480 transformer to 120 ac current what is the kva on transfomer?

The kVA rating will be listed on the transformer's nameplate, which is usually on the front of the transformer. The 480v to 120v is irrelevant, because many Transformers with different kVA ratings convert 480 volts to 120 volts. The kVA ratings can be different and thus affect the rated current through the transformer.