TTHN vs. THWN

[insparks]

I?m having a little debate with a local electrician over TTHN vs. THWN. He is saying that TTHN is rated for use in conduit in wet locations like under slabs or used in PVC schedule 40, he is saying that is basically waterproof. I?m saying if it doesn?t have a W or a U in the labeling it isn?t rated for that used and I need some code references to help prove my point.

 

[david luchini]

See Table 310.13(A)

 

[480sparky]

Look up both in Table 310.13(A). THHN is listed for dry and damp locations. THWN is listed for dry and wet locations.

Typically, both are listed on the conductor markings, but the application will determine the temperature rating used for ampacity & derating purposes.

 

[renosteinke]

Textbook answer: THHN is not rater for wet locations.

Real World answer: Just try to find a spool of THHN that is not also THWN rated.

Personally, I think the two are identical materials, and the different ratings are only a way to tell you to use different temperature columns in your de-rating calculations (THHN: 90C, THWN: 75C).

 

[Jim W in Tampa]

thhn is not for water thwn is, however many are duel rated. That is why they are confused

 

[jwelectric]

310.8(C) Wet Locations. Insulated conductors and cables used in wet locations shall comply with one of the following:
(1) Be moisture-impervious metal-sheathed
(2) Be types MTW, RHW, RHW-2, TW, THW, THW-2, THHW, THWN, THWN-2, XHHW, XHHW-2, ZW
(3) Be of a type listed for use in wet locations

 

[bobserrano]

Look at 300.9. Even above ground conduit in a wet location the interior of the conduit is considered a wet location and any conductor or cable must be listed for a wet location.