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I note that with 8 AWG wire... in NM-B (Romex) cable I can only get 40A: I am required to pull from the 60 degree C column of Table 310.15(B)(16). However, in conduit, with 75C wire (THHN) and terminations, I am allowed to use the 75C column, giving 50A on a #8 wire.

Can I do the same thing with #14 wire, using the 75C column to put 20A on the #14 wire?

If not, why not -- i.e. why #8 and not #14?

If so, am I then allowed to use 15A receptacles (with the backstabs and everything), and pass through the 20A?

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Nope, there's another section of the NEC that limits here

In copper, 14AWG is limited to 15A, 12AWG to 20A, and 10AWG to 30A not by the ampacity tables or even necessarily termination ratings, but by the small conductor rules set forth in NEC 240.4(D), points 3 through 7 (points 1 and 2 apply to 18AWG and 16AWG conductors, but no branch circuit breakers I know of are listed or marked for protecting 18 or 16AWG wires, and class CC fuses are generally not seen outside of industrial work):

(3) 14 AWG Copper. 15 amperes

(4) 12 AWG Aluminum and Copper-Clad Aluminum. 15 amperes

(5) 12 AWG Copper. 20 amperes

(6) 10 AWG Aluminum and Copper-Clad Aluminum. 25 amperes

(7) 10 AWG Copper. 30 amperes

The only exceptions to this are when the rules for specific loads in the various sections listed in Table 240.4(G) are applied, or if you have 14-10AWG tap conductors in accordance with the sections listed in 240.4(E).

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