Unless you work for the utility, and unless you are talking about the service transformer, you don?t get to use any factor of this type. If (as I suspect) you are talking about a transformer that is within a building and that supplies a portion of that building?s load, then you must perform a calculation to determine the amount of load that it will supply. The NEC gives some demand factors that allow you, under specific and limited circumstances, to reduce the amount of calculated load. This is to take advantage of the probability that not everything that is connected will be running at the same time.
But once you use all the allowable demand factors, then you must select a transformer that has a rating at least as high as your calculated load. You don?t get to tack on any ?diversity factor? or any other factor at this point.
