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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?

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    Your breaker should be no larger than the smallest wire on the circuit. Since you have 14-2 somewhere in the circuit then that means your max breaker size is 15A.
    – MonkeyZeus
    Commented Oct 4, 2019 at 15:39

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Breakers are there to protect the wiring in the wall. You can mix wire sizes, but the breaker has to match the smallest wire you used. So if there is any 14g wire on the circuit anywhere, it must have a 15A breaker.

The electrician that mentioned the fire hazard probably assumed the circuit was covered by a 20A breaker because of the 12g wire so that would be bad since there was also 14g in the wall.

When a house is first wired, mixing wire size generally doesn't happen. So, whenever you see mixed wire, you should assume it's a problem until you check the breakers and make sure it's ok.

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Consider the following scenario:

20A breaker. 12-2 to some box. 14-2 from that box to a box with a duplex 15A plug.

Plug in a microwave (12A) and a coffee maker (7A)

That 14-2 wire now has up to 19A flowing through it. (Breakers are allowed 30% over rate -- good ones trip very close to the rated capacity. You might be able to plug in a popcorn maker too and not trip the breaker. )

That is going to heat that wire up. Fire hazard.

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