210.19 Exception No. 2: Permits grounded conductors not connected to an overcurrent device to be sized at 100 percent of the continuous and noncontinuous load.

A hypothetical question:

19A continuous load:
ungrounded THHN 19x125% = 23.75A requires a #10 AWG
grounded THHN 19x100% = 19A requires #12 AWG

In a two wire circuit the ungrounded and the grounded have the same current flow. How come the Neutral can be smaller than the grounded?

Is it because the OCPD is rated at 80% ??
Is it because the #12 ampacity is 35Amps so it is within capacity?

Thanks






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