3 Phase motor electrical installations 1

[stevemechanical]

What is the correct wire callout for a 208V/3PH/60Hz packaged air conditioning unit (cooling only) with the following electrical characteristics:
minimum circuit amps = 28.7 amperes
overcurrent protection = 45 amperes
full load amps = 28
locked rotor amps = 156

Thank you

 

[EEJaime]

The minimum circuit amps is defined as the sum of 125% of the largest motor plus all remaining motors in the package unit, probably a condenser fan and possibly an economizer fan. In any case the compressor is generally the largest motor of course. By overcurrant protection I assume you mean the "Maximum overcurrent protecion-'MOCP'" which is defined as 225% of the largest fan plus the remaining loads in the unit. So the feeder and circuit protection must be in between those two values. It is odd to have the MCA and the full load amps so close together in value because the MCA is supposed to include an additional 25% of the compressor motor as noted above. In any case the circuit for these values could be from 30 to 45 Amps, I personally would size it to a 40 Amp breaker using 3# 8 awg conductors plus a # 12 ground in a 3/4" conduit.

 

[stevemechanical]

EEJaime,
Thank you very much. But I have one question. Why wouldn't you run a neutral also? Aren't there any components in the unit that use 120 volt?

 

[resqcapt19]

EEJamie,
The NEC would require a #10 EGC for a circuit with a 40A OCPD.