Wire in a conduit in a slab

[Rybenavides]

Is it acceptable to install 12-2 Romex in a PVC conduit in a slab for a kitchen island? Or do I have to install UF wire in the PVC conduit?

 

[infinity]

Depends. If the slab is on grade with dirt below it then you cannot use NM cable as the raceway interior is considered a wet location.

 

[Rybenavides]

It's is encased in the slab in top of base

 

[infinity]

Is there earth under the slab?

 

[Rybenavides]

There is a layer of base under the slab.

 

[FionaZuppa]

There is a layer of base under the slab.

It's on-grade.

Not on-grade is like 2nd story floor where the slab is not on earth.

Concrete is like a sponge for water.

 

[NTesla76]

I would say NM isn't acceptable. UF would be.

 

[tom baker]

No, as NMB is not rated for a wet location, see definition wet location. Use UF, which will be a tough pull, or 12 AWG THHW/N

 

[don_resqcapt19]

Is there a vapor barrier between the slab and the earth below?

 

[Barbqranch]

How big and how long is the conduit?
Use lots of lube.

 

[infinity]

Since it's a kitchen island I would assume that it's a short run with 2 elbows. 12-2 UF will work just size the raceway accordingly.

 

[wwhitney]

A few comments:

- I would think that compacted base rock is not the same as earth, and so the definition of "Location, Wet" does not automatically make the slab a wet location.

- As the slab is not above grade, even if the slab is a wet location, 300.9 does not tell us that the interior of a conduit in it is a wet location.

- 300.5(B) tells us that if the conduit is underground, then the interior of the conduit is a wet location. But is "underground" the same as "below grade"?

I can see an argument that grade is the top of the slab, and underground is below the slab and base rock. In which case I would say the interior of the conduit is not a wet location. But if underground starts at the top of the slab, then the interior of the conduit is a wet location under 300.5(B).

Cheers, Wayne

 

[infinity]

A raceway in a concrete slab in contact with the earth is a wet location by Article 100 definition.

Location, Wet. Installations underground or in concrete slabs or masonry in direct contact with the earth; in locations subject to saturation with water or other liquids, such as vehicle washing areas; and in unprotected locations exposed to weather.

 

[Ponchik]

without getting into base rock or the vapor barrier detail and whether or not is a wet location, you can either install UF cable and be done or ask the AHJ for clarifications. Install 3/4" or 1" conduit so it is easier to pull and will allow future expansion.

IMO, per NEC it is wet location (even though it may never get wet).

 

[don_resqcapt19]

The building codes typically require the vapor barrier for the very purpose of preventing the slab from being a wet location. If the vapor barrier is installed, the slab in not in direct contact with the earth and the slab is not a wet location.

 

[LarryFine]

- As the slab is not above grade, even if the slab is a wet location, 300.9 does not tell us that the interior of a conduit in it is a wet location.

- 300.5(B) tells us that if the conduit is underground, then the interior of the conduit is a wet location.

The interior of a conduit is not a location; the conductors are located wherever the conduit is located.

 

[don_resqcapt19]

The interior of a conduit is not a location; the conductors are located wherever the conduit is located.

It is a more specific location compared to the location of the conduit.

 

[wwhitney]

The interior of a conduit is not a location; the conductors are located wherever the conduit is located.

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

Cheers, Wayne

 

[wwhitney]

A raceway in a concrete slab in contact with the earth is a wet location by Article 100 definition.

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."

And I see no comments so far on whether 300.5(B) applies, i.e. whether in the slab itself is "underground".

Cheers, Wayne

 

[LarryFine]

Methinks the use of UF (or individual W-rated wires) would be less trouble than debating it in this case.