I just bought a new doorbell transformer (24V 40VA), to replace for current one (10V 5VA) because I bought a video doorbell.

I'm don't have any experience with electricity but I'm willing to learn (please bear with me). So I was hoping it was going to be an easy swap old for new.

Unfortunately what I see is a transformer going through an electrical outlet and no wire colors. Picture of wiring through transformer

Other side of outlet

My concern is the wiring the new transformer properly. The new transformer new transformer has the Hot(black), Neutral(White), Ground(Green) wiring. When I disconnect the disconnect the old transformer from the sides of the outlet, how can I tell where to connect the Black and White wires? In the picture of the old transformer, they are both black!

Any advice would be appreciated!



UPDATE: Thanks all for your prompt feedback! Just so I am being clear... I was just hoping to replace the old one with this new one. I hope this new diagram better illustrates what I'm thinking: I was thinking of running the the White wire from the new transformer to the silver screw on the outlet. Running the the Black wire from the new transformer to the brass screw on the outlet. Lastly running the Green wire from the new transformer to the ground screw on the outlet (pigtail it?). enter image description here

Do you see any issue(s) with my setup? Thanks again!

  • 1
    Never buy electrical gear from Amazon unless it says "Sold by Amazon.com". In particular, the guy whose company names are all in ALL CAPS is a known Chinese junk-dumper. Further, that UL sticker is missing something important - the UL file number. It's probably fake goods. Any competent HVAC supply house will have a transformer of good provenance. Commented Oct 18, 2020 at 0:55
  • 1
    @harper based on the original install and the proper connection to a box for the new transformer, I would say the new one is much better because of the mounting method under the NEC or not, because the original install did not meet the NEC or close in any way the one linked two at least has separate zones for mains voltage and low voltage as required by code.
    – Ed Beal
    Commented Oct 18, 2020 at 2:06

2 Answers 2


The ac going to the transformer doesn’t matter because the low voltage on the other side is isolated.

However there are several issues as I see it The high voltage between the transformer and metal box the wires are not in flex or rated for being in the open not a big deal but a code violation. Next with the transformer not mounted to the metal it is not grounded so a fault could end up on the low voltage side and not tripping the circuit breaker.

Most door bell transformers are made to mount on a box and the 120v connections are made inside the box, this is the case with your new one You will have knock out one of the slugs on the side of the box )make sure it will fit first) you may have to add a spacer behind the box so the transformer can mount on the side but the mounting creates the ground path I mentioned as the fitting on the side of the box makes it very tight in a single gang box I would wrap electrical tape around the terminals after you connect the wires again the high side the colors don’t matter if they are the same color.

If the receptacle doesn’t look like it will fit you may need to change to a single receptacle and use one of the knockouts on the top side or bottom side (not the ends as the cover usually caused problems on the ends)

The good news is you now have the correct type of transformer to be mounted on the box.

  • I've seen those transformers dangling by the primary wires hanging in attics. lol +1
    – JACK
    Commented Oct 18, 2020 at 0:27
  • Thanks all for your prompt feedback!
    – Marbrus
    Commented Oct 18, 2020 at 2:07
  • 1
    assuming that the old transformer is going away the plastic bit can be removed and the new one fitted in that place. if the hole doesn't line up it might need some wood added behind the box or use a larger box.
    – Jasen
    Commented Oct 18, 2020 at 3:37
  • The old transformer was just screwed to the wood, underneath the electrical outlet. I was planning to put the new one in the same place. I can easily wire straight up into the box through the same holes.
    – Marbrus
    Commented Oct 18, 2020 at 4:01
  • I understand doing it like it was I was providing the safe way to do it , it is not often that transformers fail but when they do in some cases they step up the voltage and damage things if not grounded. I can say I have only seen this a couple of times.
    – Ed Beal
    Commented Oct 18, 2020 at 15:28

NO. This is highly dangerous. That looks like some sort of 3rd world unfused plug socket and to attach a transformer like this is simply not safe. You MUST connect such a transformer via a fused spur for safety. And the cable MUST be double insulated or enclosed. See for instance my Ring install:

enter image description here

  • 1
    Uh, what are you talking about? The receptacle in the question looks like a bog-standard American NEMA 5-15, presumably on a correctly breakered circuit. Is the US now a third world country?
    – TooTea
    Commented Apr 18, 2021 at 12:43
  • UL listed NEC Class 2 transformers are inherently current/power limited -- there's no point in fusing them down from the branch circuit they're on! Commented Apr 18, 2021 at 13:55

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