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

There also is an issue of the conductor being able to dissipate heat better when in a dry location.

It gets even more interesting when you install THW conductors in a conduit with other conductors that have THHN insulation. This would have us derating from the 75° column for THHN even in a dry location.

Why would a dry location dissipate heat better. Water is a better conductor.
I think the reality is U/L is behind in the technology and covering their ass

Ya, water cooled conductors

gfretwell: It might be an issue that the insulation degrades faster when wet, as well as faster when hotter. Both hot and wet and the degradation could be greater than the accepted level. But while wet, drop from hot to not so hot and you're back at the acceptable level. This is just a guess. It might really be some other reason entirely. But based on my study of chemistry three decades ago, it is a very plausible effect.

Also, in a confined space, the water is not going to be carrying the heat away unless it goes away. If the space is tight, like solid conduit, the water is not really a factor because it just stays there as hot as everything else. Get the temperature above 100C and you have steam, and pressure, and some will go away, carrying some heat with it. But this process stops when the vapor pressure equalizes, and you still have 100C destroying your insulation.

To have water cooled conductors, you'd have to arrange for the water to literally carry the heat away. Pumping water through could do that, but that requires special piping.

Why is it so hard to believe they can make 90c waterproof insulation?
I don't suppose it is worth arguing about. If that is the rule, rules is rules.

Of course you can have wires with insulation rated to 90C when wet. Look for wires with type THWN-2 or type XHHW insulation, among many others. Ya gets what ya pay for....

Certainly not worth arguing about; but worth wondering about as to why and wondering what is available; perhaps someone here will notice that the standard THHN wire that they use just happens to be THWN-2 rated (rather than THWN rated) and we can all start asking for that particular brand. Or maybe someone has an application where hunting down XHHW insulated wire will save having to go up a wire size, and they'll remember this thread.

I generally don't have to deal with temperature ratings for building wire. But I have to deal with the temperature ratings for motor wire; which has an entirely different set of requirements. You don't pull in through conduit, so it doesn't need the same sort of abrasion resistance. But it will move around in the slots a bit. For efficiency you want to pack as much wire in as possible, and this means the insulation needs to be _thin_. Motors get hot, far hotter than most building wiring, but hot enough to stay quite dry...so you end up with wire that is rated for 220C, with very thin insulation good to high voltage, but which you simply could not use in a conduit for ordinary wiring. The temperature versus life issue is very explicit for this sort of wiring. When an insulation system is described as class 200C, that means that at the design voltage, at 200C, you can expect the insulation to last for 10000 hours before failure; this is hardly more than a year of continuous operation, so you generally aim to operate 20 to 40C below the temperature class.

-Jon

Of course they can make insulation that is both waterproof and can withstand 90C while wet. But it's going to be more expensive than the insulation that can withstand 90C dry and 75C wet. Insulation that can only withstand 75C while dry would likely be cheaper (and easier to strip, too).

The stuff I have is also 105c MTW rated so I doubt this is anything more than a listing problem.