Confused by hvac mca/mocp

[StarCat]

Yes and RLA is NOT FLA. RLA is a useless number to anybody but the manufacturer.

Quite an interesting thread.
RLA is useful to HVACR Technicians in the field. Systems at full load in the heat, that is MAX condenser entering air temperature can approach and or run right about at RLA depending on load and a few other factors. It is one of the most important parameters used in troubleshooting, especially fractional single phase gear. Most modern hermetic compressors are suction gas cooled which makes them critically charged systems in most cases. The Engineering of the motors is right on the wire anymore and they are FAR less fault tolerant than in the past. A slight overcharge on a fixed restrictor system can push a small compressor over the RLA boundary faster than thought and then it will be OFF on thermal with all other things correct. An open run capacitor on a single phase compressor can be missed if you do not read current and look for it. There is more still.....
RLA is important.
I am always informed by reading this forum.

 

[Mgraw]

Quite an interesting thread.
RLA is useful to HVACR Technicians in the field. Systems at full load in the heat, that is MAX condenser entering air temperature can approach and or run right about at RLA depending on load and a few other factors. It is one of the most important parameters used in troubleshooting, especially fractional single phase gear. Most modern hermetic compressors are suction gas cooled which makes them critically charged systems in most cases. The Engineering of the motors is right on the wire anymore and they are FAR less fault tolerant than in the past. A slight overcharge on a fixed restrictor system can push a small compressor over the RLA boundary faster than thought and then it will be OFF on thermal with all other things correct. An open run capacitor on a single phase compressor can be missed if you do not read current and look for it. There is more still.....
RLA is important.
I am always informed by reading this forum.

A compressor running at or near RLA is not an indication that the compressor is running properly or charged properly. Fractional compressors used in domestic refrigerators, for example, are charged by weight not amp draw.

 

[GoldDigger]

Yes pretty much all of them. Just look at all the dumb things they put in the NEC and how many jurisdictions just blindly adopt it.

In actuality, that wouldn't happen because the manufacturers would comply.

Even if it adds what an engineer or user might consider unnecessary cost or complexity.

 

[ericwg]

There still seems to be contention between whether the conductor size (defined by MCA) and MOCP can be defined by manufacturers or must be defined per NEC. I believe it was David that first listed the reference to 440.33. Honestly, between 440.32 and 440.33, these are what I have been referencing for a long time to size motor-compressor conductors. However, looking at my 2020 NEC Handbook, right below Exception No. 2 in 440.33, in blue, it states:

"Branch circuits for listed air-conditioning and refrigerating equipment that have a nameplate marked with the branch-circuit conductor size and branch-circuit short-circuit protective device size are not required to have the branch-circuit conductors sized in accordance with 440.33. The product standard includes the 25-percent increase for the largest motor or compressor in the group plus the other nonmotor or noncompressor load; therefore, the actual nameplate full-load amperes for the complete assembly can be used to size the branch-circuit conductors."

Is it me, or is that the NEC specifically stating that the manufacturer supplied data can be used to size the conductors and you do not have to follow the method prescribed in 440.33?

 

[Eddie702]

There still seems to be contention between whether the conductor size (defined by MCA) and MOCP can be defined by manufacturers or must be defined per NEC. I believe it was David that first listed the reference to 440.33. Honestly, between 440.32 and 440.33, these are what I have been referencing for a long time to size motor-compressor conductors. However, looking at my 2020 NEC Handbook, right below Exception No. 2 in 440.33, in blue, it states:

"Branch circuits for listed air-conditioning and refrigerating equipment that have a nameplate marked with the branch-circuit conductor size and branch-circuit short-circuit protective device size are not required to have the branch-circuit conductors sized in accordance with 440.33. The product standard includes the 25-percent increase for the largest motor or compressor in the group plus the other nonmotor or noncompressor load; therefore, the actual nameplate full-load amperes for the complete assembly can be used to size the branch-circuit conductors."

Is it me, or is that the NEC specifically stating that the manufacturer supplied data can be used to size the conductors and you do not have to follow the method prescribed in 440.33?

Yes. Just use the name plate information

 

[kwired]

I have noticed that the newer resi units have lowered the max overcurrent protective device in recent years

Probably is not across the line started when the MOCP is low.

Across the line starting you could normally expect MOCP to be at least be about twice the compressor rated current, sometimes a little higher.

Just realized this is old thead, and have no idea what was mentioned between post I replied to and where this post is.