In general, the NEC requires sizing based on connected load. Demand factors are permitted in many situations.
Perhaps the most commonly used exception is
Noncoincident Loads
"Where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest load(s) that will be used at one time, in computing the total load of a feeder or service.
Suggest a careful reading of Article 220 would be appropriate if the NEC applies in your area.
