loads of RTUs with ELec Heat

[steve66]

Isn't deciphering NEC fun sometimes?

Yes, its clear as mud. I see what you and David are saying about the difference between branch circuits and feeders.

But personally, I'm going to stick with the more conservative approach until I see something some thing in the code that's spelled out a little better.

And yes, I know the handbook isn't code. But if I ever had to defend my calculations in court, I don't want to have to say "I though I knew the code better than the NEC handbook writers."

 

[Timebuilder]

440.4(B) is leading me to believe that MCA is properly loaded.

That is correct.

The manufacturer has performed any calculations for MCA and MOCPD for unitary equipment in the UL acceptance filing. No other calculation is needed.

Take the MCA and apply it in the appropriate NEC table as your selection current, and use whatever correction and adjustment values you have, according to the conditions of use.

In my world of HVAC Electrical, I encourage you to become familiar with the heat characteristics of running raceways on a roof, exposed to the sun. This is commonplace in some jurisdictions, and this kind of installation can create the need for much larger conductors than would normally be expected. In the 2011, the factors are shown in table 310.15 B 3 c. The informational note mentions averaging of ambient temperatures, but I believe they mean ASHRAE listed design temperatures for your region.

This manufacturer has a pdf of various temps. You can see that Philly is just above 90° F. I would use 91°.

https://www.captiveaire.com/catalogcontent/fans/sup_mpu/doc/winter_summer_design_temps_us.pdf

Because this temp occurs in summer, with all compressors and fans running, you can expect to use your design temp and the above listed table for running raceways on a roof and choosing the appropriate sizing of the conductors for those conditions of use.

A mere roof penetration at an RTU with some LFMC going to a disconnect would not need this type of factoring.

Questions?