What If: Electrical Question

[jar546]

We can see the NM cable entering the 1900 box from above. What if you found NM cable coming up into the box from inside the PVC/ENT below?
They did not call for inspection yet so don't look at anything else unless it is electrical. Lets not get off topic.

 

[steveray]

Ground floor?

 

[jar546]

Ground floor?

Slab on grade single story home.

 

[steveray]

Slab on grade single story home.

So wet location...No Bueno....

 

[wwhitney]

So wet location...No Bueno....

In the spirit of the thread title, what if the ENT is in the slab itself, and under the slab there is a vapor barrier over insulation over a capillary break (drain rock)?

Seems like that would make it a dry location. But that would be an atypical condition.

Cheers, Wayne

 

[jar546]

In the spirit of the thread title, what if the ENT is in the slab itself, and under the slab there is a vapor barrier over insulation over a capillary break (drain rock)?

Seems like that would make it a dry location. But that would be an atypical condition.

Cheers, Wayne

Not per the NEC and I agree as an experienced electrician.

 

[wwhitney]

Not per the NEC and I agree as an experienced electrician.

Oh?

The definition of "Location, Wet" refers to "concrete slabs or masonry in direct contact with the earth". The implication is that a concrete slab not in direct contact with the earth would not necessarily be a wet location.

Cheers, Wayne

 

[jar546]

Oh?

The definition of "Location, Wet" refers to "concrete slabs or masonry in direct contact with the earth". The implication is that a concrete slab not in direct contact with the earth would not necessarily be a wet location.

Cheers, Wayne

When you have direct experience pulling wires out of conduit that is run under a slab but on top of the vapor barrier and then have to vacuum out all of the water so you can pull new wire, you might understand. All underground/under slab conduit fills with water, even from condensation. Direct experience in construction, specifically electrical contracting for decades teaches you that.

Now, to continue with your hyper-technical thought process. How do you know there is a vapor barrier in place when you get there as the electrical inspector? How do you know that 100% of the concrete slab is isolated from the earth and that no part whatsoever is touching the earth? Did you know that the vapor barrier you are referring to is for underneath the slab? Did you know that the sides of the slab are backfilled against with earth when you pull away the forms and grade the property? Did you know that in many areas, grade beams and pilings connect directly to the slab? Are you familiar with capillary action? How about during construction when it rains several times before 'dry in' is complete? Where do you think the water goes? Did you know concrete is porous?

Maybe in a hypothetical world, there is a possibility that this may not be a violation under the most perfect of conditions; however, in the real world, the one that counts, NM cable in the conduit that goes under a slab on grade, this is most definitely a violation and any qualified electrician with 1/4 of a brain knows better than to do that which is why the hypothetical question I posed is just that, hypothetical because the electrician on this job knows what he is doing and pulled THWN through the conduit.

 

[bill1952]

There was a thread a while back about a stair in a basement and I raised my concern that it was plain untreated lumber on the slab. I was corrected that the vapor barrier meant that the slab was not in contact with the ground. Not that I see that same requirement of not in contact with the ground with a different spin.

 

[tmurray]

Now, to continue with your hyper-technical thought process. How do you know there is a vapor barrier in place when you get there as the electrical inspector? How do you know that 100% of the concrete slab is isolated from the earth and that no part whatsoever is touching the earth? Did you know that the vapor barrier you are referring to is for underneath the slab? Did you know that the sides of the slab are backfilled against with earth when you pull away the forms and grade the property? Did you know that in many areas, grade beams and pilings connect directly to the slab? Are you familiar with capillary action? How about during construction when it rains several times before 'dry in' is complete? Where do you think the water goes? Did you know concrete is porous?

Maybe in a hypothetical world, there is a possibility that this may not be a violation under the most perfect of conditions; however, in the real world, the one that counts, NM cable in the conduit that goes under a slab on grade, this is most definitely a violation and any qualified electrician with 1/4 of a brain knows better than to do that which is why the hypothetical question I posed is just that, hypothetical because the electrician on this job knows what he is doing and pulled THWN through the conduit.

