Wiring – interpretation of conduit vs raceway wet location vs outdoor vs NM NM-B

conduitnecwiring

eventual question: for a sealed PVC conduit external to house, does that qualify as a wet or damp location preventing use of non metallic (NM or NM-B) indoor type wire? And does underground feeder (UF) type of wire need to be used within that conduit?

scenario is regarding schedule 40 PVC gray electrical conduit {outdoor rated} run on outside of house from 1st floor (load center access) to access attic above 2nd floor. And this is along side same type gray PVC conduit of the same length that encloses single phase 2-wire 240 volt feeder from pole to house leading down into the electric meter socket/box (typical U.S. setup). Both conduits are the same length about 14 feet on the outside of house. Only practical means of getting new electrical run is along outside of house and penetrate attic wall, everything above concrete foundation and above grade by at least 3 feet.

  • a) difference between "raceway" and "conduit". does this definition matter? This is regarding circular conduit, schedule 40 NEMA TC2 the type used to connect to meter sockets. I think it's PVC, the gray electrical circular conduit definitely schedule 40 or above.
  • b) using the types of outdoor conduit bodies available today (21st century) they are quite durable and water tight, they come with gaskets on their covers. With solvent welding the plastic pipe and such good conduit bodies available does this run of conduit still classify as a wet/damp location such that the typical NM-B THHN indoor romex single run cannot be run within a 2" or 3" piece of conduit, per NEC interpretation?
  • c) does drainage matter for what has been described?
  • d) does a dual rating of THWN on the romex allow it? I don't remember at the moment if the typical romex 14/2 or 12/3 wire is also THWN. What about other wire ratings such as XHH or XHHW?

NEC-2017 sections:

  • 310.B and 310.C page 70-145
  • 334.10 and 334.15 page 70-191

Best Answer

Yes, that conduit counts as a wet location

All raceways in wet locations are considered to be wet on the inside as per NEC 300.9:

300.9 Raceways in Wet Locations Abovegrade. Where raceways are installed in wet locations abovegrade, the interior of these raceways shall be considered to be a wet location. Insulated conductors and cables installed in raceways in wet locations abovegrade shall comply with 310.10(C).

This means that the answers to your questions are, in turn:

a) Conduit is a kind of raceway (see the definition of "conduit" in Art. 100) (there are also surface raceways)

b) You will still get condensation in the raceways even if the joints are sealed by solvent welding. So, you will need to use wet-location-rated wire or cable (not NM!) in the conduit.

c) Drainage does matter -- 314.15 states that boxes and conduit bodies in wet locations must be arranged so that water is not trapped. It also provides that you can field-fit drainage openings no larger than 1/4" to boxes or conduit bodies in damp or wet locations, or use listed drainage fittings in knockouts for that matter. Take advantage of this, as drainage is your friend, and so is pressure moderation!

d) NM has a paper separator layer in it that makes it unsuitable for wet locations, period, even if the inner conductors were dual-rated. In general, stuffing cable down conduit is more annoying than it's worth, so you're better off getting dual-rated (THHN/THWN or XHHW-2) wires of the appropriate gauge and using those instead of a cable.

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