IMHO.. This has to do with heat dissipation within the conductor. A conduit in a wet location can be filled with water. Water impedes the heat dissipation, therefore, 60 degree as to no damage the conductor. Even if seal tight is the wiring method.
Water has a thermal conductivity about 25 times that of air, so how would water impede the heat dissipation?
Heat Transfer.
I don't understand this part of the code. How is #14 MC permitted for 20 amps?
It has to do with 240.4(G), hermetic compressors have overloads in them so we don't have to match conductor size to breakers size. The breaker is for ground faults and short circuits. The overloads in the compressor take care of the overload. All the info will be on the nameplate of the HVAC unit.
240.4(D) says that its limitations do not apply when "specifically permitted in 240.4(E) or (G)." 240.4(G) says that OCPD for "Air-conditioning and refrigeration equipment circuit conductors" shall be permitted as per Article 440. So for an Article 440 application, you don't have the 240.4(D) limitation of 15A for #14 conductors. If your #14 conductors have 75C (or 90C) insulation, and 75C terminations, then their ampacity is 20A, and the OCPD may be even larger as usual for Articles 430 and 440.
What Wayne said. #14 at 75° C is rated for 20 amps. It's due to the limits in 240.4(D) that we typically only use #14 as a 15 amp conductor. For the AC unit those limits in 240.4(D) do not apply.
Geez. It is so clear. How could I have missed this?
That is a great question. I might need to be enlightened.
I think Jim's post #44 explains it.
While it has a higher thermal conductivity it also has a higher specific heat and will hold the heat longer than air. Air slower at thermal transfer it also doesn't hold the heat either. If you have moving water that moves from a heated source to a dissipating area and back (like your car radiator) It then will act to cool the heat source. But generally water and electricity don't mix and moving water is erosive.
My idea is that the thermal conductivity of the water transfers the heat from the conductor to the raceway and the surface of the raceway having a lot more surface area will dissipate more heat than just the conductor in a dry raceway.
If we consider the case of a constant current (so constant heating rate) that runs long enough for temperatures to stabilize, we can use an equilibrium analysis. Heat transfer requires a temperature difference. So for each thermal layer between the wires and the ultimate heat reservoir (bulk atmosphere for above ground use), the temperature difference across that layer will be determined by its characteristics and how much temperature difference is required to convey the full heating rate from the wires.
