I'm doing some simple wiring in a shed I built and will have an actual electrician connect it to the panel.
I ran 12-2 for all my outlets so they can be on a 20 amp circuit and 14-2 for all my lights on a single switch and they will be on a 15 amp circuit.
The thing I was noticing yesterday though is I ran 12-2 from the future panel to the switch, and then 14-2 from the switch to all the light fixtures.
I texted my electrician buddy and asked if that was ok and he said yes as long as it's on a 15 amp circuit only.
Then I got to thinking about a time when I was tearing apart an old house and there was some wire running that was either 12-2 to an outlet and then 14-2 from there, or possibly vice versa and the guy I was working with said "whoa that's definitely a fire hazard".
Can someone explain to me what is safe, what is not safe, etc when mixing wire size on a single circuit (I know it's not a great idea to mix, but if you were to). Is it solely related to breaker size?