HVAC Rooftop Units

[Reesh14]

Hi. I have a question concerning HVAC rooftop units. I have two units, RTU-1 (MCA:174A, 208V) and RTU-2 (MCA:174, 208V) going to a 250A, 3 phase, 120/208V Panel. How do I calculate/figure out if these two units can both be fed by this panel? Normally, I would take the FLA (mca/1.25) and convert to kva for both units, then add up the kva's and convert to total connect amps. If this value is less than 250A, it would be okay to do so. I am not sure if this is right, or if I should take a diversity factor of .8 of the connected amps to get the demand load of the panel. Please tell me how you would approach this situation. Thank you.

 

[davidbeach]

First off, MCA/1.25 will be less than the FLA of the unit. The MCA is the sum of all the motors plus 25% of the largest. Since there will be several motors, 25% of the largest can be considerably smaller than 25% of the total. If both units are three phase, there is nothing gained by converting to kVA and then back to amps, you could add the amps directly. My guess is that the combined FLA will be closer to 300A than to 250A, no diversity is available to use.