24volt power adapter/transformer,c-wire adapter thermostat,compatible with honeywell ,ecobee,nest and sensi smart wifi thermostat pro(25foot /7.6m cable) wuyelin <b>attention:to prevent no power output, you must ensure that the terminals are stripped and securely clamp the screw terminals into the thermostat to maintain good contact.<br> personal advice: required to ask the person who understands the electrician to install.<br> be advised, the initial ends on this wire are treated and if you cut them to shorten it, the copper is tougher to get into the thermostat with pinch vs. screw terminals.will result in no power output.for nest e connected this adapter to the c and r pins (r also having the hvac's red line connected),for nest learning thermostat,3rd generation connected this adapter to the c and rc pins <br></b>