How often do we do things 100% right 100% of the time? Almost never I would guess.

 

[steveray]

Oh?

The definition of "Location, Wet" refers to "concrete slabs or masonry in direct contact with the earth". The implication is that a concrete slab not in direct contact with the earth would not necessarily be a wet location.

Cheers, Wayne

Interesting...I'm still going to go with Jar...but....

 

[steveray]

There was a thread a while back about a stair in a basement and I raised my concern that it was plain untreated lumber on the slab. I was corrected that the vapor barrier meant that the slab was not in contact with the ground. Not that I see that same requirement of not in contact with the ground with a different spin.

Yeah 317 doesn't say what most of us think it does, this is the section you would have to cite for stringers I believe and it doesn't really fit that well:

3.Sills and sleepers on a concrete or masonry slab that is in direct contact with the ground unless separated from such slab by an impervious moisture barrier.

And note that the wood gets separated from the slab, not the slab from the earth....Probably because the people that write the building code are smarter than the electrical code folks...

 

[wwhitney]

When you have direct experience pulling wires out of conduit that is run under a slab but on top of the vapor barrier and then have to vacuum out all of the water so you can pull new wire, you might understand. All underground/under slab conduit fills with water, even from condensation.

I'm familiar with this idea and I have no reason to doubt it.

My hypothetical wasn't under the slab, but within the slab. The point being that the slab and the conduit in my hypothetical are all within the thermal and vapor envelope of the building. In which case I'm not seeing the mechanism that would drive condensation inside the conduit. Concrete is a pretty poor thermal insulator, meaning the entire slab will be at close to the same temperature. So if we don't have water condensing on the surface of the slab, we shouldn't have water condensing in conduit a few inches under the surface of the slab, assuming the air's humidity is the same in the two locations.

And temporary conditions while under construction are not relevant to this determination, as per the definition of "dry location" in the NEC. Once the building is dried in, you might find water in this hypothetical conduit, but if you vacuum it out, it shouldn't return. So you need to do that before pulling your NM cable.

How's this for a summary: conduit in an elevated concrete slab inside a building is not a wet location. Conversely, conduit under a slab on grade, or within a slab on grade in contact with earth, is a wet location, and that covers almost all cases on grade. But you can construct a slab on grade that would be very similar to an elevated slab in an insulated basement, just not elevated, and then conduit within that slab would not be a wet location.

Cheers, Wayne

 

[steveray]

Lets go at it from another direction....If the NEC is considering any basement damp, I am considering every slab as such...And I would likely argue that in the slab is embedded and/or similar to the "shallow chase" provision

Location, Damp.

Locations protected from weather and not subject to saturation with water or other liquids but subject to moderate degrees of moisture. (CMP-1)

Informational Note:
Examples of such locations include partially protected locations under canopies, marquees, roofed open porches, and like locations, and interior locations subject to moderate degrees of moisture, such as some basements, some barns, and some cold-storage warehouses.

(B) Type NM.

Type NM cables shall not be used under the following conditions or in the following locations:

• (1)
  Where exposed to corrosive fumes or vapors
• (2)
  Where embedded in masonry, concrete, adobe, fill, or plaster
• (3)
  In a shallow chase in masonry, concrete, or adobe and covered with plaster, adobe, or similar finish
• (4)
  In wet or damp locations

 

[wwhitney]

Lets go at it from another direction....If the NEC is considering any basement damp,

But it doesn't consider any basement damp. It considers "some" basements damp. Any new building with a damp basement has a serious design problem, but there certainly are plenty of such buildings in the existing stock.

I am considering every slab as such

No, not every slab is damp. Concrete doesn't generate moisture. If a piece of wood in the same location wouldn't be subject to "moderate degrees of moisture", then concrete in that location won't be damp.

