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Re: CQD answer published Monday, October 16, 2017 -AC Equipment Branch Circuit
I noticed the NECA Code Question of the Day for Friday concerned an article I wrote for the IAEI News magazine on AC equipment. I was hoping you might be able to give “the rest of the story” as Paul Harvey use to say.
In the July/August 2017 IAEI News magazine, I wrote an article titled, Air-Conditioning Equipment Installations. In the article, I stated the following: The minimum supply circuit ampacity is 24 amperes. Consequently, if no adjustment for ambient temperature higher than 86°F is required, all 12 AWG conductors having an allowable ampacity of 25 amperes are acceptable as the branch-circuit conductors supply this unit. This rule includes 12 AWG copper conductors that are part of a cable assembly, such as Type NM or UF, which have an allowable ampacity of 25 amperes at 60⁰C.
The 25 ampere I quoted from my article is in error (per the 2017 NEC). We can thank the Canadians for this one. Based on the 2008 NEC my statement regarding NM and UF was correct and per 334.80, could be used as a 25 ampere conductor for air-conditioning equipment as permitted by 240.4(G) (See Table 310.16 from the 2008 NEC). No problem with what I described.
Now let’s move forward to the 2011 NEC and one of the US/Canada ampacity harmonization compromises occurred where the NEC gave up some of the higher ampacities for specific conductors sizes provided by the NEC to harmonize with the Canadian Electrical Code. So the 2011 NEC contained the ampacity reductions for some conductor sizes including 12 AWG. So the 60 degree column ampacity for 12 AWG copper was reduced from 25 amperes to 20 amperes (see Table 310.15(B)(16) of the 2011 NEC).
This change in the 2011 NEC, plus the 334.80 restriction to use the 60 degree column ampacity for load connections limits 12 AWG NM and UF to 20 amperes. The bulk of the information for my article came from IAEI’s One- and Two- Family Dwelling Electrical Systems textbook (which I author as well). This one sentence never got updated in the 2011 Code cycle and moved forward in that textbook until I realized the error in the article quoted above.
Sorry for the confusion and glad the error was caught.
Regards, L. Keith Lofland, IAEI Director of Education
Hey Keith thanks for sharing the background about what happened in extensive detail and glad we arrived at the same answer. It's also a good reminder to be aware that as NEC editions are published they can change previous rules which gets more complicated if a jurisdictions is still enforcing earlier NEC editions.
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