While I have been experimenting with reverse feeding 208/120V step down control transformers in order to step up 120V 60Hz mains into 208V 60Hz mains to run North American 208V 60Hz only ballasts and Japanese 200V 60Hz ballasts, the transformers I have been using have been outputting different voltages when I was using the 120V secondary winding as the primary winding and the 208V primary winding as the secondary winding. Both transformers are rated at 3kVA for reference.

The first transformer I was using is a Cutler Hammer C340NE. When I measured the output voltage of that transformer on the 208V tap while feeding 120V 60Hz into the 120V tap, I was getting a voltage reading of only about 195V or thereabouts.

The second transformer I was using is a Matra Electric 90034131K. Although it uses 110V and 115V taps in addition to a 120V tap, I was somehow getting an output voltage of about 210V on the 208V tap when I connected the 120V tap to a 120V 60Hz supply.

I wonder why these transformers are giving out noticeably different output voltages on the 208V tap despite both being fed 120V on the 120V tap?

Manufacturers try to compensate for resistive voltage drop by choosing slightly different turn ratios instead of just Vin/Vout, so the transformer outputs slightly increased voltage at no load and some close to the nominal at full load.

When the transformer is used in reverse, this compensation obviously does not work, and actual voltage error depends on what extra correction coefficient manufacturer have chosen. Some cheapo transformers with aluminium windings for example will show a higher drop, because of actual voltage drop over windings resistance, and higher compensation, which is working in this case improperly.