RTU nameplates....

[curt swartz]

I was under the impression that these were multi motor units. If these units are only single motor. Just add the FLA or RLA of each unit and add 25 for the largest motor.

 

[topgone]

Figuring the size of the feeder isnt in question. It was how to determine the actually demand amps of all the units to determine if a 3 phase 208V 150A panel was adequate.

Let me try.
The question you want cleared up is whether or not your 150A panel is adequate. The post mentioning "feeder sizing" is actually related to your question. If your demand load is 100A for example, you will choose a conductor sized 125A (the conductor loaded 80% of its rating). The breaker will also have to be loaded 80% of its rating as a minimum(except for 100%-rated breakers). That means the breaker will be sized 125% of the demand amps which is also the same factor (1/0.8) you used when sizing your feeder conductors! Unless your connected loads are motors that drive high-inertia machines, 125% factor will be enough when sizing breakers. With high-inertia loads, you will have to refer to trip curves of the breakers or NEC Table 430.52 which will tell you by how much you are allowed to oversize you breaker to account for motor starting currents. BTW, you haven't told us about the specific types of motors loaded on your panel, so there's no quick answer to your question/s.

 

[ike5547]

I believe "rated load amps" is equivalent to "full load amps" for the compressor only. If there are other motors (fans, blowers) their ratings in "full load amps" should be on the nameplate as well.

 

[chevyx92]

Unless MCA is the same as BCSC -- which in my opinion isn't the case.

Why do you say its not the same? What else would it be?

 

[ike5547]

Why do you say its not the same? What else would it be?

I agree, they're the same. You're not the only one learning here. Air conditioning equipment is somewhat rare around here because of the climate.

 

[ike5547]

http://www.inspectapedia.com/aircond/aircond06.htm



Minimum Circuit Ampacity - MCA: MCA can be used to determine the required size of branch circuit conductors (wiring and also control switches) supplying the equipment. [For example, using NEC Table 310-16 in the 60 degree column as required in 110-14]. However some experts recommend using BCSC. [Using the "minimum permitted" sized wire rather than the optimum-size to a compressor may save a few dollars at installation but may increase system operating costs and it may be less safe than using a larger conductor.]

 

[ike5547]

http://www.lni.wa.gov/tradeslicensing/electrical/files/currents/elc0502.pdf

Multi-motor equipment or combinations of a motor(s) and other loads, supplied from a single branch-circuit, will be marked with the required supply-circuit minimum circuit ampacity (MCA) on or adjacent to the equipment nameplate. As the name indicates, minimum circuit ampacity is used to determine the required minimum rating of the branch circuit conductors.

Branch-circuit selection current and MCA are not interchangeable. If equipment is not marked with branch-circuit selection current then you must use the rated-load amperes (RLA) marked on the equipment nameplate (not MCA) to perform the calculations in NEC 440.12 when determining the appropriate rating and interrupting capacity for external disconnecting means.