It doesn;t matter what kind of wire it is ... just so long as it doesn;t get confused with any other wire. It's usually a piece cut from one of the wires in the kit so that you have the proper amount of wires... so it would just depend on the placement of your light and thermostat and which wire has extra length on it to cut off. Just make sure the RIBBED wire on the plug goes to the silver screw and the the non ribbed goes from the plug to the #1 post on the thermostat. After that... whatever piece of wire you have laying around just goes from the brass screw to the #2 post on the thermostat.