How to determine the polarity of the wires
TH1120, TH1121, TH1123, and TH1124 series thermostats have two unpolarized wires, meaning that either thermostat wires can be connected to either electrical box wires.
TH1500 series thermostats have four polarized wires. The wires used to power the thermostat are positioned at the ends and are identified “LINE” (power). The wires used to connect the heater are located in the center and are labelled “LOAD.”