I am installing a generator transfer switch in the garage.

My house is long and a main service panel feeds one half and a sub-panel feeds the other half. I want to put the master bedroom on the transfer switch. So, I will have to run it from the subpanel on the opposite side of the house.

The master bedroom is wired with #14 with a 15 amp breaker. I was thinking of doing the long run (about 80') from the sub-panel with #12 to minimize voltage drop. I would keep the existing #14 in the bedroom with only the #12 providing the feed. It would still have a 15amp breaker. Is intermixing wire size legal? I can put a label in the subpanel on the #12 that says "15amp only/#14 wire in circuit" to avoid confusion in the future.