...And I would likely argue that in the slab is embedded and/or similar to the "shallow chase" provision

Agreed that you can't run NM cable directly in the slab. But ENT is not so restricted, and inside ENT is different from being "embedded" or in a shallow chase.

Cheers, Wayne

 

[Paul Sweet]

How about NM-C?

 

[wwhitney]

How about NM-C?

Are you aware of any commercially available products that are listed as NM-C and not also listed as UF?

Cheers, Wayne

 

[steveray]

But it doesn't consider any basement damp. It considers "some" basements damp. Any new building with a damp basement has a serious design problem, but there certainly are plenty of such buildings in the existing stock.


No, not every slab is damp. Concrete doesn't generate moisture. If a piece of wood in the same location wouldn't be subject to "moderate degrees of moisture", then concrete in that location won't be damp.


Agreed that you can't run NM cable directly in the slab. But ENT is not so restricted, and inside ENT is different from being "embedded" or in a shallow chase.

Cheers, Wayne

We will agree to disagree.....

If it is conductive and wet enough (with a vapor barrier) to require GFCI, that is all I need....

 

[jar546]

Lets go at it from another direction....If the NEC is considering any basement damp, I am considering every slab as such...And I would likely argue that in the slab is embedded and/or similar to the "shallow chase" provision

That is why they use NM-C

 

[ICE]

When you have direct experience pulling wires out of conduit that is run under a slab but on top of the vapor barrier and then have to vacuum out all of the water so you can pull new wire, you might understand. All underground/under slab conduit fills with water, even from condensation. Direct experience in construction, specifically electrical contracting for decades teaches you that.

Now, to continue with your hyper-technical thought process. How do you know there is a vapor barrier in place when you get there as the electrical inspector? How do you know that 100% of the concrete slab is isolated from the earth and that no part whatsoever is touching the earth? Did you know that the vapor barrier you are referring to is for underneath the slab? Did you know that the sides of the slab are backfilled against with earth when you pull away the forms and grade the property? Did you know that in many areas, grade beams and pilings connect directly to the slab? Are you familiar with capillary action? How about during construction when it rains several times before 'dry in' is complete? Where do you think the water goes? Did you know concrete is porous?

Maybe in a hypothetical world, there is a possibility that this may not be a violation under the most perfect of conditions; however, in the real world, the one that counts, NM cable in the conduit that goes under a slab on grade, this is most definitely a violation and any qualified electrician with 1/4 of a brain knows better than to do that which is why the hypothetical question I posed is just that, hypothetical because the electrician on this job knows what he is doing and pulled THWN through the conduit.

So did you run out of amo or can you keep going?

 

[wwhitney]

So did you run out of ammo or can you keep going?

If we all agree that conduit in an elevated slab is a dry location, then clearly that is also achievable for a slab on grade with the proper detailing. [Again, in the slab, not under it.]

I think the disagreement jar546 and I seem to have here is whether that "proper detailing" is so unlikely as to be a negligible possibility (quite possibly true for a practicing building inspector) or something that an excellent builder can in fact achieve (I would consider that definitely true).

Cheers, Wayne

 

[ICE]

The likelihood of achieving a dry location with a plastic membrane is slim to none. The difficulty of pulling NM in conduit is another reason that it is not done. As an inspector, if I encounter a conduit emerging from a slab on grade, any wire has to be listed for a wet location. The contractor can make all sorts of claims about how every effort ws made to create a dry location but there's a definition in the NEC to get around and that has never happened.

 

[jar546]

So did you run out of amo or can you keep going?

Not out of ammo, just clarifying intent and common sense vs hypothetical extremes.

 

[wwhitney]

Not out of ammo, just clarifying intent and common sense vs hypothetical extremes.

The "proper" way to detail a slab on grade within the thermal envelope in cold climates (capillary break, under-slab and perimeter insulation, vapor barrier) is going to yield a dry location for conduit within it when well executed.

The actual way that slabs on grade are usually detailed in warm climates won't.

Cheers, Wayne