The main breaker panel that we are putting the disconnect in front of probably did not have enough interrupting rating either... Have to check it out tomorrow. Maybe POCO can tell me how much? It is a 750kva, I have no idea what impedance
You will also need to know conductor type, size, number of parallel conductors and length, magnetic vs non magnetic raceway does have some impact as well, then there also is motor contribution to factor in during a fault.
Two biggest things that reduces available fault current at a particular point of a system though is conductor size and length. Many times just an additional twenty feet or so of conductor will reduce fault current levels significant enough if you have equipment that is questionable if it is within ratings. That often results in a transformer right outside a building resulting in too much fault current but move it so it is a little further out and you are OK.
Easier to deal with of course at installation time than after it is done.
