Resistance welding transformer primaries are wound with many turns in order to produce the desired turns ratio. This in turn produces the conversion of the high voltage and low amperage in the primary to the desired low voltage and high amperage in the secondary. The winding material is copper and is required to conduct both current and heat at the amperage the transformer is designed for.
SUBASSEMBLY OF A TRANSFORMER
Calculation of the winding gauge size is a manufacturing question and is beyond the scope of this forum.
Contact a resistance welding transformer manufacturer for additional assistance.