Transformer Impedance Tolerance

[TSun]

Does anyone know whether ANSI constructed transformers have a tolerance associated with the impedance value stamped on the nameplate?

I am aware ANSI standards allow a +/- 7.5% design tolerance and that UL constructed transformers can have an impedance of +/-10% of the value stamped on the nameplate. Would either of these two tolerances still be applicable to the tested value stamped on the transformer nameplate?

I am asking in regards to using the nameplate impedance in short circuit calculations.

 

[TSun]

I should have clarified that I am mostly interested in two winding, 3 phase, 15kV class transformers between 250kVA and 5MVA. Both oil filled and dry type.

 

[prc]

If you are asking whether the impedance will vary with service - No. The impedance punched on name plate is the actual measured value at principal tapping.Impedance can be different at other tappings.But for the range of transformers mentioned by you the variation can be will not be significant.