WHAT AWG WIRE ?

No, 12 ga wire would not be wrong. It will cause no problems. But if the rest of the wire is 14ga then it would be overkill. If the rest of the wire is 12ga then you could be asking for problems as a possible overload could occur.

You guys are great. Thanks. I am already running the heater on this circuit just adding a second plug to a more convient location. It is a 15 amp breaker. Will go with 14 guage as it is easier to plug, or insert wire into the plug I bought. Just out of curiosity using 12 guage wire would be wrong and cause resistance heat at point of transfer? Or am I wrong about that?

The wire itself is not marked. The sheath of non-metallic cable is marked.
When in doubt go larger. #12.
If possible run back to 120 volt AC panel and install additional circuit.

If extending a circuit you need to know the existing breaker size. Probably 15a breaker and #14 wire. Use same #14 is fine.

If adding a new breaker and circuit use 20a breaker and #12 wire. 15a and #14 is only used so the manufacturer can save a nickel per hundred built.

If your circuit breaker is a 15 amp you must use at least 14 ga. If your circuit breaker is 20 amp you must use at least 12 ga. My opinion is: If I put in a circuit for a 120v. AC or heater service I would always use 12 ga regardless of the the breaker size to minimize voltage drop.

Your rig might be wired for #12. I'd pull a receptacle and look. If it is, just use #12.

If you are adding an extension to an existing circuit and plan to put a 1500 watt heater on the end of your extension, it could be very likely that you will overload that circuit and it will cause you tripping or bigger problems.

If that is your purpose, a 1500 watt heater should be wired with a dedicated plug in receptacle directly to the panel board. #14 wire could be used, but #12 could also be used if you have some.

14 ga is good for 15 amps if I remember right
12ga for a 20A breaker