A How-To Guide to Doorbell Transformer Wiring (2024)

ByAlly Childress

Step 1

Find the transformer

• Locate the doorbell transformer, if replacing.
  • The transformer may be in the garage or utility room, near the doorbell chime or in the attic.

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Step 2

Turn off the power

• Turn off the breaker that controls the doorbell.
• If you don’t know which breaker it is, turn off one breaker at a time and test the 120V power coming into the box by holding a non-contact voltage tester to the cable until you find the right one.
  
  • The power cable will be a yellow, white or (rarely) metal flexible cable sticking out of the electrical box.
  • If the tester beeps, the power is still on. Do not proceed until you find the right breaker.
• Remove the cover housing the 120V wiring powering the doorbell.
• Test all wires in the box and turn off any additional breakers, or the main power.

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Step 3

Remove transformer

• Separate line (120V) wires from the old transformer by snipping them with wire strippers near the wire nut.
  • Snip the hot, then neutral, then ground.
• Snip the low-voltage doorbell wires leaving the transformer terminals.
• Unscrew the transformer mounting screw, or use pliers to remove the locknut holding it to the box.
• Remove the transformer.

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Step 4

Replace or add the transformer

• Feed the line side wires (black, white, green) on the transformer into the box.
• Tighten the mounting screw or locknut to secure transformer to box.

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Step 5

Wire the transformer (doorbell wires)

• Strip 3/4-inch of insulation from the low-voltage wires going to the doorbell.
• Make a hook in the end of each wire.
• Attach one wire to each of two screw terminals, wrapping the wire clockwise around the screw.
  • If you have three terminals, consult your doorbell instructions to see which two you should use.
  • It doesn’t matter which wire goes on which of the two terminals.
• Tighten screw terminals with a screwdriver.

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Step 6

Wire the transformer (line wires)

• Strip 1/2-inch of insulation from the black, white and green wires.
• Splice the ground wires from the power, the transformer and the electrical box (if metal) together.
  
  • If you have a metal box but no wire present to ground it, add a grounding pigtail by screwing the green grounding screw attached to the pigtail into the metal box’s screw terminal.
• Splice the two neutral (white) wires, then splice the hot (black) wires.

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Step 7

Finish

• Tug on all wire connections to make sure they’re secure.
• Close up the electrical box housing the 120V transformer connections.
• Turn on the breaker.
• Test the doorbell.

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