ECN Forum Forums Code Related Discussion NEC & other Code issues Derating Issue with Inspector

I was told by the inspector today that When derating THHN I couldn't use the 90C column. He said I had to use the 75C column because the breaker terminations are only rated at 75C. I know the termination rating limits the ampacity when selecting the proper size conductor for a given circuit, but I thought you could derate from the highest temperature rating a conductor has. If not, what is the 90C column for?

Must be a new or uninformed inspector. If he is approachable, show him 110.14(C). There is only one thing you need to be aware of. If this wire is THHN/THWN and you are using it in a wet location, the call was correct. THHN is not good for wet locations, only dry or damp. Hence you'd derate from the 75° column.

George, why don't you think dual rated THHN/THWN is 90c rated? That is the whole point of the dual rating. The THHN part says 90c and the THWN part says "wet"

The THHN/THWN is actually two seperate ratings; when you are in dry or damp you can use the THHN rating, but when it is wet you have to use the THWN rating. By table 310.16 THWN is only rated for 75C even when doing deration.

dmattox is correct....according to the UL "White Book," the wire is marked THHN or THWN....notice the use of the word "or"....so in a dry location, start your calcs at 90 degrees.....in a wet location, start the math at 75.

Greg-The question doesn't state exactally if the wire was duel rated or if the location was wet or dry or damp. I totally agree with what the other gentlemen are saying. The point I was making was to make BigB know he needs to use the code reference and make sure the wire qualifies for derating from the 90° column by being in a dry or damp location. I'd assumed the wire was duel rated as they usually are. If it's only straight THHN he shouldn't even be using it in a wet location. Needs a "W" to do that

Yes the wire is dual rated. I is installed on a wall outside in emt so I guess that I would need to derate from the 75C column for this reason, but not for the reason the inspector stated (termination rating).
This of course assuming it is a wet location which I believe emt outdoors is a wet location, is it not?
Thanks, Brian

Table 310.13 also covers this.
So thhn/thwn in conduit underground stats derating at 75% then. On THWN says see note 4 which says if dual rated good for 90%.Its also in the UL white book ZLGR below in product markings.

[This message has been edited by Yoopersup (edited 05-04-2005).]

So common logic doesn't enter into it. The same wire that is good for 90c will suddenly burst into flames if the conduit strays outside the building envelope. hmmm

Wire insulation temperature ratings are not based upon a sudden point at which the insulation will instantly fail and burst into flames. Instead the rating is based upon how quickly the insulation will decay over time, and the desired life of the insulation system. If you take 90C insulation and run it at 120C, it will work just fine...but with a _much_ shorter life time.

It is entirely reasonably that a particular material will have acceptable life at 90C when dry, but an unacceptably short life at 90C when wet. THHN is rated to have an acceptable life at 90C when dry. THWN is rated to have an acceptable life at 75C when wet. It will _probably_ work at 90C wet, but might not have an acceptable life...or may simply have not been tested for 90C operation when wet (THWN-2)

-Jon