I am wiring a barn/shop area and have run all wiring in EMT conduit except above 8 feet where I have run some ROMEX to lights. My question is: I need one extra wire for a 3-way switch traveler wire. Can I staple this along side of the ROMEX wire above 8' in the hi bay area, or does it have to be in conduit. I was trying to avoid buying more ROMEX wire, and had this THHN left over from the conduit runs.
Thanks, Guy

Can you do it, sure you can but no electrician would ever talk to you again. Guy, its just not safe to do it this way. Put it in conduit, run another romex or buy 3 wire and save the romex you currently have.

If power is making it to one switch box and light switchleg goes to other. Then you could
Use the 12-2 w/ground for travellers. The 2 are travellers and ground is ground.

First, I said if power was available at one switch location, and switchlegg was going to other location.
THEN IT WOULD BE POSSIBLE TO USE 2 CONDUCTORS FOR TRAVELLERS.
3RD WIRE IS THE HOT AT ONE BOX, THE 3RD WIRE AT THE OTHER BOX IS THE SWITCHLEG.
REREAD MY POST, then if you can remove you little red mark, I would appreciate it.