The RC wire on a thermostat refers to "red cooling" and powers the cooling system. It is one of the wires that connect the thermostat to the heating and cooling systems in a unit. The RH wire, on the other hand, powers the heating system. In a four-wire heat/cool system and a single-stage heat system, the RC and RH terminals are jumpered together. If a thermostat has only one R wire (no Rh or Rc), the R wire can be connected to either Rc or Rh on the thermostat. However, if a unit has both an Rh and an Rc wire, it is a dual transformer system, and it is strongly recommended to contact a local HVAC professional to prevent damage to the system.