Yes, #14 THHN/ THWN could be used on an A/C Condensing Unit that requires a 13.6 minimum amperage rating based on article 240.4 (G). The overcurrent protective device (OCPD) should always be what the nameplate calls for based on 110.3 (B). It should also be noted that if the nameplate calls for 'MAX FUSE' then you'll need a fused disconnect. Those aren't as common as they used to be 20 years ago (or so they tell me).
