Thread: You make the call AC circuit rating

Jerry, keep out of this one until you disagree with me then we can hash it out. Looking to see how everyone approaches this one.

How about this case with more information given, acceptable or not?

Single Family Residence, all wiring is copper 12 awg NM-B. From the exterior disconnect to the unit they change over to 12awg copper THWN run through LFMC. Circuit Breaker in the panel is 40A.



http://www.iaei.org/subscriber/magaz...nston_fig4.gif

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"acceptable or not?"

Not

"12awg" is not rated for 24 amp min circuit much less the 40 amp breaker

Any other takers?

Come on guys, you see this every day and write it up..or do you?

The 40 amp has nothing to do with the 12 gauge wire in this case. I see it as not quite acceptable. The rating would be okay for 25 amps with # 12, but if you derate it at all for ambient temperature, you fall below the 25 amp limitation. You have to think with an AC unit, there would be ambient temperature above 86 degrees, so the wire size should have been bumped up to a # 10. 40 amp breaker is proper according to the label.

Acceptable-- 110.3(B) says it must be installed according to the listing and labeling. 240.4(G) tells you there is an "exception" to the overcurrent protection rules for Air Conditioning equipment and sends you to Article 440 for application. This is an NEC compliant installation.

Keep Your Cool When Working With A/C and Refrigeration Equipment

The site above shows that the wire size is ok as 12 THHN rateded at 25 amps. I did not find the referenced table that allows 12ga. conductor to handle 25 amp. It also shows the wire from the panel to the disconnect as the same THHN type wire. This leaves me with the question, does it have to be THNN?

Roland is correct. This is an NEC compliant installation.

We are not discussing derating any conductors, don't read into the question. If I wanted you to derate I would have given the temperature factors involved.

This situation comes up frequently and makes our profession look bad. This gets written up then an electrician goes out there and says it is OK. Then the buyer and seller argue then they call me (I do work as an AHJ too) and after I review the situation, I too say there is not a problem which then get both the listing agent and buyer's agent upset with the home inspector.

Thank you Roland.

I hate to sound so ignorant, but how can a 40a breaker protect 12ga wire?

Oh my!!
Keep Your Cool When Working With A/C and Refrigeration Equipment

This is a good place to go to understand what is happening..

Thanks for posting this question Jeff.

read some of it ...please just the why of it

My bad. I knew the wire was good for up to 25 amps, but I just assumed that if there was central air present the ambient air temp in the summer was above 87 degrees.