pvc under a slab

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RowE

Member
Location
Dumas Tx. 79029
I am installing a pvc conduit under the slab from an outside wall to a kitchen island. Should the conduit be wrapped with tape or sleeved with a foam insulation as it emerges from grade to give it a little added protection from the concrete as it is being poured? I don't see anything in the code that requires it or Is there something in the building code to reference.
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
I've never put sill seal around my conduits. I did see one general contractor put it around my conduits where they passed through a slab, I can only assume because he saw the plumbers do it, that he must think electrical conduits need it too. It can't hurt, I just don't think it's necessary.
 

klineelectric

Member
Location
FL
Occupation
electrical contractor
It is not needed. PVC only has to be below 4" of concrete so the 6" is not required either. Although I still bury it 6-10" also.
 

acrwc10

Master Code Professional
Location
CA
Occupation
Building inspector
Since it is in the slab you could run ENT if you wanted, no need for sch 80. I have seen lots of track house guys use a large pvc and run UF/NM and pex water lines and the dishwasher drain hose through the same conduit.
 

klineelectric

Member
Location
FL
Occupation
electrical contractor
Since it is in the slab you could run ENT if you wanted, no need for sch 80. I have seen lots of track house guys use a large pvc and run UF/NM and pex water lines and the dishwasher drain hose through the same conduit.
UF yes, NM no. Under the slab is considered "outside " of building and a wet location.
 

Smart $

Esteemed Member
Location
Ohio
Under the slab is considered "outside " of building and a wet location.
A wet location, yes, but not always "outside" the building.

In some instances it is advantageous as "inside" the building... but it does technically have to be within the footprint of the building, which doesn't seem to be the case here.
 

kwired

Electron manager
Location
NE Nebraska
What is tape or foam going to protect the conduit from?

If running through an outdoor slab in locations that freeze - maybe there is some protection from movement at different rates between the two, but shouldn't be much of an issue indoors especially in a habitable space.

Corrosion isn't an issue either.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
What is tape or foam going to protect the conduit from?
Concrete shrinks as it cures, which means a hole in the concrete will get smaller. So if pour concrete tight to a penetration like a conduit, then either the penetration has to crush or the concrete has to crack. I think pvc conduit is plenty strong enough to avoid crushing, so it will probably crack the concrete.

The upshot is that wrapping the conduit in crushable foam is to protect the concrete, not the conduit.

Cheers, Wayne
 

kwired

Electron manager
Location
NE Nebraska
Concrete shrinks as it cures, which means a hole in the concrete will get smaller. So if pour concrete tight to a penetration like a conduit, then either the penetration has to crush or the concrete has to crack. I think pvc conduit is plenty strong enough to avoid crushing, so it will probably crack the concrete.

The upshot is that wrapping the conduit in crushable foam is to protect the concrete, not the conduit.

Cheers, Wayne

If it shrinks wouldn't there be a gap between the concrete and the pipe after the shrinkage?

Either way - sounds like concrete guy's problem more so then the elecrician's. I see a lot of penetrations through concrete with little or no trouble.

I see a lot of "sleeves" through concrete for further addition of penetrating objects - they never wrap sleeves with foam either and those also survive.
 

acrwc10

Master Code Professional
Location
CA
Occupation
Building inspector
UF yes, NM no. Under the slab is considered "outside " of building and a wet location.

In this installationt the pvc in the slab would not be in a "wet location" the interior slab is on a vapor barrier and is a dry location.
 

Smart $

Esteemed Member
Location
Ohio
In this installationt the pvc in the slab would not be in a "wet location" the interior slab is on a vapor barrier and is a dry location.
The NEC does not define underground, but most consider anything below the upper surface of an on-grade slab to be underground... and then there's 300.5(B) "Wet Locations. The interior of enclosures or raceways installed underground shall be considered to be a wet location. ..."
 

kwired

Electron manager
Location
NE Nebraska
The NEC does not define underground, but most consider anything below the upper surface of an on-grade slab to be underground... and then there's 300.5(B) "Wet Locations. The interior of enclosures or raceways installed underground shall be considered to be a wet location. ..."
I didn't look it up but also think raceways in a slab on grade are considered wet location - but slab not on grade depends more on other details. Don't know if a vapor barrier would change the "on grade" status or not when it comes to determining if it is a wet location, but certainly would think it will be drier then if no vapor barrier - assuming top side is otherwise a dry location.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
If it shrinks wouldn't there be a gap between the concrete and the pipe after the shrinkage?
That's what you might think at first glance, but it's not how it works. When you have a hole in a material that shrinks, like green wood or green concrete, the hole gets smaller during the shrinkage.

There are a couple ways to visualize this. The one I like is suppose instead of making a hole you just mark out a circle. The material inside that circle is going to shrink, which means the circle itself is going to shrink. Now if that circle was actually the boundary of a hole, the hole just shrunk.

Cheers, Wayne
 

acrwc10

Master Code Professional
Location
CA
Occupation
Building inspector
I was thinking PVC "in the slab" not under the slab. In new construction, if the slab (interior kitchen floor) was not a dry location the finish flooring would have all kinds of issues like mold, etc.
 

klineelectric

Member
Location
FL
Occupation
electrical contractor
I was thinking PVC "in the slab" not under the slab. In new construction, if the slab (interior kitchen floor) was not a dry location the finish flooring would have all kinds of issues like mold, etc.
The slab may be dry but have you ever pulled a snake through a conduit under a slab and find moisture within? I have and I glue caps on all of my pipes so they are protected until rough in. Condensation will be in the conduit at some point under a dry slab.
 

jap

Senior Member
Occupation
Electrician
What is tape or foam going to protect the conduit from?

If running through an outdoor slab in locations that freeze - maybe there is some protection from movement at different rates between the two, but shouldn't be much of an issue indoors especially in a habitable space.

Corrosion isn't an issue either.

It's so when someone trips over it and breaks it off you can dig the foam out around it and get a coupling back on it easily......:D

JAP>
 
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