%IZ vs %Z


Senior Member
IZ% Percent impedance. Voltage drop due to impedance at rated current expressed in percent of rated voltage.
The formula to convert to short circuit impedance .... I will look for it
Last edited:


Senior Member
Cleveland, Ohio
Fel Flores
I am hoping to add to your question if you don't mind.

Expanding on original question....
If given IZ, IX, and IR for a specific transformer.
How do you convert ALL into actual Z, X, and R values ?
Are these values always associated with the SECONDARY side ?
In addition to the three values mentioned above, the data sheet also provides 2 resistance values in ohms @20d C.
HV Coil R & LV coil R, is the LV coil R the ACTUAL R value from first question above ?
HV coil R actual LV R reflected to the primary side ?


Senior Member
Vancouver, WA
One and the same

One and the same

%IZ and %Z are the same. When a transformer impedance is measured and scaled to be on a per unit basis, it is expressed as a %. IZ is the voltage rise created by a short circuit of one winding and the voltage impressed on the other winding. Since it is a %, it is the same as %Z.

By measuring the DC resistance of the windings or the power loss from the windings, you can calculate %IR or %R. Since Z is the vector sum of R and X, you can calculate X and the X/R ratio.

Be careful because the % values are based on the OA or base rating of the transformer. For transformers with multiple ratings, 55C vs 65C rise or with and without fans, the % will change depending on the rating used.