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A needed explanation for a common discussion on this forum. Good stuff.
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Thank you, Jim Krumm and Lon Henderson, for contributing to this new inspection article.
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Very good article but slightly confusing. When referring to Minimum Ampacity, you kind of skip over Romex. Isn’t Romex the most common type wiring seen these days? “If Romex® is used, according to NEC 334.80 Ampacity, the 60°C column should be used. But let’s use the 75°C column for this article.”
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That is correct since NM cable is limited to its 60° C ampacity in NEC Table 310.16. The 75° C wiring methods such as MC cable or conduit and wire can be used at their 75° C ampacity.
To further confuse this MC cable or conduit and wire in this case are not subject to the limitations of 240.4(D) which would normally limit branch circuit ampacities to:
#14 AWG to 15 amps,
#12 AWG to 20 amps,
#10 AWG to 30 amps.
So when wiring an AC unit {under NEC Table 240.4(G)} with a 75° C wiring method:
#14 AWG can be used as a 20 amp conductor,
#12 AWG as a 25 amp conductor,
#10 AWG as a 35 amp conductor.
As an example:
MCA=25 amps
MaxOCPD=40 amps
#12 MC cable would be code complaint.
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Thanks, @rmeier2. If you have any suggestions to improve the article, we’d love it. You’re the best. Please feel free to email me ben@internachi.org.