How to Replace a Doorbell Transformer

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What You'll Need

When you replace a doorbell it is also a very good idea to replace the doorbell transformer. A transformer will be in the basement or attic and will be connected directly to a power source or electrical junction box. This is not overly difficult, but it is important to make sure you have the wires connected properly for the doorbell to work. Things can get a little more tricky if you have multiple doorbells connected to the same transformer. Here’s how to go about it.

Step 1 - Electricity

Even though doorbell transformers use low voltage you do want to turn OFF the electricity for this point. You can turn off the circuit breakers to the area you are working in. However, if you are connecting the transformer to the main power source it is best to turn off the main power.

Step 2 - Remove the Old

junction box

Locate the old transformer and disconnect the white and black wires from the junction box. These are found on one side of the transformer. You also want to disconnect the wires that are on the opposite side of the transformer.

Step 3 - Wire the New Transformer

Once the old transformer is out then you can install the new transformer. The black wire is the power wire and will be connected to the black lead in the junction box. The white wire is connected to the white lead. Your transformer may also have a green wire attached to it as this is a grounding wire. If you do not have a grounding site to attach the wire then simply cap this wire. Now your transformer is connected to the electrical junction box.

Step 4 - Connect the Doorbell

You will also need to wire the doorbell to the new transformer. The two doorbell wires will be connected to the low voltage side of the transformer. This will be the screws that are open on the opposite side of the transformer. It does not matter which screw each wire goes to as long as they are on the same side.

Step 5 - Wiring for Two Doorbells

If you have two doorbells, then the wiring is not too difficult. You will be working with two sets of black and white wires. It is important that the wire pairs are connected to the correct side. Many times if wires are crossed then the doorbell will not ring.


Some transformers can be plugged into an electrical outlet. There are also doorbells with built-in transformers that work wirelessly. To keep things simple you can disconnect one wire from the old transformer and then attach it to the same place on the new transformer. Keep transferring wires until the old transformer is unconnected and the new one is completely wired.

Too Many Wires

It can get confusing when you have many wires for the front and rear doorbell. A voltmeter can come in handy. To determine which are the correct paired wires, you will need to touch each wire pair with the voltmeter. If you get a reading of about 10 volts then you have the correct pair of wires.