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using two 3 phase multi tap autotransformer outputs simultaneously?


Sep 15, 2002
Shop is 240 volt 3 phase. I have two edm machines using separate 3 phase multitap autotransformers to get from 240v up to 380v. Transformer says 12kva, machines say 10kva.

Just added a third edm machine with the same multitap autotransformer setup. Getting the 380v out is no issue. The question is the chiller unit for this new to me machine needs 480v 3 phase where my other older chillers were 240v and were on the shop standard volts.

On the same multitap autotransformer as the 380v edm can I also pull the 480v for the chiller from the 480v taps as long as I don't exceed the total kva or amp rating for the transformer with the combined loads?

I have used an isolation transformer that has 480/240 with a center tap on one phase so I can also get 110v which is in effect pulling multiple voltages from a sort of multi tap scenario but wasn;t sure about the autotransformer multi tap.

The alternative is to investigate converting the 480v chiller to 240v if its dual wound or provide a separate transformer for it.


Aug 16, 2014
bainbridge island
what sort of 480v 3 phase chiller?

if its a 10 ton compressor then the starting amps are going to be in the 120 amp range, this will brown out your edm machines on the same transformer and possibly your whole shop.

I would look into a 480v vfd to run the chiller if needed.


Cast Iron
Sep 25, 2012
I don't know a lot about auto transformers, but I would think you could. I could be wrong.

What I do when I get into a situation like this, I try to call the manufacturer of the unit. Usually I have very good luck getting proper answers from them.


Jun 16, 2001
St Louis
Multiple taps "can be used".

There are details, though.

You can draw as much total as the kVA rating of the transformer, adding up the taps. Since any tap is usually good for the entire kVA, that should not be an issue so long as the total is not more than the rating.

The problem is what was mentioned above, the inrush. That can and will cause "interactions", since the inrush has to come through the impedance of the transformer, and some of that total impedance "links" the two (or more) outputs. A big inrush on one will drop the voltage to another more directly than if you used two separate transformers.

If your usage cannot tolerate that, separate the loads on different transformers, or use a VFD etc to reduce the inrush.