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I would like to install fans and and ceiling lights at different locations on a concrete ceiling and not have the wire or the conduit visible. I've seen previous posts recommending a trench as a solution and like this idea. Is there a minimum depth to the trench? (I also tried to find out if the OP of a similar question had successfully pursued any of the solutions offered but that query was deleted for some reason).

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  • Can you link some of those other posts so all three of us can go yell at them too? – Mazura May 30 '20 at 4:57
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    Does this answer your question? How to run electrical wire in concrete ceilng? – Mazura May 30 '20 at 11:37
  • What type of concrete ceiling are we talking about here? Cast-in-place slab? Composite slab on metal deck? Precast/prestressed hollowcore plank? – ThreePhaseEel May 30 '20 at 15:01
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If you cut a "trench" in a concrete ceiling, you potentially weaken the strength of the structure. Not a good idea. Better to use already available surface mount wiring channel (trade name "Wiremold" for reference only) and if it bothers you, change your attitude about it. Or hide it beneath some architecturally unnecessary structure, like a faux beam made of stryrofoam and painted to look like the concrete (people sell this for that exact reason). Compromising safety for aesthetics is never a great plan.

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  • Point taken but while trenching is the word associated with what I'm proposing to do, I envisage a very small channel at a distance of no more than 4 feet for each fixture. I would still like to know if anyone has done this successfully and how they gauged the depth of the trench/channel. Obviously the thickness of the conduit, but plus what? – Marky May 28 '20 at 21:26
  • @JRaef Actually, the OP asked for a minimum depth. Your answer would make an excellent comment, but it’s not an answer. – Lee Sam May 29 '20 at 18:48
  • Maximum depth : zero. +1. Anyone who has done this unsuccessfully is dead. – Mazura May 30 '20 at 4:32
  • I'm no expert, but I fail to see how a small trench for an electric wire could have any meaningful effect on the strength of the building. Can you explain/justify this? – einpoklum May 30 '20 at 10:17
  • @einpoklum -- part of the problem is that it could reduce minimum cover depths to the prestressing tendons or rebars, depending on the type of ceiling, which could lead to a serious fire-rating compromise point – ThreePhaseEel May 31 '20 at 17:40
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The wire would have to be run through conduit, and I don't believe there is a minimum "depth" that the conduit needs to be "buried" in the concrete. So, if using 1/2" conduit, you would need a channel that is at least 1.25" deep by 1" wide. The outside diameter of the conduit is close to 0.75" and you need extra room because I doubt this will be a precision cut, and you need some depth so the concrete patch doesn't immediately crack and fall out. Oh, and you need to fasten the conduit in place so you need to take that into account as well.

Now, that all said, I really doubt you should cut anything into a concrete ceiling. In a multi-unit residence, you likely don't have the legal right to do it, and in any other case, the ceiling is probably concrete because it's structural. And engineer would need to tell you it is safe, in writing.

The normal way to hide electrical in a concrete ceiling would be to add furring strips to the ceiling, run all the electrical, and then build a new ceiling on top of everything. Either that, or hide it with faux wooden beams or similar.

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  • Doesn’t the Electrical Code require a minimum “cover”? – Lee Sam May 29 '20 at 18:52
  • @LeeSam, that's what I'm not sure about. All the cover rules I know of are for buried conduit under concrete. If you can attach conduit to the surface of the concrete, I don't see why there would be a minimum depth under it. – JPhi1618 May 29 '20 at 18:55
  • NEC 300.5 does state that cable in a raceway "under a building" requires a cover depth of zero, so that's what I was basing this on. – JPhi1618 May 29 '20 at 19:01
  • Maybe if it’s in conduit there is no minimum cover. – Lee Sam May 29 '20 at 22:45
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In the ground with the utmost minimal exception: (otherwise it's at least 4", usually 6", if not more)

5: Where solid rock prevents compliance with the cover depths specified in this table, the wiring shall be installed in metal or non-metallic raceway permitted for direct burial. The raceways shall be covered by a minimum of 2 inches of concrete extending down to rock. – adamselectric.coop, PDF

In a concrete ceiling, in a trench after it was poured :

Absolutely zero.

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