Greg,

First you must calculate the Duty Cycle of the load you will be placing on the equipment. That formula is:

DC={(N x t)/(60 x f)} x 100

N = Production Rate (transformer energizations/min)
t = weld time
f = frequency (power line frequency I.E. 60 Hz)

Then calculate the kVA demand of of the load. That formula is:

kVA-demand = (secondary voltage X welding current) / 1000

It is important to rememeber that transformers are rated at 50% DC in a 1 minute integrating period. So, the kVA 50% DC would be:

KVA-demand X sqrt(DC / 50)

Hope this helps!