15A breaker tripping when Air Conditioning comes on
Could the motor failing be the cause of the overload?


Normally, there is an overload fuse or breaker on the air handler / evaporator that protects the motor at more like 3 amps. This is supposed trip if the motor is overloaded. The 15 amp breaker is supposed to protect the conductors to the air handler / evaporator.


  What is the likelihood that both the blower and the thermostat are on the same circuit?


Yes. The thermostat is actually 24volt on the secondary side of a 120volt to 24 volt transformer fed by the 15 amp circuit.

If the breaker keeps tripping that is telling you something is broke.

Call a reputable HVAC or electrical contractor to take a look at it.

Good luck!