1) If the UR unit has a thermostat control, the extra lead wire is for the connection between the control box and the power source. The thermostat "lead" from the Heated Well to the control box is only available in 3’ (914 mm) standard length, or 6’ (1829 mm) optional length. 2) If the UR unit has an infinite control, the extra lead wire is for the connection between the control box and the Heated Well. This can be ordered in any length in 1’ (305 mm) increments. The lead wire from the control box to the power source is provided by the customer/installer.