Fascinating question!

Why can't you use Table E3604.2(1) where it says for fixed motors, the full-load current of motors plus 25% of the full load current of the largest motor.

The A/C has a "fixed motor" and since it is the only motor in this question, it is also the largest motor.


Therefore:

((1512/120))/.75 = 16.66 Amps which gets you to the same answer, 20 amps, when you round up.

Or is my logic totally flawed?