In response to DC voltage not being able travel longer distances. If you are just using DC lights and the amp draw is under a few amps the 12ga romex is waaay oversized. The way to fight the voltage drop over longer distances is to drastically increase the wire size therefore 12 romex should be ok for moderate distances. I use this method in building low voltage control systems on large machines and sometimes the draws can reach 5 or more amps.
