An air-conditioning system uses a condensate pump when a standard condensate drain system will not work. When an air conditioner removes humidity from the air, condensate or water forms. The water flows out of the coil's drain pan into the condensate pump's holding tank. Once the water reaches a set level in the holding tank, the condensate pump turns on and forces the water through a drain pipe. Two sets of wires leave the pump. The high-voltage wires power the float switch and pump motor, and the low-voltage wires turn off the air handler if the pump fails.
Turn off the circuit breaker to the air-conditioning system's inside unit. This circuit breaker usually has a "Furnace" or "Air Handler" identification tag.
Examine the condensate pump's wiring chart. The chart identifies the high- and low-voltage wires. If the condensate pump has a "NC" and a "NO" low-voltage option, choose the "NC" low-voltage position.
Run the condensate pump's low-voltage wires into the air handler's electrical connection box. The condensate pump's low-voltage wire set contains two thin wires, usually 18-gauge wire.
Remove the wire nut holding the thermostat's red wire to one of the transformer low-voltage wires and separate the two wires. The transformer has two low-voltage wires: one connects to the ground lug, and the other sends power to the thermostat.
Twist one of the condensate pump's low-voltage wires to the thermostat's red wire. Twist the second condensate pump low-voltage wire to the transformer's low-voltage wire. Twist a wire nut over each pair of wires.
Connect the condensate pump's high-voltage wires. If the condensate pump model has a high-voltage cord with a plug, then insert the plug into a wall receptacle. If the condensate pump hard-wires into a high-voltage power source, then run the condensate pump's high-voltage wire into the air handler's electrical box and connect one high-voltage wire to the ground lug and the other wire to one lug on the air handler's terminal block. The wires from the circuit breaker enter the air handler and connect to the air handler's high-voltage wires at the terminal block.