Everytime i go through this process i reach out to the electrical utility. They have a chart for the infinite buss max available fault for transformers of each voltage type. If its existing building they have the transformer info already, if not you'll need to tell them the anticipated load and service size.
Ask them what the max Available Fault Current (AFC) is at the secondary side of the transformer.
Locate distance from transformer to building disconnect or panel.
Enter in utility provided info, feeder sizes, and number of conductor info into calculator. For distances i would use point to point (hypotenuse) not likely route for installed conductors cause shorter distance is worst case scenario (I do typically add 5' on either end for rise and drop of conductors into equipment).
If your AFC at MDP comes in at 66,340A... 65,000 AIC breaker wont work so someone might over shoot and put 96,000 AIC rated breakers. Personally I would do your own verification on the numbers. Every once in a while I'll be doing a design on an existing building, and see all breakers are called out for 65kAIC to be conservative. Then find out the utility max fault is only 19kA? Technically yes 65KAIC works but pretty overkill. Its like wearing -40° winter gear in the fall cause it might get cold.
One other interesting thing people dont always grasp, the rating of the panel is equal to the rating of the lowest AIC rated breaker in the panel unless series rated combinations are used. Meaning if they have a main breaker thats rated for 96KAIC, but the remaining breakers in that panel are only 42KAIC, 65KAIC, and 50KAIC, that panel is technically only protected to 42KAIC unless the manufacturer tested and approve the branch and main breakers in combination and shows their combined higher rating.
Rarely have I ever seen a project needing higher than 65KAIC but it has happened, once. There were 3 parallel 14kV transformers inside a building with exposed bus. If the utility comes back with a max of 32kA fault, your available fault will only go down from there.
