Transformer Impedance

What I think you are asking is what the loop impedance would be if you had really short leads to the transformer. That you can work out from the regulation, which represents the voltage drop at full load. (so a 2.5% regulation would mean 2.5 volts in every hundred drop at full load, the 400V phase to phase would fall by 10V and the phase to neutral voltage would fall by 230*2.5 = 5.75V )

You could work that back to however many ohms at the load current, but be aware that it is not simply resistive- if it was transformers would need rather larger cooling fins than they do already - it is best modeled as a mixture of series L and R to represent losses and the un-linked magnetic flux. In general the magnetic losses do not change much between on and off load, while the resistive (winding )losses are load dependent.

If you can see the transformer rating plate you can get the % reg ulation rating direct but to estimate between 2% and 4% is a good start.

Modern ones will meet GN35-1 but there are plenty in service that pre-date that.

Mike.

IEC 60076-5 recognises a minimum short-circuit impedance of 4.0% for 25-630kVA and 5.0% for 631-1250kVA for two-winding transformers at nominal tap, with a +/-10% tolerance

Actual impedance specified by DNOs may vary, and the upstream network fault level is sometimes signifcant.

However I think what you really want is table F5 of the IET book Calculations for Electricians and Designers