Wire in a conduit in a slab

[ggunn]

Indeed, which immediately raises the question of whether a slab cast again compacted base rock, rather than native soil, is "in direct contact with earth."

What difference would that make? Compacted base rock is just as permeable to water as are native soils, possibly more so than some.

 

[wwhitney]

What difference would that make? Compacted base rock is just as permeable to water as are native soils, possibly more so than some.

My question was primarily terminological, whether the word "earth" implies "soil" or includes other fills like compacted base rock.

And while I mentioned compacted base rock, which to me connotes a graded mixture of sizes including fines and so therefore could transport water via capillary action, I believe it is also common for the topmost layer to be 3/4" crushed rock without fines as a capillary break. Which would matter. So I was just being a bit imprecise.

Cheers, Wayne

 

[petersonra]

I don't see how some foreign material could be considered "earth".

 

[ggunn]

My question was primarily terminological, whether the word "earth" implies "soil" or includes other fills like compacted base rock.

And while I mentioned compacted base rock, which to me connotes a graded mixture of sizes including fines and so therefore could transport water via capillary action, I believe it is also common for the topmost layer to be 3/4" crushed rock without fines as a capillary break. Which would matter. So I was just being a bit imprecise.

Cheers, Wayne

If there is standing water around the foundation, capillary action isn't necessary for water to get to the slab.

 

[wwhitney]

If there is standing water around the foundation, capillary action isn't necessary for water to get to the slab.

Certainly true, but for a slab on grade under living space (the kitchen mentioned in the OP), having standing water around the foundation would be a failure condition. The foundation drainage and site grading are supposed to prevent that.

Also, given the wording in 300.5(B) and 300.9, the question is just whether (a) the conduit is underground, (b) the conduit is in a wet location above grade, or (c) the interior of the conduit is considered a wet location for some other reason (maybe a section I missed?). If you buy the idea that a conduit in a slab on grade is both not "above grade" and not "underground" (not sure that I do, but it's worth considering), then the slab itself could be a wet location, but the interior of the conduit might not be a wet location.

At least, as an exercise in logic.

Cheers, Wayne

 

[wwhitney]

That is an approximate rule of thumb; the actual rules are in 300.5(B) and 300.9. (Any others I missed?)

Just to follow up on this, those sections are silent on the topic of damp locations. So say you have a damp location such some regions under an open porch. NM cable is not allowed in damp locations per 310.10(B). But if you install conduit in the damp location, does anything say that the interior of that conduit is considered a damp location? If not, you could use NM cable inside of it.

Cheers, Wayne

 

[ggunn]

Certainly true, but for a slab on grade under living space (the kitchen mentioned in the OP), having standing water around the foundation would be a failure condition. The foundation drainage and site grading are supposed to prevent that.

Also, given the wording in 300.5(B) and 300.9, the question is just whether (a) the conduit is underground, (b) the conduit is in a wet location above grade, or (c) the interior of the conduit is considered a wet location for some other reason (maybe a section I missed?). If you buy the idea that a conduit in a slab on grade is both not "above grade" and not "underground" (not sure that I do, but it's worth considering), then the slab itself could be a wet location, but the interior of the conduit might not be a wet location.

At least, as an exercise in logic.

Cheers, Wayne

Well, if 'twere my project, I think I'd just run wet tolerant conductors and move on.

 

[Jon456]

Personally, I would consider any conduit embedded in a concrete slab that is not fully within a conditioned space (both above and below the slab) to be a wet location. The reason is that moisture in the air will condense inside that conduit and accumulate over time. It really doesn't matter if the concrete itself gets wet.

 

[102 Inspector]

Just my view, if conduit is embedded in the concrete, but the concrete is separated from the subgrade with a vapor barrier it is not a wet location. If the conduit is placed in the sub-base and then a vapor barrier placed over the top of it, it is a wet location. Luckily everyone around here uses UF cable for these applications.

 

[FionaZuppa]

Mixing of terms going on.

IN or UNDER the slab?

2nd, IN a slab where slab is on earth (rock or whatever) IS still a wet location.

