I am considering getting a Nest Hello doorbell. The requirements state:

Nest Hello needs to be powered by wires delivering 16 - 24 V AC, and at least 10 VA (in North America)

I had a brief look at the electrical panel, and I see that there are two transformersLeft: 16V-10VA, Right: 10V-5VA

The one on the left (with red, green, yellow wires) is a 16V-10VA transformer. The one on the right (with black and white wires) is 10V-5VA.

As far as I suspect (and I haven't confirmed yet), the right one is the one that is likely used for the doorbell. So obviously using that with Nest Hello won't work.

My question: do I need to get a new transformer to replace the existing 10V-5VA, or can I just move the black and white wires over onto the left transformer?

  • Can you follow the wires leaving these transformers and learn anything?
    – Tyson
    Feb 17, 2019 at 18:14

3 Answers 3


The other transformer is for some other system - another doorbell, an alarm system or (small possibility) HVAC. So even if you can move the wires and power the NEST with that transformer, it is possible that (a) the combined loads of the two systems may be too much for the transformer, as it is rated at exactly the value recommended for the NEST by itself, and (b) there could be some unexpected interaction between the two systems - i.e., one switching on/off might somehow induce a problem into the other one.

Transformers are cheap. A quick search finds them at around $10 on Amazon, and you may be able to get one in a regular store (which has overhead but doesn't have to build in the Free Shipping to every little product) for less. Just make sure it is UL Listed as one side of the transformer is connecting to 120V AC.

  • 1
    It's not an HVAC transformer, they are always 24v.
    – Joe Fala
    Feb 17, 2019 at 17:09
  • 1
    It's probably doorbell and alarm. Or doorbell and old doorbell.
    – Joe Fala
    Feb 17, 2019 at 17:11
  • Thanks. You're right - I just checked and 24V is near (but not absolute) universal for HVAC. Feb 17, 2019 at 17:12
  • 1
    Also HVAC transformers are not located on the panel. Almost always in the mechanical room and usually within the appliance itself.
    – Joe Fala
    Feb 17, 2019 at 17:20
  • @JoeFala As far as "in the mechanical room" - that could well be the case here since we only have one small picture (in my house everything is in one small room - in my parents' old house they were two totally separate rooms). "usually" is the operative word - there have been plenty of questions regarding situations where the transformers are separate. But end result - I agree it is more likely a transformer for "something else". But my concern about running both devices (whatever they are) on one transformer remains. Feb 17, 2019 at 17:27

Just search 24v plug in transformer there is usually a receptacle by the electrical panel and you can use it for the plug in transformer. Very easy. No need to go into the panel. https://www.google.com/aclk?sa=L&ai=DChcSEwim8N3_o8PgAhWUW4YKHT4zAO8YABAgGgJ2dQ&sig=AOD64_1fkgjEftgWpuxK-u5OL4KaoDlvgw&ctype=5&q=&ved=2ahUKEwjRt8z_o8PgAhVnqlkKHZ7WCpYQwg96BAgLECI&adurl=

  • Very often there are not any spare receptacles near the electrical panel - or there are, but the FIOS or Cable box or some other device has already grabbed that convenient receptacle. But a straight replacement here should be easy enough. Feb 17, 2019 at 17:28

You don't need to guess. You can test.

If you temporarily disconnect the wire(s) on one terminal of a transformer, it will break whatever that transformer is powering. You can then go search the house for what broke. Putting the wire back will fix it, obviously.

Once you know what those things power, the answers become a lot easier. Obviously if it's the 16V 10VA transformer, you should be good to go.

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