Motor HP and FLA

[fifty60]

The numbers I gave earlier are for 3 seperate compressors. They are not 3 different input amps for the same compressor. Sorry if that caused confusion. I have some equipment that will use a cascade refrigeration system, so it will have 2 compressors. The system 2 compressor wil draw way more amps, at times, than the system 1 compressor. What is the proper way to represent the amps and HP on this equipment?

Seems like i would be better off finding the max operating current for system one, the corresponding KW, and then converting that to HP and using the tables in 430. Then, do the same thing for the other system.

The compressors do not come with an RLA on the nameplate, there is an RLA published by the manufacturer along with the Max operating current. There are actually two different RLA's, one for sizing conducters and overcurrent protection, and one for sizing contactors. The Max operating current is where the internal overload is set.

What is the correct way to size the mains and mains overcurrent protection? Using NEC 440, i am directed to the nameplate RLA. There is not a nameplate RLA, but there is one published. The equipments Running Load Amps can exceed this Rated Load, and also fall below the running load.

I have found that using the manufacturers publish RLA, and then using the FLA for other heaters and motors in the equipment is , spot on the FLA of the equipment. In fact, this method has never produced an FLA below the measured worst case FLA. I am just not sure that using the RLA of a compressor to get a HP, and then using that HP to size the feeder circuit breaker is correct. Would i need to take the max current the compressor draws and then convert that to HP and then use that as my largest motor HP?

 

[Smart $]

Does the compressor have a nameplate, and if so, what is on it?

If the rated-load current is not marked on a/the nameplate, the compressor does not comply with NEC provisions. It's pretty much moot to proceed beyond that. There is nothing anyone here can do to bring it into compliance.

If you are including these compressors in a manufactured assembly, you can use the NEC as a guideline for what you put on your nameplate. But ultimately, the NEC isn't for this purpose. Chances are, these compressors are manufactured to some standard. If you can determine what standard that is, then you have a true basis for determination. The NEC required nameplate rated-load current is actually determined under other standard(s). Which, IDK.

Your use of RLA in preceding posts is confusing when you are talking about both rated-load amps and running-load amps. IMO, you should only use the RLA acronym to designate nameplate "rated-load current". Published load data is not the same, even if the manufacturer dubs it such.. and I don't believe the NEC uses the term running-load amps.

Where Article 440 applies, an HP rating is seldom a true motor HP rating. It's a derived value based on all the connected loads... essentially an HP equivalent. It's used for determining HP ratings of upstream equipment, as electrical distribution is classed as a motor load. There is no need to establish an HP rating then convert it back to amperes.

 

[GoldDigger]

Pretty much Full Load Amps, FLA, which implies running; or Locked Rotor Amps, LRA.
Other common values for appliances with motors are Maximum OCDP or Max Fuse and Minimum Circuit Ampacity.
With that pair the electrician knows exactly what is optimal, namely to size OCPD to the Max and size the wire to at or above MCA.

 

[fifty60]

The compressor manufacturer lists 3 things: RLA for sizing conductors/OCPD, RLA for sizing contactors, and max operating current. Non of this is listed on the compressor. What is listed on the compressor is the locked rotor amps.

Since they are pointing out that it is used for sizing conductors and OCPD, then this must be what 440 is referring to as "Branch circuit selection current", right?

I am just trying to think through that the fact the RLA is one thing, but the running amps can be higher (resulting not only in higher amps, but also higher HP output), should that be a factor in sizing the disconnect?

What if the load is broken by the disconnect when the compressor is drawing its maximum running load amps? This leads me to another question which may answer my question a little bit. The service (line of sight) disconnect is not meant to break the load when it is running at full load amps right? It is meant to remove the ungrounded conductors from the supply right, but not as an emergency stop?

 

[GoldDigger]

A disconnect can either be load breaking (CBs, bolted pressure switches, knife switches with arc chutes, and others) or non load breaking (many if not all removable fuse blocks, some cord and plug configurstions).
Rather than generalize about what type a disconnect will be based on its location I would find out what it is by its listing.
Some disconnects, such as service disconnects, are required to be safely load breaking. Others like A/C disconnects are not.

 

[Smart $]

The compressor manufacturer lists 3 things: RLA for sizing conductors/OCPD, RLA for sizing contactors, and max operating current. Non of this is listed on the compressor. What is listed on the compressor is the locked rotor amps.

As I said, the compressor itself is non-compliant. The NEC only refers to rated-load current marked on nameplate... and that value is constant for determining the rating of all associated equipment.

Since they are pointing out that it is used for sizing conductors and OCPD, then this must be what 440 is referring to as "Branch circuit selection current", right?

Perhaps... but with it not being on the nameplate, and marked as branch circuit selection current, I cannot say one way or the other.

I am just trying to think through that the fact the RLA is one thing, but the running amps can be higher (resulting not only in higher amps, but also higher HP output), should that be a factor in sizing the disconnect?

IMO, no.

What if the load is broken by the disconnect when the compressor is drawing its maximum running load amps? This leads me to another question which may answer my question a little bit. The service (line of sight) disconnect is not meant to break the load when it is running at full load amps right? It is meant to remove the ungrounded conductors from the supply right, but not as an emergency stop?

Typical disconnect switches are not rated for break under load... but there are disconnect switches that are rated for break under load. You wouldn't be asking if you knew the disconnect was rated for break under load.