How to Install or Replace a Doorbell Transformer

Knoji reviews products and up-and-coming brands we think you'll love. In certain cases, we may receive a commission from brands mentioned in our guides. Learn more.
How to replace or install a new doorbell transformer using basic electrical skills.

Standard doorbells are not powered by 120 volt electric typically used in the home. Most doorbell chimes use a transformer to step down the voltage from 110/120 VAC (volts alternating current) to 10 to 24 VAC. A small transformer is installed in the basement near the door or on the side of the panelboard to power the device. Older 10 volt transformers are used for buzzers and 16 volt and higher are used for doorbell chimes. If the distance from the transformer to the button and chime is long, the transformer is usually increased to 24 volts.

If you are installing a new doorbell, the transformer typically comes with the doorbell chime. If you are replacing the transformer, you need to locate the transformer and check the voltage marked on the side of the frame. If you have a hot water heating system you may have a transformer to power zone valves, or the transformer may supply power to the heating system and doorbell, so make sure that you have located the correct transformer.

Before you start a replacement, verify that the transformer has power by checking the terminals with a voltage tester. If the meter is reading over 10volts, you may have a damaged wire between the transformer and the doorbell.


Testing the transformer for output voltage


Tools and Materials

4-in-1 screwdriver

Electrician’s pliers

Voltage tester

Wire stripper

Wire connectors

Electrical tape

Fish tape

Cordless drill


18 AWG wire, 2 colors usually red and white


Simple wiring diagram

Check the instructions that are included with your doorbell chime to confirm the wiring requirements.

1. Location

Since the transformer is a low power device, it is often installed on an existing circuit on the side of a junction box. The transformer should be located near the doorbell. If you are installing it onto an existing junction box, make sure that the circuit or circuits feeding that box are off and test the wires.

2. Mounting the transformer

Remove a knockout on the side of the junction box by tapping the edge with a flat screwdriver and twisting it off with a pair of pliers. If your transformer has a threaded nipple you can slide this into the hole and secure it with a locknut. Other transformers need to be mounted to a new electrical box sized for a switch.


3. Wiring

Remove the wire nuts from existing circuit and connect the black wire from the transformer to the black circuit wire and connect the wires with a larger wire nut. Repeat for the white and green wires respectively. Carefully bend the wires into the box and replace the junction box cover plate.


4. Run low voltage wiring

The wiring from the transformer to the doorbell button and chime are usually 18 gauge (AWG) or larger depending on the voltage requirements. For existing systems 2-wire thermostat wire is often used to complete this circuit.

A. If you are replacing the transformer you simply need to reconnect the wires to the terminals on the outside of the transformer. (Go to #5)

B. If you are installing a new doorbell, you will need to run the low voltage wire to the door bell and the chime. If the transformer is in the basement you can drill a 3/8-inch hole up through the floor into the wall cavity where you want the chime located. Cut a 2 inch square hole in the drywall and push a fish tape up from the basement to the hole. Attach 2 pieces of 18 AWG wire, the same color, to the end of the fish tape and secure it with a piece of electrical tape and pull the tape back down through the hole. One wire should reach to the doorbell button and the other to the transformer.

Running the wire for the doorbell to the outside can be more difficult. You need to drill a ¼-inch hole through the trim on the side of the door for the button. It is best to remove the trim and run the wiring behind it. Drill a 3/8-inch hole down through the plate of the wall and use your fish tape to pull one wire from the chime and another 18 AWG wire of a different color from the transformer through the hole. Leave the wire stick out about 4 or 5 inches. Slide the wire through the hole in the trim and reattach the trim to the door.

5. Complete the wiring

Strip ¾-inch off the end of each doorbell wire using a wire stripper. It is important to keep track of which wire is going to the chime and which is going to the button.

For the doorbell button – Connect the white wire to one terminal screw on the back of the button and the red wire to the other terminal. Screw the doorbell button to the trim.

Connect the red wire from the button to the “hot” terminal of the transformer.

The white wire from the button is connected to one terminal on the doorbell chime. The other white wire is connected to the other terminal on the chime and then to the neutral terminal on the transformer.

Loosen the two screws and wrap one wire around each screw post. Tighten the screw down to secure the wires in place.


Restore the power to the circuit and test the doorbell operation.