I have a question on residential load calculation.Do you use a window ac unit as the largest motor in the standard calculation if it's larger than all of the other motors
It would stand to reason. The largest is the largest. The tricky part is that an A/C unit might have several things drawing power, and it might even have more than one motor. If you don't know exactly what part of the total load is the single largest motor, then you are stuck with using the entire A/C load as though it were one motor.