A 24 VAC (volt alternating current) transformer is a step-down type of transformer. The device typically converts 120 VAC to a lower voltage for use in push buttons. The most common application for a 24 VAC transformer is to activate a doorbell chime. The lower voltage is sent to the push button to engage the chime mechanism. The 24 VAC transformer is also employed in many thermostats for home heating, ventilation and air-conditioning systems.
Inspect the wires connected to the 24 VAC transformer. There are usually four wires--two black and two more that are yellow, green or blue. The two black wires will be connected to the 120 VAC input, while the yellow, green or blue wires are the 24 VAC output.
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Remove all electrical power to the device the transformer is being installed for. This may entail shutting off the circuit breaker or pulling the fuse that supplies power to the device.
Mount the transformer to a firm surface using the provided screws or enclosure. Most transformers will be mounted directly inside the mechanism they are supplying for the push button or thermostat power.
Strip 3/4 inch of the outer insulation from the transformer's four wires with your wire strippers. This will reveal the bare copper under the insulation.
Connect the transformer. Attach the two black wires from the transformer to the 120 VAC power supply wires. One supply wire will be the black voltage wire, attached to the fuse or circuit breaker. The other power supply wire will be white--this is the electrical systems neutral or common.
Attach the low-voltage side of the transformer to the push button of the doorbell or the thermostat wiring. If it's a thermostat, consult the wiring specifications, as manufacturers use different wiring configurations.
Twist the bare ends of the wires together using your wire pliers. Cover the connections with a plastic wire nut. Turn on the electrical power to the device.