310-10 Temperature Limitation of Conductors. No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved. In no case shall conductors be associated together in such a way with respect to type of circuit, the wiring method employed, or the number of conductors that the limiting temperature of any conductor is exceeded. FPN: The temperature rating of a conductor (see Table 310.13 and Table 310.61) is the maximum temperature, at any location along its length, that the conductor can withstand over a prolonged time period without serious degradation. The allowable ampacity tables, the ampacity tables of Article 310 and the ampacity tables of Annex B, the correction factors at the bottom of these tables, and the notes to the tables provide guidance for coordinating conductor sizes, types, allowable ampacities, ampacities, ambient temperatures, and number of associated conductors. The principal determinants of operating temperature are as follows: (1) Ambient temperature — ambient temperature may vary along the conductor length as well as from time to time. (2) Heat generated internally in the conductor as the result of load current flow, including fundamental and harmonic currents. (3) The rate at which generated heat dissipates into the ambient medium. Thermal insulation that covers or surrounds conductors affects the rate of heat dissipation. (4) Adjacent load-carrying conductors — adjacent conductors have the dual effect of raising the ambient temperature and impeding heat dissipation.
Very good answer, Joe. But? I would contact my local HVAC engineer for an ambient temperature to use. That's where we get those things from here in the office. Even in a residential attic, the temp's are probably up in the 150-160 F range. How many people derate for the lighting conductors installed there? For 90 Deg C wire, the temp derate is at 0.41 for 160 Deg F. That means your #12 wire is good for about 15 Amps., with a 20 Amp breaker ahead of it. Legal? As Joe pointed out, any part derates the entire length of conductor. Just a few thoughts, and the kind of things we in the design end of things have to consider also.