THHN Underground ???

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Looking through the archived articles, I found an article about underground conduit and THHN/W wire. "THHN Underground?" The posts never really defined "underground" or if burried conduit is a wet location.

Is PVC conduit burried in soil always considered
a wet location? Could it be a damp location? If this is true, I can't use THHN except indoors where the conduit is always 100% dry? Not talking continuously submerged. The nylon jacket of the wire is gasoline and oil resistant indicating liquid resistance?

Guess I'm a bit confused. I can burry a UF cable, code depth, in my backyard with a GFCI and be ok. OR lay PVC (SCH 80) conduit, code depth, with THHN + GFCI and get tagged? A metal shovel cutting through a live UF cable is less of a danger than a 99% dry conduit with nylon covered gas/oil resistant wire?

Trying to understand, maybe I didn't grasp the concept fully. Any NEC references on where to start researching would be great! Thanks.
 

ron

Senior Member
Re: THHN Underground ???

Look in the definitions Article 100 - "Location, Wet" which defines it as installations under ground .....
For you to use THHN underground, it must be dual listed as THHN/THWN (most are dual listed).
 
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