MCA vs FLA/RLA for Load Calc?

[thump3r]

I have heard varying opinions regarding how mechanical equipment should be incorporated into the service load calculation (MCA vs FLA/RLA) and would like your take on the matter. I have read an old related thread but would like clarification in the form of an example.

Example of a rooftop unit with electric heat:

208V-3?
MCA: 122A
MOP: 125A
Electric heater: 36 kW
Blower motor FLA: 3.5A
Compressor #1 RLA: 27.6A
Compressor #2 RLA: 20.5A
Evaporator fan FLA: 10.6A
Condenser fan #1 FLA: 3.2A
Condenser fan #2 FLA: 3.2A

My understanding is that the MCA is solely for wire sizing, MOP is solely for breaker/fuse sizing, and the rest should be used to calculate the actual system load as follows:

Heat Mode: 100 + 3.5(1.25) = 104.5A
A/C Mode: 3.5 + 27.6(1.25) + 20.5 + 10.6 + 3.2 + 3.2 = 75.5A

Because this analysis is of a single unit, the two modes are not coincidental. The load is greater in heat mode so 104.5A should be used for the actual system load.

Please correct me if any of this is incorrect.

 

[JoeStillman]

I'm not sure what the difference is between the evaporator fan and the blower. I would expect the evap fan to run when you are in heat mode. The blower could be a return or exhaust fan that's needed for outside air. I recommend using them both in your heat-mode calculation.

 

[JoeStillman]

In answer to your question about the service calc, I always use FLA/RLA and not MCA. For a service calc, I consider the whole unit as a continuous load, so it all gets multipled by 1.25 along with the lights and receps.

 

[augie47]

I can't locate supporting documentation off hand, but I am of the opinion that the scenario you posted is the correct approach.
Many of us tend to use MCA for everything as it simplifies things