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I'm replacing a door chime transformer with a new 16v 30va transformer to accommodate a new Ring doorbell. The existing transformer is attached in our circuit breaker panel through a knock out at the bottom of the panel. This morning I shut off power to the breaker panel, removed the panel cover, and found that the old transformer has two red wires and no green/ground wire -- the new transformer has a white, black and green/ground.

I'm trying to figure out whether I need to attach the new transformer's green/ground within the breaker panel or can I cap it off. I'm asking because the new ground wire is too short to reach the neutral bus higher up in the panel where I assume it should be attached.

If I do need to attach the new ground wire to the neutral bus, can I/should I extend it? If so, can you tell me an acceptable way to extend -- e.g., can I use a standard wire nut or lever nut, or is there something more I need to do? Any other suggestions as to what to do with that new ground wire? The only other thing nearby the transformer location on the panel is a neighboring panel outlet which shares the circuit with this transformer.

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You should follow the instructions provided with the transformer here. Most I've seen have the threaded portion that goes through the knockout made of plastic, and therefore require the ground wire to be used for bonding. There may be some transformers that can be grounded via the knockout to the metal box directly. The instructions should tell you for sure.

You can make splices inside your breaker panel the same way you would in any other junction box. If you have wire nuts or lever terminals and some wire on hand, use that to extend the ground so you can reach the ground terminal. For the ground, you should only use green or bare copper wire.

And it seems you probably already know this, and you didn't ask, but for the sake of completeness, you will want to make sure the white wire on the new transformer is connected to the neutral bar, and the black to the hot of wherever your previous transformer was connected.

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  • Really helpful -- thank you, and thank you for the additional clarification on the black and white wires. Dec 1 '20 at 14:03
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There is already an excellent answer about the new transformer, but a little explanation about the old transformer may help clarify things a bit:

the old transformer has two red wires and no green/ground wire

In the olden days, grounds were not required. But even when they were, one option was, and still is for some situations, to have the metal case (or yoke in the case of switches) securely fastened to a grounded metal box. This was often the case for transformers mounted directly to either a junction box or, as in this case, to the breaker panel box. Arguably this method of grounding is more reliable than a little wire. Newer transformers are (a) more likely to have plastic cases and (b) more likely to be installed on plastic boxes (simply because plastic boxes have become more common).

As far as two red wires instead of a black & white, a simple transformer does not know or care about hot vs. neutral. In fact, a transformer (but not typically a doorbell transformer in the US) can just as easily work off of 240V as 120V - the only change is that the output voltage is doubled to match the doubling of the input voltage. Provided there is no connection (which there should not be anyway) between either of those wires and the transformer case (which is grounded), it just doesn't matter.

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