HVAC Circuit Sizing

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ESolar

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Eureka, CA Humboldt County
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Electrician/Contractor
1994 60 Amp breaker in main panel feeding SER AL going compressor. Currently wired with a 40A slow blow.
Compressor was upgraded to a more efficient unit. New efficient unit label:

Compressor: RLA - LRA = 21.2 - 96.
Minimum circuit ampacity 28A.
Max Fuse/Breaker HACR = 45A.

It seems that this compressor could be on a 30A breaker. If so, why not put run this circuit to a 60A subpanel, run the compressor on a 30 amp circuit from subpanel.
That would free some capacity for a mini split. If possible, what is the largest circuit for the mini split? (I assume 30 Amps)
 
The unit will draw the same current regardless of the OCPD so what does changing the CB to 30 amps accomplish?
 
You needs conductors to the unit with minimum ampacity of 28 amps per the MCA shown. The breaker can be anything up to 45A. I would be cautious of using a 30A breaker though as you may have occasional trips. BTY, the breaker size has nothing to do with the load the panel sees-it sees the compressor load no matter what breaker size you are using.
 
texie said:
You needs conductors to the unit with minimum ampacity of 28 amps per the MCA shown. The breaker can be anything up to 45A. I would be cautious of using a 30A breaker though as you may have occasional trips. BTY, the breaker size has nothing to do with the load the panel sees-it sees the compressor load no matter what breaker size you are using.
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Of course changing the breaker does not affect the demand. I didn't mean to imply otherwise. However, changing to a subpanel and a 30A (or 35A) breaker provides for another circuit on the same feeder. If nuisance tripping is an issue with HVAC it could go on 35A breaker.

BTW, the current breaker is 60A (with 40A slow blow at the unit). Isn't the breaker oversized?
 
infinity said:
Not if the SE cable is rated for at least 60 amps.
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Right. But I was thinking about the Max Fuse of 45. I guess that's handled by the slow blow. I guess it's the demand issue that I was having problems with. Often HVAC has oversized breakers, relative to the NP min. I was thinking that perhaps running this compressor and adding a mini split, might cause issues. For example: 28A compressor on a 35A breaker, and a ~20A mini split on a 20A breaker - both start at the same time.
 
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ESolar said:
Right. But I was thinking about the Max Fuse of 45. I guess that's handled by the slow blow. I guess it's the demand issue that I was having problems with. Often HVAC has oversized breakers, relative to the NP min. I was thinking that perhaps running this compressor and adding a mini split, might cause issues. For example: 28A compressor on a 35A breaker, and a ~20A mini split on a 20A breaker - both start at the same time.
Click to expand...
I don't think the 60A breaker would trip just because the mini split started at the same time. There would be a high amperage due to inrush current but would have to maintain that high current for a while before it would trip on overload. The inrush only lasts for a few seconds.
 
ESolar said:
Right. But I was thinking about the Max Fuse of 45. I guess that's handled by the slow blow. I guess it's the demand issue that I was having problems with. Often HVAC has oversized breakers, relative to the NP min. I was thinking that perhaps running this compressor and adding a mini split, might cause issues. For example: 28A compressor on a 35A breaker, and a ~20A mini split on a 20A breaker - both start at the same time.
Click to expand...
Changing the OCPD does not give you any more current on the circuit for a second load.
 
ESolar said:
Right. But I was thinking about the Max Fuse of 45. I guess that's handled by the slow blow. I guess it's the demand issue that I was having problems with. Often HVAC has oversized breakers, relative to the NP min. I was thinking that perhaps running this compressor and adding a mini split, might cause issues. For example: 28A compressor on a 35A breaker, and a ~20A mini split on a 20A breaker - both start at the same time.
Click to expand...
I agree with Bill the OCPD shouldn't trip. So you could use the 60 amp feeder to a sub-panel then install a 45 amp OCPD for the existing unit and whatever the MaxOCPD is for the mini split.
 
infinity said:
I agree with Bill the OCPD shouldn't trip. So you could use the 60 amp feeder to a sub-panel then install a 45 amp OCPD for the existing unit and whatever the MaxOCPD is for the mini split.
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The 45A breaker would be > the demand. So the demand calcs would still use the NP rating of 28A (compressor) and 20A (mini split).
 
ESolar said:
The 45A breaker would be > the demand. So the demand calcs would still use the NP rating of 28A (compressor) and 20A (mini split).
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What demand calc's, do you mean for a sub-panel? The size of the circuit breaker is irrelevant to the amount of load. The unit MaxOCPD is 45 so you can use that. If the mini split is MaxOCPD 25 amps you use a 25 amp OCPD.
 
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