Correct breaker size for an AC unit

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mathcoin

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California
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Electrical Engineering
Dear all,
I am installing an AC unit with a minimum circuit ampcity of 17.9 A, and a max fuse size of 25 A. I am installing a dedicated branch using 1/2" EMT and 12 AWG THHN-THWN-2. The ac unit is connected to a fused disconnect box with 25 A fuses. My question is if a 30 amp breaker (cannot find a 25 A GFCI) is okay for this branch or would I need to switch to 10 AWG THHN? I know 240.4(D) says 12 AWG is limited to 20 A, but NEC 310.16 says 12 AWG THHN has a 30 A ampcity. I am also aware since NEC 240.4(G) points to NEC 440 for AC units which may overrule 240.4(D). Thank you for the help.
 

don_resqcapt19

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Location
Illinois
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retired electrician
The 30 amp number is in the 90°C column and you can't really use that.
The part of 240.4(D) that is over ridden by 240.4(G) is the OCPD limit of 20 amps for 12 AWG.
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
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EC
The minimum circuit ampacity of 17.9 A, and a max fuse size of 25 A is set by the fused disconnect. Whatever you have before the disconnect is a feeder and normal rules apply according to the OCPD in the panel. So #10 to the disconnect and you might as well use it for the few feet from the disconnect to the A/C unit too.

-Hal
 

mathcoin

Member
Location
California
Occupation
Electrical Engineering
Even thoughtthe MCA is 17.9 A, a 12 AWG will not cut it since the feeder circuit breaker should be larger than the fused size of the disconnect? In this case >25 A, so a 10 AWG and a 30 A breaker is needed.
 
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mathcoin

Member
Location
California
Occupation
Electrical Engineering
The minimum circuit ampacity of 17.9 A, and a max fuse size of 25 A is set by the fused disconnect. Whatever you have before the disconnect is a feeder and normal rules apply according to the OCPD in the panel. So #10 to the disconnect and you might as well use it for the few feet from the disconnect to the A/C unit too.

-Hal
I pretty sure a 20 AMP breaker would be too small for the AC unit, so I will just size the wire to AWG with a 30 amp breaker.
 

infinity

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New Jersey
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I pretty sure a 20 AMP breaker would be too small for the AC unit, so I will just size the wire to AWG with a 30 amp breaker.
IMO a 20 amp OCPD would not be too small. The MCA already has 125% added to it so the running current is likely less than 15 amps. You can use #12 AWG conductors with a 25 amp OCPD.
 

Dennis Alwon

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Chapel Hill, NC
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You can install a 30 amp gfci overcurrent protective device and install a fused disconnect near the unit with 25 amp fuses
 

mathcoin

Member
Location
California
Occupation
Electrical Engineering
I think 20 A is fine as well since the inverter compressor uses less current than its MCA rating. I just find it weird to use a 20 A breaker and a 25 A fuse.
 

mathcoin

Member
Location
California
Occupation
Electrical Engineering
Okay, so If I use a 20 amp breaker and fuse, that should be fine with NEC? It does say max fuse/brekaer size, so I could go a bit lower to 20 A.
 
The minimum circuit ampacity of 17.9 A, and a max fuse size of 25 A is set by the fused disconnect. Whatever you have before the disconnect is a feeder and normal rules apply according to the OCPD in the panel. So #10 to the disconnect and you might as well use it for the few feet from the disconnect to the A/C unit too.

-Hal
Hal, I have not checked the code wording, but I am skeptical that that is correct. While I agree that the circuit before the fuses is a feeder, don't the "motor rules" also apply to feeders?
 

infinity

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New Jersey
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Hal, I have not checked the code wording, but I am skeptical that that is correct. While I agree that the circuit before the fuses is a feeder, don't the "motor rules" also apply to feeders?
Isn't there something in the NEC that states that if there is protection in the fused disconnect at the unit that does not automatically make the circuit a feeder?
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
This is one of those questions that comes up from time to time regarding a feeder to a single disconnect/overcurrent device. Hal is correct.
 

mathcoin

Member
Location
California
Occupation
Electrical Engineering
Thanks for the good information. Based on that, should I install a 20 amp 12 awg branch or a 30 amp 10 awg branch? I have both 12 awg and 10 awg wire, and still need to order the breaker, so I am flexible. I cannot find a 25 A GFCI breaker that fits my panel which is why I I do not mention one, and have a the fuse for the unit.
 
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texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Thanks for the good information. Based on that, should I install a 20 amp 12 awg branch or a 30 amp 10 awg branch?
While the code will allow the 20 amp you need to consider the ramifications if on the off chance it occasionally trips on start. The first thing the HVAC guy is going to do is point the finger at you saying the OCPD is too small and you really have little defense. Call backs like this can be very expensive to fix in some cases. And while you may have just one unit, what if this was an entire apartment building with many units? Pretty dicey to me.
 
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