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Total amateur here. I’ve watched multiple video tutorials on replacing an old doorbell transformer with a new one, and none seem to address my particular set-up.

My old transformer is attached to the breaker panel. The first point of confusion is that the old transformer has only two black wires coming out of it, which doesn’t match the video tutorials showing three wires so it’s unclear to me how to attach the 3 wires coming out of the new transformer (green, white, black) I purchased because I cannot match them up with the existing set-up.

Second point of confusion is I can’t figure out how to detach the old transformer because the wires run directly from inside the old transformer to (1) a breaker switch inside the panel and (2) to a metal part inside the panel. The tutorials I watched showed wire nuts holding the connection from the old transformer, and it’s a simple matter of unscrewing the wire nuts and attaching the new transformer via the same wire nuts. But there are no wire nuts here. Am I supposed to cut the two black wires to remove the old transformer?enter image description here

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    STOP! Did you shut-off the main feed breaker at the top of this panel? If so, then remove the wires by unscrewing the terminal screws they are connected to. If you are unsure whether this panel is de-energized, then you should call an electrician.
    – Glen Yates
    Aug 11, 2023 at 14:53
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    @manassehkatz-Moving2Codidact Sure, but currently OP thinks his best option is to cut the wires, so I think its best to err on the side of caution here.
    – Glen Yates
    Aug 11, 2023 at 15:11
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    @manassehkatz-Moving2Codidact I agree with Glen Yates. Even with some experience I turn off the main when working in the panel. I imagine some of us have had oops when working on a switch/outlet we thought was dead or thought we would be careful enough to work with it live.
    – crip659
    Aug 11, 2023 at 15:17
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    Yeah, total amateur here, caution is warranted. I did turn off the main breaker and all power before doing anything. Also have a volt tester to check if anything is live before touching, per all the video tutorials I watched.
    – user171779
    Aug 11, 2023 at 15:24
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    The condition of that neutral bar looks suspicious to me. Is there any evidence of any moisture getting in there?
    – brhans
    Aug 11, 2023 at 15:52

3 Answers 3

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The most common setup is a junction box with wire nuts. However, in your case the original installer did a mostly good thing by installing it on the panel. The only bad part, in my opinion, is that they left the black wire as is (didn't mark it to indicate neutral, which is normally white).

Removal is easy:

  • Turn off the breaker
  • Remove the low-voltage wires (probably screw terminals, but can't see) from the transformer
  • Loosen the screw on the breaker and remove the hot wire
  • Loosen the screw on the neutral bar and remove the neutral wire
  • Remove the transformer from the panel

For the new transformer, you do the above in reverse. For the wiring:

  • Black to breaker
  • White to neutral bar
  • Green to ground bar or neutral bar. You can't always put grounds on the neutral bar, but since there are already many grounds on the neutral bar, it is presumably a true main panel where that is allowed.

Your existing transformer is grounded by contact with the panel. Your new one will most likely be grounded that way as well. The green wire is provided to make sure it is grounded in case it is installed on a plastic box. However, unless the new transformer directions say that the green wire does not need to be connected when used with a grounded metal box, you should still connect the green wire.

Note that typically you can put multiple ground wires under a single screw but not multiple neutral wires. Check the panel label. I think yours is similar to mine (definitely same brand based on breaker handle color), in which case you can put up to 3 small grounds on one screw. But the label is the rule, not some random person on the internet.

There is a second option, but really if you 100% just can't get the neutral bar screw undone:

  • Turn off the breaker
  • Remove the low-voltage wires (probably screw terminals, but can't see) from the transformer
  • Loosen the screw on the breaker and remove the hot wire
  • Cut the neutral wire as close as possible to the transformer
  • Strip the transformer end of the wire to an appropriate length for use with a wire nut
  • Wrap white electrical tape over the entire black portion of the wire (i.e., everything except the part you stripped).
  • Remove the transformer from the panel

Then when you install the new transformer:

  • Black to breaker
  • White to old neutral wire with an appropriately sized wire nut
  • Green to ground bar or neutral bar. You can't always put grounds on the neutral bar, but since there are already many grounds on the neutral bar, it is presumably a true main panel where that is allowed.

However, I would be concerned about the condition of the neutral bar for both current use (if the corrosion affects the connections) and future use (if one screw won't come out, you will probably have problems with other screws on the bar).

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    That's a CH panel, so it allows at least double and maybe triple grounds. Do note they have to be the same gauge, and the label rules over all.
    – KMJ
    Aug 11, 2023 at 15:13
  • Thanks for writing out the explicit steps. I tried to unscrew to loosen the existing wires, but the screw tops are so rusted and corroded they won’t loosen. Is it dangerous to cut the wire?
    – user171779
    Aug 11, 2023 at 15:18
  • The breaker screw does not look rusted at all. If it did then I would replace the breaker (and for that I would turn off the main breaker first - my electrician doesn't, but he also makes me stand back when he does that...). So I assume you are referring to the neutral bar. In theory you could cut it, but then you've lost that spot on the neutral bar. For best results, make sure you use the proper sized square/Robertson drive bit. If you use a philips bit it will mostly work but not as effectively, particularly if it is a little rusted. Aug 11, 2023 at 15:32
  • Yes, I was referring to the neutral bar. Almost all the screws on the bar look rusted. Good tip, I have been using a Philips bit and I will now try to look for the proper sized bit. Thank you!
    – user171779
    Aug 11, 2023 at 15:35
  • I learned from my electrician that basically all modern receptacles, breakers, neutral/ground bars, etc. use one particular square/Robertson bit. There is an ongoing debate as to which is the correct terminology (they are nearly identical) and I just don't remember the particular size (but all the small stuff uses the same size). Aug 11, 2023 at 15:43
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To remove the old transformer:

With the power off, carefully remove the black wire from the breaker, (circled in your pic).

Remove the other black wire from the neutral bar.

Remove the old transformer. REMEMBER THE BOX STILL HAS POWER, work carefully.

To install the new transformer:

Mount the new unit to the side of the breaker box as per instructions.

Attach the neutral (white) wire to the neutral bar

Attach the ground (green) wire to the neutral bar. ( grounds and neutrals can share the same bar in the main breaker box. The old transformer did not have a ground wire because it relied on the mounting to the box to be the ground.)

Attach the black wire to the breaker, ( same breaker as the old unit)

Turn the breaker back on. Done!

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They just shorten the circuit up a lot.

Seeing it is inside the panel, it will be safer to turn off the main breaker. Usually the big one on top of all the smaller ones. It should have marking saying it is a 100 or more amps.

Withe the main breaker off just unscrew the old wires.

The new black wire goes to the breaker. The white wire and the ground wire goes with the the white/ground wires. The white wire usually has it's own screw. The ground wires can usually be double up under a screw.

Turn on the main breaker and you should be good.

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