New Article: Inspecting Oversized "AC" Breakers on Small Gauge Wires

inspecting oversized ac breakers small gauge wires internachi gromicko


A needed explanation for a common discussion on this forum. Good stuff.


Thank you, Jim Krumm and Lon Henderson, for contributing to this new inspection article.


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.


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