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RE: You sure about that?...

Posted by sayntjack on January 10, 2017 at 12:57:56:
This is pretty basic stuff. To establish the rated full load current of the windings of a two winding single phase transformer, take the VA rating of the transformer and divide it by the rated winding voltage.
For example, for a 50VA transformer with a 120V primary and a 6.3 volt secondary, the rated full load currents are: Primary, 50/120 = 0.42A, Secondary, 50/6.3 = 7.9A.
So, if you wanted to make a buck transformer out of this by connecting the secondary in series with the primary in subtractive polarity (120-6.3 vs 120+6.3) and you needed it to be capable of supplying 3 amps to your load then the transformer would need to be rated 360VA minimum (120V x 3A) to have a primary winding capable of 3 amps.
To verify, I took a 135VA transformer, 120V pri, 6.3V sec. With my line voltage I got 128V additive polarity & 114V subtractive. Wired in subtractive polarity I connected a 60W light bulb and measured 474mA in the wire to the bulb. In the incoming wire to the transformer primary from the wall outlet I measured 480mA - a little higher as expected because of the transformer magnetizing current.