hermetic refrigerant motor-compressor units

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h1h2h3

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When sizing the branch circuit for a hermetic refrigerant motor-compressor unit, is it more appropriate to max out the installation requirements or find some middle ground between the max breaker requirement and the minimum circuit ampacity? I understand that the min circuit requirement is determined by (RLC x 125% + all other concurent loads) and that the max circuit requirement is determined by (RLC x 225% + all other concurent loads). What I don't understand is if anymore calculations need to be performed to determine the branch circuit size (ie: min circuit requirement x 125% or is this redundant?) Also, will the disconnect switch be rated based on the branch circuit ocpd or the starting protection (based on locked rotor current) for the motor? http://forums.mikeholt.com/images/smilies/confused.gif
 
h1h2h3 said:
When sizing the branch circuit for a hermetic refrigerant motor-compressor unit, is it more appropriate to max out the installation requirements or find some middle ground between the max breaker requirement and the minimum circuit ampacity? I understand that the min circuit requirement is determined by (RLC x 125% + all other concurent loads) and that the max circuit requirement is determined by (RLC x 225% + all other concurent loads). What I don't understand is if anymore calculations need to be performed to determine the branch circuit size (ie: min circuit requirement x 125% or is this redundant?) Also, will the disconnect switch be rated based on the branch circuit ocpd or the starting protection (based on locked rotor current) for the motor? http://forums.mikeholt.com/images/smilies/confused.gif
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Folow the label requirements for MCA and Max OCPD 110.3(B)
Size the disconnecting means per 440.12
 
h1h2h3 said:
When sizing the branch circuit for a hermetic refrigerant motor-compressor unit, is it more appropriate to max out the installation requirements or find some middle ground between the max breaker requirement and the minimum circuit ampacity? I understand that the min circuit requirement is determined by (RLC x 125% + all other concurent loads) and that the max circuit requirement is determined by (RLC x 225% + all other concurent loads). What I don't understand is if anymore calculations need to be performed to determine the branch circuit size (ie: min circuit requirement x 125% or is this redundant?) Also, will the disconnect switch be rated based on the branch circuit ocpd or the starting protection (based on locked rotor current) for the motor? http://forums.mikeholt.com/images/smilies/confused.gif
Click to expand...


Welcome to the forum, as you pointed out, the MCA already has the 125% built into it. So it would be perfectly fine to size your branch circuit to that. Many times all of us would love to (over)build things the way we want,,,however,, if you know your competition is gonna build bare minimum, sometimes you have to do the same thing.
 
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That 225% is used to size the short-circuit ground-fault protection which is gonna be high because of the starting current. NEC 250.122(D) state's that this value would be used to size the EGC and not the circuit conductors. I would size it for the minimum. Your overload is sized according to NEC 440.52(A).
 
"Folow the label requirements for MCA and Max OCPD 110.3(B)"

This is actually a hypothetical question from my industrial wiring book in class and that route is not an option. However, 440.12 still applies for the disconnecting means so I thank you for that.
 
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