I removed a fireplace and there are exposed 12 gauge wires. My question is can I put in a junction box and wire nut together 12 gauge with 14 gauge to wire a plug. I'm asking because I have a roll of 14 g. no 12 g.

Only if the breaker is rated at 15 A. It's probably not good practice, but OK. Mark the panel with a note.

Reminder, junction boxes must be accessible.

You could but then you would have to change the breaker from 20 amps to 15. Cheaper and better to just buy some 12 gauge wire.

And you cannot hide that junction box.

No, you can't mix gauges, even if you use a 15 amp breaker. Someone at sometime will see 12 ga wire and install a 20 amp breaker, not knowing it has reduced ampacity due to the smaller wire size, and this is a potential fire hazard. Pull all 14 or all 12 and use the proper size breaker.

Where in the code does it say you can't mix wire sizes? As keep said, you can do this, but in most cases, it is not a good practice! If two different sizes of wire are connected together, then the OCPD shall not be greater than required for the smallest wire.

Just an added example (Not specifically related): A mixture of wire sizes is permitted per section 210.19(A)(4)

Wash, we agree it's not good practice... but I don't see the reference in the NEC which permits it. Are you referring to the "other loads" comments?

Don't want to hi-jack the thread, but was giving a small example of a tap rule that permits mixing. Just was throwing it out there!

Also see 240.4

Ddhigg,
In your case, If two different sizes of wire are connected together, then the OCPD shall not be greater than required for the smallest wire. Ignore the tap comment. It was only presented to make a point.

"smallest wire" means the smaller diameter. Remember that 14 AWG is SMALLER than 12 AWG, yet 14>12. Go figure.

Not to add any more confusion to the mixed wire size issue... but the "tap rule" specifically refers to wires within an appliance. NOT to be confused with the issue of mixing wire sizes in a circuit, which is not addressed by this paragraph and is not an acceptable wiring method.