To get precise values of voltage drop you have also to take into consideration the ground temperature, which has an influence on the cable resistance.
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>I am aware of the 12 times for 0.1 sec inrush factor.
>I'm not sure how to apply that to voltage drop.
Well, 3winding, if you know the inrush-current and the impedance of your grid you have to apply the formula:
U = I*Z
