residential AC

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stjohnbarleycorn

Senior Member
Is there any other code reference on residential AC that explains how you can use MCA such as #12 and put a 30 amp breaker for the MOCP. I am trying to explain it to someone who references 240.4 and I show him 240.4(G) which leads me to this. Thanks.

440.22 Application and Selection.
(A) Rating or Setting for Individual Motor-Compressor. The motor-compressor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor. A protective device having a rating or setting not exceeding 175 percent of the motor-compressor rated-load current or branch-circuit selection current, whichever is greater, shall be permitted, provided that, where the protection specified is not sufficient for the starting current of the motor, the rating or setting shall be permitted to be increased but shall not exceed 225 percent of the motor rated-load current or branch-circuit selection current, whichever is greater.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
There are sections in the code that allow you to size the conductors based on 125% of the load or with a/c's you just use the minimum cir. amp which has the 125%. You quoted the section that allows for a high OCPD.

Try this as an explaination. The breaker or OCPD does not function as overload protection in the case of an a/c because the unit has built in overload protection. The function of the breaker is simply for ground fault or short circuit protection.
 

stjohnbarleycorn

Senior Member
Thanks, he wants #12 on a MCA of 13 amps, because the unit needs 20 amps ( in his mind) for the unit to start. I told him I can put a 20 as allowed. He insists that you can't put a 20 on #14. He is a combo elec/havc in PA. and says there they have to match the wire size to the breakers as per 204.4. Maybe its a local code in lancaster PA. thanks again. kevin
 

mcclary's electrical

Senior Member
Location
VA
Thanks, he wants #12 on a MCA of 13 amps, because the unit needs 20 amps ( in his mind) for the unit to start. I told him I can put a 20 as allowed. He insists that you can't put a 20 on #14. He is a combo elec/havc in PA. and says there they have to match the wire size to the breakers as per 204.4. Maybe its a local code in lancaster PA. thanks again. kevin

It's not a local ordinance. He is just wrong. Period. Your #14 on a 20 amp breaker would be legal
 

busman

Senior Member
Location
Northern Virginia
Occupation
Master Electrician / Electrical Engineer
Agree 100% The guy is wrong.

Almost got into a fistfight once over this. The HVAC tech said it was against code and "mickey-mouse" to put #10 on a 40A breaker. These guys have just memorized #14=15A, #12=20A and #10=30A and NOTHING ELSE. Wouldn't know how to apply motor or compressor rules. Or derating or temp adjustment. Don't even begin to think they'll understand termination temperatures.

Mark
 

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
Thanks for the replies I will print him out some code and calculations on MCA and MOP.,
I have never seen a residential A/C unit that only has a single motor. All equipment has at least 2 motors. You should not need to do any calculations since the manufactures have already done them and put the information on the nameplate.
 

Jim W in Tampa

Senior Member
Location
Tampa Florida
Nothing better than unqualified inspectors. Send him to 310.16 and he will see that #14 is rated 20 amps and #12 at 25 amps. The star next to that number sends him to 240.4 D
He then goes to G for air conditioners and your finished. One word of caution that might need watched is if this is a package unit such as RTU or mobile homes is the electric heat strips if any. Even heat pumps have them.
In practice i prefer to oversize the wire so that if it gets replaced with larger unit it will not need rewired. But that is strictly an option not required by nec.
As to local rules they must be in writing.
 

macmikeman

Senior Member
I've never had to concern myself about heat strips even once yet. There would be no reason to, I haven't ever seen a unit which has any built in. I guess this means I will never be able to relocate...:cool:
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
People try to make A/C's harder than they are.

Everything you need to know is on the nameplate, min ckt ampacity and max ocpd. That's what you go by.

Don't try to size them per the motor section which is where most people get confused.
 

stjohnbarleycorn

Senior Member
Nothing better than unqualified inspectors. Send him to 310.16 and he will see that #14 is rated 20 amps and #12 at 25 amps. The star next to that number sends him to 240.4 D
He then goes to G for air conditioners and your finished. One word of caution that might need watched is if this is a package unit such as RTU or mobile homes is the electric heat strips if any. Even heat pumps have them.
In practice i prefer to oversize the wire so that if it gets replaced with larger unit it will not need rewired. But that is strictly an option not required by nec.
As to local rules they must be in writing.

the units I have been replacing now a days are actually less MCA because of the new efficiency ratings, so that might not be needed for the future. thanks for the info.
 

Jim W in Tampa

Senior Member
Location
Tampa Florida
I've never had to concern myself about heat strips even once yet. There would be no reason to, I haven't ever seen a unit which has any built in. I guess this means I will never be able to relocate...:cool:
I been in FL too long to think about going up north. We do hit 30's but not often. Problem is a heat pump will not work in 30's. Just pointing out that if it has heat strips your number 12 is back down to 20 amps.
 

macmikeman

Senior Member
I been in FL too long to think about going up north. We do hit 30's but not often. Problem is a heat pump will not work in 30's. Just pointing out that if it has heat strips your number 12 is back down to 20 amps.

Never goes below 60 that I can remember unless you drive uphill a good bit. Upcountry different than down here at the beach....
 
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