How do you calculate bus bar voltage drop?

I believe there is no simple way to calculate this based just on the transformer structure; this is best calculated once the transformer is built, on the basis of actual measurements with currents. You would have to measure input and output

currents and voltages, calculate the input and output power, and divide output power by input power. This might give a number like, say, 0.98. Subtract this from 1 (1 - 0.98 in this example), and you have the transformer loss (in this example, 0.02, or 2%).