Question about hermetic motors (condensers) and sizing disconnects, breakers and wire

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LouisianaApprentice

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Forest Hill, LA, USA
On a job i worked on i was reading the FLA off of units and calculating wire size. I told my jman all of them were under 30 when taken at 125% of FLA. Well he took that as only needing a 30 amp disconnect. The problem is the breakers used are oversized for the inrush so they are the MOCP rated on the unit. After digging through forums i saw where some people size a disconnect for calculated loads others say u have to use the size of which is greater (this being the breaker size which is 60). Will these 30 amp disconnects be acceptable for the condensers or do i need to size to the breaker.
 
"440.12(A)(1)The ampere rating shall be at least 115 percent of the nameplate rated-load current or branch-circuit selection current, whichever is greater."

The branch circuit selection current is something that is provided for in the code but I have never seen it on a nameplate. This is not the same as the minimum circuit ampacity. It is simply the rated load current with not multipliers.
 
don_resqcapt19 said:
"440.12(A)(1)The ampere rating shall be at least 115 percent of the nameplate rated-load current or branch-circuit selection current, whichever is greater."

The branch circuit selection current is something that is provided for in the code but I have never seen it on a nameplate. This is not the same as the minimum circuit ampacity. It is simply the rated load current with not multipliers.
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Yes i believe u are allowed to calculate the MOCP at 225% as well. And this is what was on the nameplate.
 
LouisianaApprentice said:
On a job i worked on i was reading the FLA off of units and calculating wire size. I told my jman all of them were under 30 when taken at 125% of FLA. Well he took that as only needing a 30 amp disconnect. The problem is the breakers used are oversized for the inrush so they are the MOCP rated on the unit. After digging through forums i saw where some people size a disconnect for calculated loads others say u have to use the size of which is greater (this being the breaker size which is 60). Will these 30 amp disconnects be acceptable for the condensers or do i need to size to the breaker.
Click to expand...
I'm in the midst of redoing an installation someone made almost identical. They have 4 Units listed as MOCP of 45A and they installed 50A breakers and a 30 amp pullout disconnects. The disconnects are melting down. And breakers not tripping. Units are listed as FLA 36.3Amp
 
LouisianaApprentice said:
Yes i believe u are allowed to calculate the MOCP at 225% as well. And this is what was on the nameplate.
Click to expand...
No calculations when you have a nameplate on the hermetic compressor unit. Wire with an ampacity equal to or greater than the minimum circuit ampacity and the OCPD equal to or smaller than the maximum overcurrent protection.
 
Fred B said:
I'm in the midst of redoing an installation someone made almost identical. They have 4 Units listed as MOCP of 45A and they installed 50A breakers and a 30 amp pullout disconnects. The disconnects are melting down. And breakers not tripping. Units are listed as FLA 36.3Amp
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A bit surprised that they are melting down with that amount of overload, but not surprised that the breaker is not tripping...no reason for the breaker to trip until the melting process results in a short circuit or ground fault.
 
don_resqcapt19 said:
No calculations when you have a nameplate on the hermetic compressor unit. Wire with an ampacity equal to or greater than the minimum circuit ampacity and the OCPD equal to or smaller than the maximum overcurrent protection.
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Can you back your answer to the code book where you may be allow in this case to have smaller size wire than its OCP?
 
don_resqcapt19 said:
The branch circuit selection current is something that is provided for in the code but I have never seen it on a nameplate. This is not the same as the minimum circuit ampacity. It is simply the rated load current with not multipliers.
Click to expand...
440.2 has a definition that explains branch circuit selection current, it is slightly different than your explanation:

"Branch-Circuit Selection Current. The value in amperes to be used instead of the rated-load current in determining the ratings of motor branch-circuit conductors, disconnecting means, controllers, and branch-circuit short-circuit and ground-fault protective devices wherever the running overload protective device permits a sustained current greater than the specified percentage of the rated-load current. The value of branch-circuit selection current will always be equal to or greater than the marked rated-load current."

Does this have something to do with "Service Factor" of motors and designs of HVAC equipment that regularly run their motors into the Service Factor?

Cheers, Wayne
 
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