To wire a humidifier to a furnace, run two 12-gauge wires from the motor control contractor of the furnace to the electrical box on the humidifier. Attach the neutral wire to the neutral lug of the furnace and the black power wire to the bottom of the motor contractor.
Begin wiring a humidifier to a furnace by switching off power to the unit. Remove the protective cover of the electrical control box of the furnace to expose the wiring area that contains the circulation fan motor contractor. A name tag placed below or to one side of the switching mechanism identifies the motor contractor.
Connect the motor contractor to the electrical control box of the humidifier using two 12-gauge wires. If needed, make a hole in the electrical box of the humidifier to let the wire in. In a 120VAC humidifier, attach these two wires to the two wires of the humidifier, using one as the power wire and the other neutral wire. Attach the black power wire to the bottom side of the motor contractor and the neutral wire to the neutral lug of the furnace.
If it is a 220VAC humidifier, use both the wires as power wires and connect them to the bottom of the motor contractor. In both cases, twist the wires together and cover with wire nut. Finally, replace the protective cover and switch on power to the furnace.