I recently ran 12-3 wire off a 15 amp breaker as a main feed for my 2nd floor, only problem is it connects to 14-3 at the closest junction box to spread out to all the other connections on the 2nd floor. Is this okay? Also I prob. Only had to run 12-2 anyway as there is no other connection between the circuit breaker and the junction box...

It all depends on what the 14/3 feeds. If it splits off into two circuits then yes, you did good by running 3-wire.
How did you hook it up in the panel? Did you splice it wire for wire in the junction box?
Who ran the 14/3 and where does it go?

You should not have run #12, or should I say you did not have to. MAKE SURE you keep it on a 15A breaker. I would out a label on the wire in the panel and mark it "15A ONLY". This way no one later will see a #12 wire and put it on a 20A as is typical.

If anything you have overkill. You might save this Wire sizes, gauges, for breakers

Thanks guys! I just wanted to make sure it wasn't a fire hazard, someone had told me the only thing is the 12-3 wire might run hot and send spikes threw the current, also the junction box I ran the 12-3 into had multiple circuits coming out of it so I figure now that the 12-3 was needed. Im so far ahead after drywalling the 12-3 into the ceiling that it would be eaisier for me to replace the 14 gauge wire now instead of redoing the 12. Also to answer stan I made a short cut down at the panel by tapping the 12 into the 14 that origanlly came out of the panel... kind of rigged right now but I was not comfortable just yet with making connections directly into the panel. That will come later.

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Originally Posted by ballengerb1

If anything you have overkill. You might save this Wire sizes, gauges, for breakers

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I agree!

I also agree with Stan, make sure you label the wire at the panel.