Depends on your exhaust fan, but mainly yes you can use 14/2 on a 15 amp breaker. It doesn't have to be dedicated but you want to make sure your not adding it into a circuit that is close to it's limits.
Little about a lot and a lot about a little.
Every day is a learning day.
food for thought - I know 12 gauge wire is expensive - all copper is. used to be it wasn't but times have changed. you can wire an entire dwelling with 12 gauge and fuse or protect the circuits with either all 20 amp breakers or undersize a circuit by using a 15 amp breaker. then if you find you're blowing the 15 you can upgrade to a 20 amp breaker and know you are not over fusing. the 12 will already be there. just don't go higher than a 20 that is looking for trouble!
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