Transformer manufacturer standard for %Z

So enter into discussion with the manufacturer over the telephone for further advice, though do turn off CAPS lock before doing so, please; it is considered akin to SHOUTING, and quite rude.

4% to 7%is the short circuit impedance of the transformer as per IEC 60076-5

Usually, for voltage drop calculation we shall take 7.5% more the rated short-circuit impedance.See

IEC 600076-5 4.2.7.4 a) Transformers of categories I and II 5) the short-circuit reactance values, in ohms, evaluated for each phase at the end of the tests, do not differ from the original values by more than: – 7,5 % for transformers with non-circular concentric coils having a short-circuit impedance of 3 % or more. The value of 7,5 % may be reduced by agreement between the manufacturer and the purchaser, but not below 4 %. – 7,5 % for transformers with non-circular concentric coils having a short-circuit impedance of 3 % or more. The value of 7,5 % may be reduced by agreement between the manufacturer and the purchaser, but not below 4 %.

– category I: up to 2 500 kVA;

– category II: 2 501 kVA to 100 000 kVA;

– category III: above 100 000 kVA.