I know some locales where earth is blanketed with a vapor barrier before slab is done, but I do not recall seeing any exception from it still being a wet location even if it's IN the slab, or under slab between barrier and slab.

I believe this topic has presented itself here on MH Forums a few times already. The last one I recall was someone digging a trench in slab to an island, using conduit to reach between island and wall cavity, and then encasing the conduit in trench with concrete, the question was, was that a wet location in trench, answer was YES, still a wet location for the conduit/wire.

 

[FionaZuppa]

Just my view, if conduit is embedded in the concrete, but the concrete is separated from the subgrade with a vapor barrier it is not a wet location. If the conduit is placed in the sub-base and then a vapor barrier placed over the top of it, it is a wet location. Luckily everyone around here uses UF cable for these applications.

Barrier is not a waterproof thing. Just like NEMA cases/cabinets rarely carry a "waterproof" label.

The easiest way, conduit it and pull THWN-2 between two locations (island jbox to wall cavity or other jbox, accessible of course), then switch to romex at the jboxes to finish the wiring.

 

[FionaZuppa]

What difference would that make? Compacted base rock is just as permeable to water as are native soils, possibly more so than some.

And to note, water migrates from wet to dry locations, just like heat moves from hot to cold areas. water WILL eventually get there, and with no air circulation to be had.

 

[petersonra]

The definition of wet location covers installations "underground" or in concrete slabs or masonry in "direct contact" with the earth.

There is not direct contact with the earth and the slab when the slab is installed on top of insulation or a vapor barrier. It's hard for me to see that something could be considered to be "underground" if it is contained in a slab that is above the ground.

On the other hand, if you are running conduit why not just use THWN?

 

[Fred B]

The definition of wet location covers installations "underground" or in concrete slabs or masonry in "direct contact" with the earth.

There is not direct contact with the earth and the slab when the slab is installed on top of insulation or a vapor barrier. It's hard for me to see that something could be considered to be "underground" if it is contained in a slab that is above the ground.

On the other hand, if you are running conduit why not just use THWN?

Exactly, way easier to pull through, and no questions or objections as to "can it be used" in the conduit, by inspector or AHJ.

 

[FionaZuppa]

The definition of wet location covers installations "underground" or in concrete slabs or masonry in "direct contact" with the earth.

There is not direct contact with the earth and the slab when the slab is installed on top of insulation or a vapor barrier. It's hard for me to see that something could be considered to be "underground" if it is contained in a slab that is above the ground.

On the other hand, if you are running conduit why not just use THWN?

But it's not a "waterproof membrane". It's just a vapor barrier. Not all locales require such barrier for slabs on earth. And if one has to cut into the slab to put in the conduit, you'll likely compromise the barrier.

And to note, I think "on earth" means that earth is supporting the PSI of the slab.

THWN-2. Extend using romex as needed. Done.

 

[LarryFine]

On the other hand, if you are running conduit why not just use THWN?

Agreed. Each end must terminate in a box, but tat's not hard to do, even if you need a bit of flex at one or both ends.

 

[FionaZuppa]

Agreed. Each end must terminate in a box, but tat's not hard to do, even if you need a bit of flex at one or both ends.

But now I ask, if it's wet location does the full BC need to have insulated EGC? If so then switching to romex at a jbox won't meet code. If this is the case, then homerun UF using conduit where needed as a sleeve might be the only ez way?

 

[LarryFine]

But now I ask, if it's wet location does the full BC need to have insulated EGC?

No, only the wet portion of the circuit.

 

[Buck Parrish]

We all know conduit is a wet location. But here's a reminder . A little video where we are blowing out the conduit to suck the mouse in. Humor for Thursday.

 

[ggunn]

We all know conduit is a wet location. But here's a reminder . A little video where we are blowing out the conduit to suck the mouse in. Humor for Thursday.

I have similar iPhone video from one of our guys where enough water collected in a conduit to the point where it flooded a junction box and shorted everything out. When his helper removed the cover on the elbow at the lower end of the conduit he was shooting the video because he knew what would happen. It looks pretty much like this one.