I am replacing a manual elcectric heat thermostat. In the instructions it says to connect L1 to the hot wire. Now I have three wires. One is the ground ( bare copper wire ), a black wire and a white wire. Don't know if it is black or white. Please help. Thanks for your time...

Is this a thermostat built into an electric heater? If so the black wire, or any other color except white and green, will always be the hot wire. If it is a 230 volt heater, the white will be hot too.

Most residential wiring has black - hot, white - neutral, and bare - ground. 230 volt appliances always have 2 hot wires, black, red if there is one, otherwise, the white.

No, its not a built in. I should have mention that it is wall thermostat for a electric baseboard heater. Thank you for the reply and for your help.