If a homeowner asked an electrician to upgrade a 15a breaker in the box connected to 12g wire an electrician wouldn't do it? I highly doubt that.
An experienced electrician would not change a lower amperage with a higher amperage overcurrent device.
Now maybe an inexperienced Do-It-Yourself Idiot might.
Rule 1- Never change an overcurrent device for a lager one.
I've had people post that their electrician didn't even run GFCI for an outside outlet
Remember that at one time they wasn’t required to install them. Then again just because someone calls theirself an electrician does not mean that they are one
I never downgrade wire in a run (12g or 14g), just asking for trouble down the road.
This is a choice that you make but I can promise you that someone who is making installations for a living will do so simply because it is called making a profit.
I can see doing it with #10 for a long run to a pump
So why would it be alright for one installation and not for another?
And if the lower gauge wire is hidden, how is anyone to know its there?
Ever thought about looking at the breaker size. This will give it away each and every time
The majority of breakers in my old panel were 15a breakers. Wire was too old to properly judge the gauge, one circuit was 2 wire only, no ground. About 90% of my house has been rewired & brought up to code
Why would you need to judge the gauge of the wire when you have a perfectly clear 15 amp overcurrent device telling what the amperage of the circuit is?
I would never replace a breaker without verifying what was connected & the wiring run. But again if the lower gauge wire is hidden in the wall your kinda screwed. But I doubt most homeowners would think that someone "went cheap" & used mixed wire in a run. Most know nothing about voltage drop. They see 12g wire, that can handle 20a
Without knowing you are making the perfect argument for not allowing the Do-It-Yourselfer to enter the realm of electrical work. You are saying in a very loud voice that you don’t understand electrical installations.
The problem always is, not necessarily what is wired/ completed know - but what might be changed years down the road by a new owner or another electrician
Again Never change out an overcurrent device for a larger one. This is the mark of ignorance. If one is going to just go around changing out overcurrent devices for larger ones why not just go all the way and do away with the darn thing completely. Shucks, it had a #12 so it must be good for at least 50 amps so lets just change it out, Silly, Silly, Silly.