The normal method
of expressing transformer impedance is as a percentage voltage drop in the
transformer at full-load current and this reflects the way in which it is seen by
system designers. For example, an impedance of 10% means that the voltage
drop at full-load current is 10% of the open-circuit voltage, or, alternatively,
neglecting any other impedance in the system, at 10 times full-load current, the
voltage drop in the transformer is equal to the total system voltage.
Formula written by MH is perfect.