Transformer backwards

[T.M.Haja Sahib]

The FLC of 480v side is 31.2A and load current is 50A which puts only around 60% overload on the transformer.Depending on the duty cycle of the welder,there may or may not be any loss of life for the transformer.

Now, all that said, a 15kVA single phase transformer will be capable of the following:
240V side, 62.5A FLC
480V side, 31.2A FLC
Your mileage may vary slightly, but less than 5% in most cases.

You cannot get 50A from this transformer, moot point.