HVAC wire size?

ptonsparky said:
Nope, all the time, for me.
60 degree wet vs dry. Makes no sense to me .
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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.
 
The limitations of #6 AWG NM cable in particular seems to come up at least once a week on here if not more often.
Until the 1950's #5 AWG wire was in the NEC ampacity tables, probably a easy code change to add that back to the table, then NM cable manufacturers could make a true 60Amp 60C cable, 5/2 - WG and 5/3 -WG.
I'd bet a milkshake its easier and cheaper for the factory to add two new sku's for 5AWG than bother with changing the UL standard to require THHN insulation on NM then getting every manufacturer to change their PVC plastic formula.
 
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xformer said:
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.
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Water has a thermal conductivity about 25 times that of air, so how would water impede the heat dissipation?
 
don_resqcapt19 said:
Water has a thermal conductivity about 25 times that of air, so how would water impede the heat dissipation?
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Heat Transfer.
Stagnant water does nor remove heat as well as circulating air. Very few raceways do not have some amount of chimney effect.
 
Greg1707 said:
I don't understand this part of the code. How is #14 MC permitted for 20 amps?
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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.
MCA would be the conductor size
Max breaker/fuse would be the max size of the OCPD
 
Greg1707 said:
I don't understand this part of the code. How is #14 MC permitted for 20 amps?
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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.

Cheers, Wayne
 
Greg1707 said:
I don't understand this part of the code. How is #14 MC permitted for 20 amps?
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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.
 
wwhitney said:
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.

Cheers, Wayne
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Geez. It is so clear. How could I have missed this?
 
don_resqcapt19 said:
Water has a thermal conductivity about 25 times that of air, so how would water impede the heat dissipation?
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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.
 
Fred B said:
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.
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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.
 
don_resqcapt19 said:
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.
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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.

That means the question about the layer between the conductors and the conduit (air or water) is just which material transfers more heat per unit temperature difference; the one with a higher heat transfer rate will cause a lower temperature difference between the conduit and the wires, and hence a lower wire temperature. [The conduit temperature in equilibrium can be determined ignoring what's inside of it, it just has be sufficiently higher in temperature than its environment to reject heat at a rate equal to the heating rate.]

While water is a better thermal conductor, I expect air can start convecting at a lower temperature difference. It could be that which material yields a lower delta T depends on the heating rate: e.g. at low heating rates, we have only conduction, and water is better than air. Then at higher heating rates, air convects and the water doesn't, so air is better than water. And then at even higher heating rates, both convect and water is not better. However, I'm just speculating and don't really know; is anyone familiar with this question?

Cheers, Wayne
 
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