My newly installed Ring Doorbell Pro 2 wasn't functioning properly due to low voltage and Ring sent me a new transformer (specific product here here). When I went to remove the doorbell chime from the wall I saw that the transformer wasn't located behind the chime. I believe I was able to track the wire to the circuit breaker and identify the existed doorbell transformer (see attached photo below).

existing configuration of doorbell transformer that is to be replaced

I ended up not proceeding with the installation because (1) I wasn't 100% sure if what I had identified was the appropriate transformer and (2) the existing transformer only had 2 wires coming out of it while the new transformer had an additional ground wire.

I am seeking guidance on how to complete this installation. Specifically, I am looking for additional confidence that I identified the appropriate transformer to replace and I trying to understand how to account for the "extra" wire in the new transformer. I'm under the impression white would go to white and black would go to black I just don't know how I'd handle the green wire from the new transformer. I'd also like to know if electrical tape is required given the videos I've seen only use the wire caps.

I've looked into other threads I wasn't satisfied with the responses, so I decided to post an independent thread.

  • it probably has two voltage outputs, can you measure it
    – Traveler
    Aug 1, 2023 at 17:20

2 Answers 2


A way to test the transformer to see if it is the doorbell transformer rather than the HVAC transformer (before disconnecting anything) is to clip or touch the leads of a multimeter (in a/c voltage mode) to the output terminals of the transformer.

You will read a constant a/c voltage of about 20 V. Then have someone ring the doorbell while you observe the a/c voltage reading. If the voltage dips momentarily, then this transformer is powering the doorbell. Mine momentarily dropped from 20 V to 14 V when the doorbell was rung.

I believe that it is unlikely that the same transformer powers both the doorbell and the HVAC low voltage thermostat wire. But to test this, disconnect one wire from the low voltage side of the transformer, and confirm that the doorbell no longer rings when the button is pressed but the HVAC continues to cycle off and on in response to the thermostat.

If there is only one set of wires connected to the low voltage side (~20 V screws), then this would indicate that the transformer is only powering one system, but you could confirm that when you have the transformer disconnected (either one wire or both wires) the HVAC thermostat is controlling the HVAC system. This is assumming you have a central HVAC.

  • Thank you for the detailed response! Few quick follow-up questions if possible (1) should the power be on when I perform the multimeter test? (2) I don't have an HVAC system it's a boiler / thermostat / baseboard heating. I believe our thermostat is battery powered (3) for the second test do I need to perform if I don't have HVAC? If yes, do I do that test with power to the breaker on or off?
    – AndrewC
    Aug 1, 2023 at 19:01
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    Yes the power must be on when you perform the multimeter test. You are testing the doorbell under operating conditions. Since you do not have a central HVAC, you do not have to perform further tests. If you do not want to mess with a voltage measurement just disconnect one of the low voltage wires from the transformer without letting it contact anything else. Bend the wire away so it is not in contact with anythinng. Now press the doorbell button (with the power on). If the doorbell fails to ring, then you know this is the transformer for the doorbell. Aug 1, 2023 at 19:13
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    The green ground wire is supposed to be connected to the metal box. Did your metal box have a cover on it or is it just open? One easy way to connect the ground to the box is with a little clip that hold s the ground and slips over the edge of the metal box. If you have a cover, you could just hang the bare end of ground over the edge of the box and screw the top down on it. Aug 1, 2023 at 19:21
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    Andrew, a multi meter is a good thing to have, but you don't need one to do this installation. If you are going to get one, get a decent one like a Fluke brand. What you do need to be safe is a non-contact voltage tester to verify that power is off when you think you have turned it off. Aug 1, 2023 at 19:24
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    That connection should work. Aug 8, 2023 at 15:59

The new transformer shows a black, white, and a green wire. The colour green(or bare) is only allowed to be used for ground.

Having a green wire is only there for your safety. It does not change how the device works.

It is not recommended, but not using/connecting the green wire to ground will not affect how the device works.

Do turn off the breaker/s providing power to that box and check there is no power in the box before sticking your fingers in it. There might be a second breaker for that box, that you do not want to find out the hard way.

  • 4
    If it is a typical box-mountable transformer (like the old one, except the old one isn't mounted properly) then transformer case to metal box takes care of the ground whether the wire is connected or not. It doesn't look like there is a practical way to mount the transformer on the top or side or bottom because of the limited clearance. But should be able to use a cover like this one - just check the size of the box and the transformer knockout size requirement. Aug 1, 2023 at 18:13
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    @manassehkatz-Moving2Codidact According to the installation, you just use a screw to mount it to a box. It looks like they use the same screw holes as what is used to hold that cover onto the box.
    – crip659
    Aug 1, 2023 at 18:35
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    There is not enough clearance to screw the new trnsformer to the hole the wires are going through right now. But you could get a short 90 deg bend fitting that would connect to that hole and allow the transformer wires to go through that hole. This would make a mouse proof installation. If you would get new cover with a knockout in the center, then you would need to get a pug or cap to cover the original hole. This is in order to make the installation mouse proof. Of course the original installation is not mouse proof. Ideally you want to prevent any mice getting to the wires. Aug 1, 2023 at 20:00
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    The old one is improperly installed hanging out of a hole on top of the box. It needs to be attached to one of the knockouts. There are usually knockouts in top (in use, no space), bottom (in use, no space), sides (not enough clearance) and back (not accessible). So that leaves the front. Since the box is only a junction box (the same type of box can be used for switches, receptacles, etc.) instead of using a plain cover, use a cover with a knockout and attach the new transformer to that knockout. Ground is automatic metal to metal. "Ground to neutral bar" refers to a panel, not a box. Aug 1, 2023 at 20:00
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    @AndrewC According to the installation instructions, you can use a knockout hole in the box or cover, or the transformer has a tab with a screw to mount the transformer to one of the screw holes in the box corners.
    – crip659
    Aug 1, 2023 at 20:05

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