50 AMP Breaker on a 10 Gauge Wire.

Here is a great example where you could have a 10 gauge wire protected by a 50 amp breaker and it would be… CORRECT… for that specific AC unit.

On AC units the most important thing is to look at the manufacturer's label/name plate.  This will give you the information you need to make a quick evaluation in the field of required wire size and overcurrent protection (OCPD)

Link to more information on this subject-

http://www.ncwhomeinspections.com/Heat+Pump+and+Air+Conditioner+Home+Inspection