I'm trying to debug why one of my doorbells isn't working, but to keep things simple, today I'm just trying to understand the reasoning and implications of what I see at the electrical box.

Background: The house has two doorbells; one original location (1967) and one added with an expansion (1980s?). When I moved into the house in 2019 I installed a Ring Doorbell Pro at the Expansion door; it worked; I was happy. The other doorbell does not work, although there is electricity in the wires at the doorbell (wires spark / read 12-20V with a multimeter).

Looking at the electrical box I see two transformers attached to the side. They appear to be wired in parallel? The silver transformer is marked 16V and the other isn't clear. There is only one wire pair (red/white) leaving the transformers and going off into the house. (Click for larger version)

Two transformers connected

So my questions are as follows:

  1. What is the effect of this wiring setup? Higher current?
  2. Why would two doorbell transformers be wired together like this? Cheaper than buying a bigger one when the expansion was added?

3 Answers 3


You mention serial transformers in the title and parallel in the text so we'll cover both. Transformers of the same type in series would double the voltage but have the same current. Those same transformers in parallel would have the same voltage but double the current. You could find either scenario in doorbell situations, although not the best way to do it.

Your two are more than likely wired in parallel due to the one transformer being 16 Volts and your multimeter readings being 12 to 20 volts. You'd be better off taking your reading at each transformer after isolating each one to get more accuracy as 12 to 20 V is a big difference. This would also insure each transformer is working. Then you could better determine series or parallel.

Whoever did this tried to save a buck, and didn't get the proper size for the chimes they purchased or had a voltage drop problem.

  • 1
    I've removed 'serial' from the subject to limit confusion for people searching/reading this later, since it does appear to be a parallel setup. Thank you for covering both, though!
    – gowenfawr
    Commented Jul 10, 2020 at 14:28

Yes, adding a second transformer to make higher current available would be cheaper than buying a bigger transformer. But not by much, likely done because that is what someone had available.


Ok if this were DC the answers would be correct but this is ac and even 1 wind difference creates a phase imbalance, since they happen to be on the same side it is not a direct short but this is the wrong thing to do with an AC transformers. It is both dangerous and not energy efficient. The proper thing would be to get a new transformer That is properly rated.

Adding the 2 kva values will be more than is needed at the same voltage.

This is bad and if your home burned this may be cited as a possible cause if the fire started in this area.

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