Kwired, the reason I mention it (mass 334.80) is that it is my understanding that the NEC would require you to derate nm from the 60c ,and that we in Mass get to derate NM from the 90c or 40 amps provided the temp is ok and there is no therm insulation .. and that stupid note (2) from Climatemaster only refers to the wire ,, how do I prove the NM wire has a 40 amp rating you may ask? I offer Mass 334.80 and ask If I did need to derate for amb.temp what ampacity do I start from ? Answer = 40 ....so since that stupid note only mentions the wire and not the circuit ampacity I still say that the "wire" does indeed have a capacity of 40 amps.. Yes ?
From your quote of local amendments:
334.80. Delete the second paragraph and revise the first paragraph to read as follows:
334.80 Ampacity. Type NM, NMC, and NMS cable shall have conductors rated at 90?C
(194?F). Where installed in thermal insulation, the ampacity of conductors shall be that of 60?C
(140?F) conductors
All I see that was accomplished was deleting the second paragraph:
Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be sealed with thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(3)(a) and the provisions of 310.15(A)(2), Exception, shall not apply
which eliminates the recent NEC change of having to derate when passing multiple NM cables through a framing member that is sealed with foam, caulk, or similar material. The rest of what was re-written looks to me like it just paraphrased what is already said in the NEC and no actual requirements in the NEC were changed. That said lets go through what the NEC actually says a little more.
The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The allowable ampacity shall not exceed that of a 60?C (140?F) rated conductor. The 90?C (194?F) rating shall be permitted to be used for ampacity adjustment and correction calculations, provided the final derated ampacity does not exceed that of a 60?C (140?F) rated conductor.
334.112 does say that conductor insulation must be 90?C rated, so the first sentence in your amendment is also already covered as is in the NEC already.
334.80 basically says NM cable must be sized according to 60?C rating, but can use 90?C rating for ampacity adjustments. So does your local amendment.
If thermal insulation is involved then you do have some difference - your amendment seems to imply that you use
60?C rating for selection as well as ampacity adjustment - but that does not apply to your case here.
So selecting an NM cable from NEC we need minimum of 25.3 ampacity at 60?C that gives us 10AWG minimum no matter what any other derations that may be allowable at higher temperature ratings may leave us at.
Your note (2) complicates the situation since the allowed 40 amp max OCD is higher than the ampacity of the conductor as that note says. So your only choices to comply with that is to either use a 30 amp OCD or use a 40 amp conductor if you want to use the 40 amp OCD.