrete out easily. And practice on some scrap wood first to get the angles and spacing correct. May 25 at 14:55
  • I have no problems cutting the ranch deeper - I'm just concerned about breaking its integrity b/c of soil shifts in my area. So I thought that 1 inch deep won't harm it.
    – Rainmaker
    May 25 at 14:57
  • I meant "cutting the slab deeper"
    – Rainmaker
    May 25 at 15:20
  • How thick is the slab? May 26 at 4:36
  • How can I find out? I've read it's 4-6 inches for residential houses.
    – Rainmaker
    May 26 at 13:12

FMC "flex" isn't allowed in concrete. In 362.10 ENT (smurf) is allowed to be embedded in concrete, so I think it would OK, but the inspector could have issue with meaning of "embedded" with such shallow cover, which could be up to interpretation of "similar finish" by AHJ.

NEC 300.4(F) Cables and Raceways Installed in Shallow Grooves. Cable or raceway-type wiring methods installed in a groove, to be covered by wallboard, siding, paneling, carpeting, or similar finish, shall be protected by 1.6 mm (1∕16 in.) thick steel plate, sleeve, or equivalent or by not less than 32-mm (11∕4-in.) free space for the full length of the groove in which the cable or raceway is installed.

Exception No. 1: Steel plates, sleeves, or the equivalent shall not be required to protect rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, or electrical metallic tubing.

Exception No. 2: A listed and marked steel plate less than 1.6 mm (1∕16 in.) thick that provides equal or better protection against nail or screw penetration shall be permitted.

  • Can I just use 1/2 PVC pipe then?
    – Rainmaker
    May 25 at 18:32
  • 1
    I think I would use sch80, so it would be Listed for use where subject to physical damage (NEC 362.12(c)). May 25 at 18:45
  • Makes sense. And you think it's gonna be OK if I cover it with only 1/2 inch of concrete and then thinset+tile?
    – Rainmaker
    May 25 at 19:56
  • I think with sch80 you'll be fine. Most places you can call the inspectors and ask for their interpretation. May 26 at 1:17
  • @Rainmaker -- make sure you're using PVC conduit, not DWV pipe! May 26 at 22:16

That should be fine. I'm not sure the depth requirements apply to interior wiring encased in conduit and concrete.

You might, however, find PVC to be cheaper than flexible conduit.

  • isn't a slab considered to be "an outside"?
    – Rainmaker
    May 25 at 17:53
  • @Rainmaker The catch here is that I read the NEC rules about depth to apply to open ground. You absolutely do not want electrical cable (conduit or not) 1" underground inside a flower bed, under your yard, etc. But nobody is going to be sticking a shovel into your kitchen concrete, and conduit with THHN wires are sufficient for a "wet" zone.
    – Machavity
    May 25 at 18:02

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