I think there was a discussion about this in the IAEI magazine a while ago. It has to do with the thermal conductivity of the wire as I recall. If I have an area where the ambient requires derating I can cheat for the first 10' but if that gets to the termination, I have to respect the termination temperature. ... or something like that. It never really made much sense to me, particularly when they got so excited about an inch and a half of Romex in spray foam. 334.80
George, that's exactly right. Derating for the cable run and derating for terminations are separate factors. Copper is a good heat sink, but it's not going to transfer much heat over 10' of length. 10' allows enough space that cabling approaching 90C in design conditions in the raceway will not overheat the terminals past 60C. You just have to make sure that the cable you installed meets all derating factors for both sections.
1) Calculate the minimum ampacity for the terminals and first 10' using the 60C table and all appropriate derating factors.
2) Calculate the minimum ampacity for the rest of the run using the 90C table (if 90C cable) and all appropriate derating factors.
3) The larger cable of the two is the one you must use.
Greg: 310.15(B)(2) Exception 3 says you can ignore derating factors for "nipples" less than 24" in length. I'd say this applies to running through foamed studs and other short pieces of raceway.
Say you have a long run (300Ft) part of that run goes over a Hot area blast furance ect. The part over the blast furance Must be taken into account for the total run (Derated because of Temperature). See table 310.16 Correction factors at the bottom) See Exceptions for areas Beyond the point of transition . 10Ft or 10% or less of circuit total lenght the highter Amerage rateing can be used. If run is 10ft or less or Hotter area less then 10% 2005 Handbook has example & clearly explains it. As far as 90 degree rateing on conductors Except for HV cable that Amperage is to be used only for derateing of conductors.(see 110.14(C).
Steve, I understand the distinction for <24" nipples but 334.80 doesn't seem to. The way the code is written and enforced it applies to a 2x4 header over a hollow bay that has a foamed draft stop in the bored hole. The NEMA report on foam and NM-B seems to dispute this assertion that there is a problem but the rule still lingers on in 2008. In fact they point out the 310(A)(2) exception does not apply. I will go look at the ROP when I get a minute and see what they were thinking.
I just looked and now I remember what happened. Jim Pauley asked if you just used the material on the bottom of the 2x4 would that still be a problem and all they took away was they forgot to say "caulk". Mike Holt said the whole thing was silly with several substantiations and was rejected 14-0. The other proposals were of "the sky is falling" variety and were adopted (including specifically saying the 310.15(A)(2) exception did not apply). Have we really had a rash of fires from this that I haven't heard about? The result is we end up with more holes drilled and more of that toxic smoke producing foam in the wall.
I think I've got it now. If the high ambient area is more than 10 feet or 10% of the total run we are to use the lower ampacity capacity of the conductor. The part that is still unclear is the part where we tag a piece of wire on to the lower ampacity wire so we can terminate on the 60 degree terminal. Example:
THHN # 1cu. is good for 150a. When I have to derate for ambient of 110 degrees I end up with a conductor derated to 130a. Since my terminations are only good for 75 degrees my conductor is derated to 107a.(I think).
Since my THHN is capable of 130a. and I need to terminate on a 125a. breaker, what gauge and length 75 degree wire do I pigtail onto the 90 degree wire so my terminations will be capable of 125a.
Thanks Ernie for your reference to the handbook- very helpful.
George No need to pigtail we terminate 90 degree wire Every day on 75 degree terminals (#12 , #10, ect 90 Degree rated wire xxhw, thhw ect on 75 degree rated Breaker terminals. We just can't use the 90 drgree Amperage rateing (Example #10=40amps at 90. # 10 = 35 amps at 75 degree But can never be put on over a 30 amp breaker . (see 240.4(D) The terminal rateing is what determians per 114.c The higher Amp rateing on wire meant for derateing purposes & when required in equipment for higher temps such as some fixtures. you follow what I;m saying ?? I give confusing answers some times. 310.16 110.14(C) 240.4(d) covers most of this.
Okay let's try this again. Feeder for 100a. panel passes through a boiler room with an ambient temperature of 125° F. The feeder over current breaker with 75° terminals is located outside the boiler room in a 80° F ambient environment. I'm running #3 THW to a j-box where I convert to THHN just before entering the boiler room. After exiting the boiler room I convert back to #3THW conductors for connection to the 75° F MLO panel located in an 80° F environment. I'm saying that we can use a #2 THHN running through the boiler room and a #3 THW outside the boiler room. All conductors are copper.
My question really was: Is there a minimum length of THW wire that needs to use for a heat sink? I think it is 5 feet but not sure.