To calculate the neutral load, it says to add all the120 volts circuits, and take 70% of the range and dryer circuits. Also it shall be possible to apply a demand factor of 75 % for four or more fastened in place appliances,other than range, clothes dryer, space heateing equipment or A/c.
Generally Aluminum SEU cable and the copper come wiht the neutral already downsized.
Somebody referenced switching power supplies inherant to computers, I have only seen this problem on 208/120 3-phase circuits where triplen harmonics are generated and this is usually in offices where many computers are used and transformer shielding and supersized neutrals have to be installed to help eliminate this problem.
The only problem with downsizing a neutral in a residence is future expansion. but circuit balance is also important such that there is as small amount of current on the neutral as possible.
