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# XFMRs in parallel

Member Posts: 34
I have 2 50VA HVAC control transformers & need to place them in paralell to be used on the same T-stat.How is this done?Thanks in advance for your help!

• Member Posts: 177
To find polarity on unmarked transformers, measure from the hot 120 volt wire to each 24 volt terminal. One will be high and one will be low. The actual numbers are about 120 plus 24 and 120 minus 24; or about 144 and about 96. Mark both transformer low voltage terminals as high or low, and hook like terminals together when making parallel connections (both high sides together and both low sides together. Make sure that they are fed from the same breaker so that they are on the same leg in the panel.
• Member Posts: 23,275
I'm conservative, and I'd put fuses in the secondaries of both transformers... fast blow, rated at the output current of the transformer.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 462
I use this method. Never tried Noels. Just connect the primaries to power and connect one of the ends of each of the secondaries together. Use a meter set to test the voltage between the disconnected ends of the secondaries. If you have them in phase, you should measure very little voltage between the two. If they are out of phase, you will measure around 48vac.

If they are out of phase, you can swap either the primary or secondary connections.

I find it odd that the transformers are not marked. I always phase my stuff to TACO specs, their Zone controllers are always in phase with each other so I just phase to that.
• Member Posts: 3,623
Why do you need two transformers? Wouldn't one 100 VA transformer be easier?
• Member Posts: 2,437
Noel is right and I have used that method, however, I use the simple method. There are only two methods.

The Simple Method:

Wire the primaries of the two transformers together to a 110V ac source. Then connect together one secondary lead from each transformer. The take the other unconnected leads from both transformers and quickly brush them past each other and look at the spark. This must be done quickly as Class 2 transformers have fuse wire in the center of the primary windings and you don't want blow that fuse wire. So don't linger.

If the spark is bright reverse the secondary leads of one transformer and brush again and look to see if the spark is dimmer.

If the spark is bright the two transformers are out of phase. If the spark is dim the two transformers are in phase and should be ganged together with that connection. Simple.
• Member Posts: 2,437
Ya! You sure don't want to connect the 2 transformers to different buss bars. They call the 220V.
• Member Posts: 11,060
edited January 2018
You will have just over 4 amps available for your T-stat.
What are the max amp ratings for your T-stat?
Some are only rated for 1 amp max continuous.

Or is this for pilot duty?
• Member Posts: 23,275
Fuses in the secondaries, dang it! They are cheap. The holders are cheap. The transformers aren't.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 1,048
Brite spark, dim spark, spark spark spark . Had to sorry, just couldn't resist. I prefer the meter method, watch power sources carefully, keep same leg at breaker box.
• Member Posts: 1,048
Brite spark, dim spark, spark spark spark . Had to sorry, just couldn't resist. I prefer the meter method, watch power sources carefully, keep same leg at breaker box.
• Member Posts: 2,437
This is for you Dennis, I know you are a stickler for doing things right. I dug it out of my library.