Sizing A/C Disconnecting Circuit Breaker at Condensor

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srainsbury

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Hello,
I would like to confirm if sizing the A/C Disconnecting Breaker is solely based on the manufactures name rating plate…. The scenario that has come up is:
An A/C Condensors name rating plate specifies a maximum circuit breaker of 30 Amps which typically would be at the A/C disconnect. At the A/C disconnect they have a 60 Amp rated Circuit Breaker. At the main service panel they have a 30 Amp rated Circuit disconnect.
1) Does the Circuit Breaker @ the A/C Disconnect need to be rated at 30 Amps per rating plate or can they use a 60 Amp rated Breaker ?
2) Can you confirm that you would use/apply the NEC conductor ampacity tables for sizing the conductors from the main service to the A/C disconnect since the rating plate information does not consider distances, (voltage drop, etc.,). Thanks for you assistance.
Steve R
 
A typical 240 volt house AC unit had an operating range of about 50 volts so if you're applying 240 volts at the branch circuit breaker so voltage drop isn't much of an issue. The disconnect can be larger than the branch circuit device ahead of it.
 
Steve, welcome to the forum.

HVAC equipment operates under different rules. See 240.4 and the sections mentioned therein.

The Minimum Circuit Ampacity determines the minimum conductor size.

The Maximum OverCurrent determines the maximum breaker (or fuse) size.

A 30a breaker will not likely handle the compressor unit's starting current.
 
As long as you have a overcurrent device meeting the nameplate requirement somewhere in the circuit, all is good.
In your case if the nameplate calls for a maxc 30 amp breaker and there is one at the panel, it is properly protected. The breaker at the unit can be just a disconnect.
 
I apologize. I misread the OP.

Yes, the breaker at the compressor can be larger, or just a switch or pull-out.
 
srainsbury said:
The scenario that has come up is:
An A/C Condensors name rating plate specifies a maximum circuit breaker of 30 Amps which typically would be at the A/C disconnect. At the A/C disconnect they have a 60 Amp rated Circuit Breaker. At the main service panel they have a 30 Amp rated Circuit disconnect.
Click to expand...

I hope you are confusing "circuit breaker" with "disconnect". Here, let me rewrite that for you and you tell me if it's right.

An A/C condenser name rating plate specifies a maximum circuit breaker of 30 Amps which typically would be at the A/C disconnect. At the A/C disconnect they have a 60 Amp rated Circuit Breaker. At the main service panel they have a 30 Amp rated breaker.

If the circuit is indeed fed from a 30A breaker in the panel, the fuses or breaker in the disconnect are irrelevant (as long as they are 30A or more, of course.)

Typically the disconnect at the A/C unit would not be fused. It would be a non-fuseable disconnect. The circuit would be fed from the main panel with, in this case, no larger than a 30A breaker. Wiring could be #10 suitable for 30 amps BUT it only has to be able to handle the MCA (Minimum Circuit Ampacity) as stated on the name plate. Sometimes that value allows you to go to a smaller size wire. So if you could use #12, don't be alarmed if you see it on a 30 amp breaker.


Also, distances unless unusual do not cause concern. It's up to the installer to calculate voltage drop if there is. We like to limit it to 5%. Plenty of on-line calculators out there which will do that for you.

-Hal
 
Just to open another can of worms, these are sometimes used and they look suspiciously like circuit breakers, but they're not.

www.homedepot.com

Square D QO 60 Amp Nema 3R Disconnect Molded Case Switch QO260NATR - The Home Depot

General duty safety switches are designed for residential and commercial applications where durability and economy are prime considerations. Typical loads include lighting, air conditioning and appliances.
www.homedepot.com

They have no OC protection, and operate only as a two-pole switch.



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