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Connecting Transformers in Parallel
I had an old-timer tell me that you can never connect two transformers together because they will “fight one another”.
If you are anything like me (and heaven help you if you are), whenever someone says something like that, a cartoon in your head starts playing.
In this case, I imagine two transformers with boxing gloves on duking it out to see which one “wins”.
The truth is you can connect two transformers together so long as you are careful, but you need to know why you're doing it and then do it properly.
Transformers have a VA (Volt-amps) rating that dictates how many volt-amps (volts x amps, which is watts simplistically but there is a more complicated reason it is called VA in transformers that we won't get into here) the transformer can handle on the secondary.
Above we show two 75VA transformers with 24V secondary windings.
So with a 75VA transformer, you can run a maximum of 3.125 Amps, if you needed more power you would need to either go get a larger, more expensive transformer or…. you could connect another identical one in parallel. If you connected two 75VA transformers in parallel you would then have 150VA of secondary capacity which can be necessary in some cases with multistage commercial units or some large accessories.
In this case, parallel simply means connecting the two primary and secondary windings together in the exact same way as we show above… Pretty easy
It is SUPER important to get the polarity exactly the same and use two transformers with identical winding turns in the primary and secondary and identical secondary coil impedance (resistance).
In fact, it is so important that I advise that you only do this if you have two identical model transformers.
To be even safer, connect the primary windings first and check the secondary's against one another with a voltmeter before actually connecting them to the system. For a typical 24v secondary you can connect the two common wires to ground to act as a stable reference first then check the two R or Hot side leads to one another and then to common. They should read 0v to one another and 24v to common. If you get anything other than 0v from hot to hot then you want to recheck your primary wiring and ensure that they are exactly the same.