So I was replacing an old light fixture with a newer one, when I noticed that the light was wired with 14ga wire, but is on a 20amp breaker.
Upon checking a few outlets, it seems that the entire upstairs of my house (which is all on 1 20amp breaker, just 2 rooms) is wired with 14ga. Is this a big problem? I thought you had to use 12ga on a 20amp breaker?
That was probably someone's way of fixing a constantly tripping 15 amp breaker - install a larger breaker! (Dangerous and a good way to burn your house down!)
And you may find the new 15 amp breaker trips. The easiest way to properly fix that is to run new wires (12 ga. / 20 amp) to power hogs. Like a refrigerator, plug for vacuum, bathroom, etc. (Instead of rewiring everything.)
Another more complicated fix, if there are junction boxes for wires going to various rooms in the attic, is to split the circuit and then use several 15 amp breakers for parts of the circuit. (Break the one large circuit into several small circuits. Run new wires from the panel to those points in the attic.)