Let's say I have an air conditioner with a minimum circuit ampacity of 28.2 amps. It then says that the max breaker is 45 amps. Am I compliant using #10 nm-b from the panel to the unit and having a 45 amp breaker or am I still required to have a 30 amp breaker? I always thought you choose the wire size by the mca of the unit and then use the max breaker size.
Sizing wire and circuit breaker for air conditioner
- Thread starter crtemp
- Start date
- Status
- Occupation
- Licensed Electrician
You are correct.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I agree with Dave. Since your using NM cable you would use it's 60° C ampacity which is 30 amps. The OCPD could be 45 amps with the 30 amp conductor.
So 440.6 is where it says to choose the correct wire for the MCA and 440.12(a)(1) is where it says to put it on the max breaker?
But the minimum breaker I can do is a 35?
28.2 x 115% = 32.4
- Location
- New Jersey
- Occupation
- Journeyman Electrician
The minimum OCPD would be 30 amps. The Minimum Circuit Ampacity is 28.2. A 30 amp circuit breaker is still larger than the Minimum circuit required. No calculation needed, the 28.2 amp already has the 125% factor added in.
So why is there a max breaker size of 45 amps when a 30 amp breaker is sufficient?
JFletcher
Senior Member
- Location
- Williamsburg, VA
But is a max required or just a good practice? Is a 30 amp breaker compliment?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Yes, but it is possible that as the unit wears starting currents could creep up causing the 30 amp OCPD to trip. I would just use the max of 45 amps.
So if I had to quote a code to my inspector that tagged the #10 nm-b on a 45 amp breaker it would be 440.6?
JFletcher
Senior Member
- Location
- Williamsburg, VA
Max is just that, maximum. In this case I'd use a 40A.
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What code section allows a 45A breaker for #10NM-B?
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
240.4(D)-- refer to (G)
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
The overload protection in the a/c protects the wire the breakers is there for ground fault and short circuit protection
kwired
Electron manager
- Location
- NE Nebraska
440.6 is for minimum conductor size, 440 part III is for short circuit and ground fault protection.
Motors and air conditioning equipment don't follow the same general rules as most other applications when it comes to overcurrent protection, you can possibly find situations at times where a 10 AWG could be on a 80 amp breaker with these kinds of loads.
- Location
- Simi Valley, CA
Yes 440.6(A)
cookerobert21
Member
- Location
- East Yaphank New York
If you have a 45 amp breaker you need wire that can handle 45 amps you cannot put 10 nm on a 45 amp breaker put the 30 amp breaker in with 10 wire if anything
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- Location
- Connecticut
- Occupation
- Engineer
For a unit with a 28.2 MCA and 45 MOCP, #10 nm is acceptable on the 45A breaker.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
As David stated this is incorrect. The conductors are not required to match the size of the OCPD. In this case they're only required to be sized large enough to meet the MCA of 28.2 amps. The 45 amp OCPD has no relation to the conductor size when using the nameplate values.
Leespark57
Senior Member
- Location
- Boston, MA, USA
cookerobert21, start by looking at 240.4(d) Small Conductors. This says "unless specifically permitted"... Table 240.4(G) lists AC and refrigeration equipment (440 Parts III, VI) as one of those times where it is specifically permitted.
kwired
Electron manager
- Location
- NE Nebraska
To add to that the major reason it is permitted is the breaker is only providing short circuit and ground fault protection for this type of load. The motor(s) supplied have overload protection that not only protects the motor, but inherently will protect the branch circuit conductors from overload as well.
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