MCA vs Running Current

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darko

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For the ACU, we are using MCA and MOCP as advised by the manufacturer, in sizing the individual wires and circuit breakers respectively. This is actually acceptable. My concern is how to size the Main breaker and the wire for the entire system. Should i still be using MCA? Because MCA is almost twice the value of running current, so the Main Breaker is actually very very large when MCA is used not to mention the cost.
 
Size wires per MCA. This is the sum of RLA (compressor running), fan unit, controls, and 125% of largest motor or compressor. Breaker can be sized between MCA and MOCP, and it best to get closer to MOCP than MCA.
 
When sizing feeders and breakers upstream of an AC unit branch circuit, I always try to use the RLA and FLA of the unit instead of the MCA. Compressors are listed with Rated Load Amps, fans with Full Load Amps. Often with 3? AC units, the compressor and indoor fan will be 3? while the condensor fan or fans are 1?. In this case, I treat the condenser fans as 3? anyway because I don't know what phase they're on.

The mechanical guys in my office always think the only info I need is MCA and MOCP. That's ok for the branch circuit, but it can add waste upstream, especially when there are multiple units. I usually wind up going to the manufacturer's web page with the AC unit model number so I can look it up myself.
 
Art 440 does not address feeders well, however, 440.3 does tell us Art 440 augment Art 430. Most folks I know use Part II & Part V of Art 430 to assist in sizing ACU feeders.
 
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