% Impedance

I can not tell you the exact percentage impedance amount but the following information may help you.

please check the transformer nameplates and it is usually marked on most nameplates. This percentage impedance is because of the winding resistance and leakage reactance at voltage drop on full load.
you have to do short-circuit test to measure this percentage impedance.
If you want to measure the percentage impedance secondary side of transformer, it must be shorted. The voltage on the primary side is increased step by step from zero to till the secondary current reaches the transformer's rate value.
This percentage of transformer can be calculated with the following formula

Z% = ( Impedance Voltage/ Rated Voltage ) * 100

Suppose, you have a transformer with a primary rating of 110v which requires a voltage of 10v to circulate the rated current in the short circuit secondary would have an impedance of 9%.

It is well explained answer, but why the percentage impedance is needed to be known ? useful for what ?

Yes,It determines the maximum current value which flows during fault condition and this percentage impedance of transformer has a great effect on system fault level.

Thanks,so what is the formula used to calculate the short circuit current if the percent impedance is known ?

Please see these references.

http://www.bussmann.com/library/docs/EPR_booklet.pdf

http://www.brainfiller.com/documents/TransformerandSourceImpedance_000.pdf

%Z will probably be in the range of 10-13%, typically closer to 10% unless specified otherwise. Some users (utilities especially) will specify a higher %Z value when manufactured if the system fault duty is high. The manufacturer can adjust for the desired impedance in their design.

Are you refering a step-up transformer?