As an engineer, if I do a design for, say a pumping station, I will perform a Transient Motor Starting (TMS) Analysis on the system in its worst-case scenario to determine the worst-case voltage drop (VD) using SKM PowerTools software.
I will then indicate on the print what I recommend as the minimum size transformer to adequately serve the load.
But, as mentioned in the earlier posts, the Utility provider will install whatever they see fit, knowing that if there ends up being a problem, they can just replace it with the next size up.
I did a pumpstation once that had two (2) 480 V, 75 HP across-the-line started motors. The secondary cable was approximately 250 feet long. I recommended a minimum size of 500 kVA, based on one motor running, and the other one coming on-line.
The Utility installed a 300 kVA transformer, which in the words of the Utility engineer, has been what he's done for years. In this particular case, the motor circuit protectors tripped on startup. I was called out as having undersized the secondary feeder.
So I re-ran my SKM TMS analysis, paranoid as hell that I had screwed up and would be liable for an upsize of the secondary feeder. I increased the number of conductors by a factor of 10, which of course is easy to do in the program. The starting VD was only negligibly improved, so I rested easy that the Utility transformer was undersized.
I was called to task by the Civil engineering firm that I did the design for, so I faxed 10 pages of my TMS analysis to the Utility, defending the selection of a 500 KVA unit. They reluctantly increased the size of the transformer , not to 500 kVA but to 750 kVA since that's all they had in stock and apparently the footprint for the 300 is the same as the 500 and 750.
It resolved the problem.
A conservative criteria to use for the VD is the article 695 fire pump limits which, off the top of my head, is a maximum 15% VD on startup.
A rule of thumb I developed from that experience is that the transformer shouldn't be saturated much past maybe 150% of it's rating during startup. In this case that means 75kVA (running motor) plus 6-8 times 75kVA (starting motor) = 525-675 kVa.
John M