I think I asked the question wrong. I plan to wire a room with #12 wire everywhere and feed the canned lights with #12 wire so that the entire room is on a 20 amp breaker. But it occurred to me that internally the light is most likely #14 wire. In other words when I splice in the canned light to the #12 wire, I am actually attaching a #12 to a #14. Is this a problem? If it is, then why are bathrooms usually 20 amp circuits including the lights, and bathroom fan?