when or is a #14 wire allowed on a 20 amp circuit ?
14 AWG is rated
20 amps @ 60 C & 75 C, even 25 amps @ 90 C which helps when needing to derate.
The only thing that normally keeps 14 AWG on a 15 amp breaker is 240.4(D)
240.4(D) Small Conductors. Unless specifically permitted in 240.4(E) through (G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.
Notice the first part of that says:
"Unless specifically permitted in 240.4(E) through (G)"
(E) Tap Conductors.
(F) Transformer Secondary Conductors.
(G) Overcurrent Protection for Specific Conductor Applications
(G) has a table that lists about 10 applications that may allow 14 AWG on a breaker larger than 15 amps.
As Peter (CTwireman) pointed out HVAC equipment is one of those times, so are motors.
With the following nameplate data 14 AWG would be fine on a 30 amp breaker
Min circuit ampacity: 19 amps.
Max fuse/breaker: 30 amps