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I’m wiring a new smart doorbell camera (Eufy brand) It’s intended to replace an existing doorbell, but I don’t have one, so I will be wiring it from scratch. The manual says “make sure the transformer is delivering 16-24V, 30VA of power.”

The doorbell transformers for sale in my local hardware store show slightly different specs. And of course I don’t fully understand them. I see why we need electricians! But I’m wondering if you can tell me which transformer would be compatible with my doorbell cam so I can buy the right one.

There are 2 doorbell transformers for sale at Home Hardware (in Ontario, Canada)

Transformer #1 reads = 8/16/24 Trivolt Doorbell Transformer * Primary: 120 VAC * Secondary: 8 VAC/10 Watts, 16 VAC/10 Watts, 24 VAC/20 Watts

Transformer #2 reads: 16 Volt 10 Watt Doorbell Transformer * For use with wired doorbells * Transformer-16V/10W * Primary: 120 VAC * Secondary: 16 VAC, 10 Watts

I just don’t see the ‘30VA’ on the package of either of these, and I don’t know how to get that out of the specs provided. I also read an article here:

https://medium.com/@johndavidback/a-lesson-about-doorbell-installation-e5a3b8ba01c9

where the guy tried hooking his doorbell up on his own and ended up frying it after 2 weeks. These fancy gadgets aren’t cheap so I want to do it right.

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OK, you need a transformer that delivers 16-24 volts and 30 VA. The 30 VA is equal to 30 watts. The first transformer you're looking at have windings that will give you three different voltages, depending on what voltage you need. None of the configurations deliver 30 watts or 30 VA. The second transformer gives one voltage, 16 Volts and 10 watts or 10 VA. Keep looking for a transformer that will deliver what your specifications call for. the smart video systems require more power than the old hard wired ones.

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    With 30 being the requirement that is the minimum needed. Going to 40 va / watts at 24v will be easier to find as it is a bit more common. Will this waste power? No the doorbell will only use what is needed your house has many times your needed load available but you only pay for what you use. – Ed Beal May 6 '20 at 20:46
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Roughly speaking, VA = Watts. So neither of these transformers are adequate since since is 20W and the other is 10W.

Strictly speaking, VA = V * I where P (in Watts) is V * I. What's the difference? It has to do with the reactive nature of some loads and the most common example of this is a motor. A common induction motor presents a "complex" load to the power supply where part of the load appears as resistive and another part appears as inductive.

The resistive part of the load is actually where the power goes while the inductive part literally swaps its power back and forth with the power company. In an ideal world this could be ignored but a portion of that swapping current going back and forth ends up heating up the wires and other components and disappears. Residential users rarely consider this aspect called POWER FACTOR but industrial/commercial users do since the power company will charge a premium for exceeding a certain power factor level.

If you look at the specs for an induction motor you will often see their power rated in VA instead of W. The VA is the "complex" power where the W is the "real" power.

In your case I can't imagine the doorbell has much of a "complex" loading so for all intents and purposes it needs 30W.

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  • Thanks! I did not know that VA and Watts were pretty much the same. Ok, I just discovered I have a transformer on my panel already. I remember it’s from an alarm system that I took down when I bought the house. It’s an “ATC-Frost” transformer and it says “Input 120v, 60Hz, Output 16.5v, 37VA” There’s a date stamped on it and the year is 1999. First, is this compatible? Second should I risk using an old transformer, that was not installed by me. It does seem like most people are just swapping out their old doorbell when they do these installations. Thanks again! – Banana May 6 '20 at 20:04
  • @Banana Yes, that transformer is compatible. Check the voltage just to make sure. Yes, most people just swap out their old doorbell and then they come to this site and ask why their doorbells stopped after a few days. – JACK May 6 '20 at 20:26
  • Thanks! So I will just need a multimeter, and check that it is indeed 16.5v as the label states? – Banana May 6 '20 at 20:48

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