# THHN applications...



## wildleg (Apr 12, 2009)

thhn is almost always dual rated thhn/thwn


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## BBQ (Nov 16, 2010)

THHN is not rated for damp locations.

THWN or THWN/THHN is rated for wet locations and very likely what you buy will be THWN/THHN-2


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## rexowner (Apr 12, 2008)

BBQ said:


> THHN is not rated for damp locations.
> 
> THWN or THWN/THHN is rated for wet locations and very likely what you buy will be THWN/THHN-2


That's THHN/THWN-2


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## rexowner (Apr 12, 2008)

THHN/THWN and THHN/THWN-2 are both widely available.

If all else is equal, THHN/THWN-2 is better because it is still rated at
90 deg in a wet location, whereas you have to use 75 deg for wet for
THHN/THWN that you typically see at Home Cheapo.


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## chicken steve (Mar 22, 2011)

i thought the *W* meant _'wet'_, as it does in most wiring letter designation....?

~CS~


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## Big John (May 23, 2010)

Seeing it more and more triple listed: THHN/THWN/MTW 

The rate we're going we'll just be able to use one multipurpose wire for everything. 

"Yeah, I'd like 500' of THWN/MTW/UF/XHHW/MI/SEOO/MV90 thanks."


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## erics37 (May 7, 2009)

Like others said, most building wire these days is dual-rated. The thing to watch out for is temperature rating. If you get a spool of THHN/THWN/MTW you need to keep an eye on the location it's actually getting installed in if derating is a concern.

THHN, for instance, is allowed to be derated out of the 90C column in 310.15(B)(16). If the wire is also rated as THWN, then you're allowed to use it in wet locations, but you're limited to derating from the 75C column. So the same piece of wire can potentially have different temperature rating limitations depending on where it's installed. If your THHN is also rated as THWN-2, though, then you're good, as that is rated for 90C even in wet locations.

Table 310.4(A) lists off most of the common insulation types and their temperature/location limitations.


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## vizzolts (Sep 10, 2009)

The inside of a raceway is considered a 'wet location' if the raceway is installed in a 'wet location'


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## butcher733 (Aug 4, 2012)

This may be a case of needing to start looking more, but. Is the 90c rating for thhn really that big of a deal? I don't believe that I see alot of terminations that are rated for 90c. I would like some insight on this because it has come up on the jobsite recently. Most panels and receptacles are rated for 75c are they not?


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## mbednarik (Oct 10, 2011)

most wirenuts and 90c rated i believe. The 90c rating becomes more important when derating comes into play.


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## erics37 (May 7, 2009)

butcher733 said:


> This may be a case of needing to start looking more, but. Is the 90c rating for thhn really that big of a deal? I don't believe that I see alot of terminations that are rated for 90c. I would like some insight on this because it has come up on the jobsite recently. Most panels and receptacles are rated for 75c are they not?


The 90C becomes important if you're derating for one of the various reasons you might need to derate. However at the end of your calculation, you still have to take the lower of: 1) your derating calculation, or 2) the value in the 75C column.


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