Condensing unit disconnect

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rgrogg

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I need help sizing a disconnect for a condensing unit. The nameplate rating for the unit is as follows: MOCP = 40A, MCA = 23.5A, 208V 3phase. Based on this information I will have 40A circuit breaker in the panel using #10AWG wire (could have used #12) and a 3pole non-fused disconnect at the unit. In this case do I use a 30A disconnect based on a MCA = 23.5A or do I use a 60A disconnect base on a MOCP = 40A?

Thanks
 
Re: Condensing unit disconnect

Yes, you size the disconnect based on the MCA. Since the MCA is 23.5, the 30 amp disconnect is fine.
 
Re: Condensing unit disconnect

You need to look at 440.12(B) as this condensing unit is a combination load.

The disconnect is not based on the MCA it seems like it should be but that would be to easy. ;)

This is some of what you find in 440.12(B)

The horsepower rating of the disconnecting means shall be determined from the sum of all currents, including resistance loads, at the rated-load condition and also at the locked-rotor condition.
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The ampere rating of the disconnecting means shall be at least 115 percent of the sum of all currents at the rated-load condition determined in accordance with 440.12(B)(1)
Click to expand...
You may very well end up with 12 AWGs on a 40 amp breaker and a 60 amp disconnect. :p
 
Re: Condensing unit disconnect

Originally posted by rgrogg:
I need help sizing a disconnect for a condensing unit. The nameplate rating for the unit is as follows: MOCP = 40A, MCA = 23.5A, 208V 3phase. Based on this information I will have 40A circuit breaker in the panel using #10AWG wire (could have used #12) and a 3pole non-fused disconnect at the unit. In this case do I use a 30A disconnect based on a MCA = 23.5A or do I use a 60A disconnect base on a MOCP = 40A?

Thanks
Click to expand...
Disconnect switches are rated by HP, not current for motor use. I am guessing you have a 7.5 HP motor. I checked the ITE catalog for a typical NEMA style switch and it indicated 240V/3phase switch is good for 7.5HP. It did not list a 208V/3 phase rating. You might have to do some investigation to see what the rating is at 208V. The AB catalog shows 208V and 240V 3phase ratings as the same (7.5 HP), so likely a 30A switch is OK, since I think they are all made to the same standards.
 
Re: Condensing unit disconnect

Originally posted by petersonra:
Disconnect switches are rated by HP, not current for motor use.
Click to expand...
For Air-Conditioning and Refrigerating Equipment the disconnects are rated by HP and 115% of the amperes.

440.12(B)(1) and 440.12(B)(2)
 
Re: Condensing unit disconnect

Oooops, sorry for the misinformation. I should have know this was outside of my area of expertise. :eek: I'll stick to lurking.
 
Re: Condensing unit disconnect

Wire for min. and fuse for max. it`s allowed.
 
Re: Condensing unit disconnect

I think Mike Holt covered this in one of his tapes i own.Yes a #14-2 romex might end up protected by 30 amp breaker.I dont like the idea and would not do it but yes its legal.
 
Re: Condensing unit disconnect

Iwire is right. If the AC Condenser has a branch-circuit selection current of 40 amps (MOCP = 40), then you must work backwards using Table 430.150, Full Load Current - 3 Phase Alternating-Current Motors (since HVAC motor-compressors are not HP rated). The closest 208 volt motor to 40 amps is a 15 HP motor. Based on the information Rgrogg provided, we would need a 15 HP disconnect for the AC Condenser. This would be the best estimate. The minimum Square D 15 HP rated disconnect I could find is a 60 Amp Disconnect.
 
Re: Condensing unit disconnect

To me the size disc would have to be sized by the OCPD but the wire supplying is sized to min ampacity listed on data plate.Since 60 amp disconnects are so readily available thats the way I would go.
 
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