I B kon fusededed about branch circuit selection current

[jeff126]

A single-phase hermetic refrigerant motor compressor has a rated load current of 24 amperes and a branch circuit selection current of 30 amperes. The branch circuit conductors are copper, Type TW. They operate at 80 degrees farenheit, and they are the only conductors in the conduit to this compressor. The smallest possible branch circuit conductors must be at least size ______ AWG.
A. 12
B. 10
C. 8
D. 6

The correct answer, according to the Master's Electrician Review, is C. - is there anyone out there who can explain why it needs to be 125% of the branch circuit if it's already rated at 125% of the motor FLA?
24 x 1.25 = 30, right? Why does it need to go another 25%?
Reference Article 440-32 2005 NEC

 

[al hildenbrand]

The NEC Handbook adds this explanatory text after 440.3:

Article 440 provides special considerations necessary for circuits supplying hermetic refrigerant motor-compressors and is in addition to or amendatory of the provisions of Article 430 and other applicable articles. However, many requirements, such as disconnecting means, controllers, single or group installations, and sizing of conductors, are the same as or similar to those applied in Article 430.
Article 440 does not apply unless a hermetic refrigerant motor-compressor is supplied. Article 440 must be applied in conjunction with Article 430.
Note the terms rated-load current and branch-circuit selection current, defined in 440.2. When a branch-circuit selection current is marked on a nameplate, it must be used instead of the rated-load current to determine the size of the disconnecting means, the controller, the motor branch-circuit conductors, and the overcurrent protective devices for the branch-circuit conductors and the motor. The value of branch-circuit selection current is always greater than the marked rated-load current.

In your example, it is only coincidence that 30 is 125% of 24. The coincidence doesn't mean anything.

 

[jeff126]

Al,
Thank you for your help and clarification. Much appreciated!
Jeff

 

[al hildenbrand]

Welcome to the Forum, Jeff.

 

[milwaukeesteve]

OUTSTANDING ANSWER, AL!!
That was very well put and answered his question perfectly.

 

[finhead]

I agree with everything Al says but still don't know why the correct size is not 10 AWG.

Please help.

 

[kc8dxx]

30 amperes * 1.25 = 37.5 amperes. T310.16 TW column requires 8AWG with an ampacity of 40 amperes to satisfy this requirement. I second milwaukeesteve, Al has presented the answer very clearly. Well Done!

 

[infinity]

I agree with Al's answer but I find the question misleading. It does not say that the information provided is on the nameplate.

 

[finhead]

Rats!
All these years I've been using only the branch circuit selection current to size conductors.

 

[Bjenks]

Don't get branch circuit selection current confused with Min Circuit Ampacity. I hardly ever see Branch Circuit Selection Current on the nameplate of any residential HVAC. However, everyone I see has Min Circuit Ampacity and RLA. With just about all the manufactures if you take 1.25 of the RLA you will see that it matches up with a rounded Min Circuit Ampacity. And if you take 175% of RLA and go one step down you will usually get the Max fuse or breaker size. However, if they did include a higher overload protection in the unit then they will reference the branch circuit selection current which must be used over the Min Circuit Ampacity. But even then if the Min Circuit Ampacity is there then that over ridets everything.

I bet you did all of those right by using the Min Circuit Ampacity.

 

[daiashi]

A single-phase hermetic refrigerant motor compressor has a rated load current of 24 amperes and a branch circuit selection current of 30 amperes. The branch circuit conductors are copper, Type TW. They operate at 80 degrees farenheit, and they are the only conductors in the conduit to this compressor. The smallest possible branch circuit conductors must be at least size ______ AWG.
A. 12
B. 10
C. 8
D. 6

The correct answer, according to the Master's Electrician Review, is C. - is there anyone out there who can explain why it needs to be 125% of the branch circuit if it's already rated at 125% of the motor FLA?
24 x 1.25 = 30, right? Why does it need to go another 25%?
Reference Article 440-32 2005 NEC

Actually 440-32 says 125% of the larger.In this case,branch circuit selection
of 30 amps.30 X 1.25=37.5