Yes, but why? Most modern homes now have many solid state switching devices in all the microelctronics in every appliance, computers, etc. These cause the neutral to have harmonics that can overheat the neutral. You will save very little by downsizing the neutral and may cost more to you and the homeowner to replace it later.
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.
Yes you can if you follow the above advice but as Warren1 said WHY. The cost saved will not pay for the trouble calls later. I have a customer where this was done in an apartment complex. Most of the residents who use computers there are having problems from monitors that will not go full screen to lost data and very slow response times. The owner will not pay for ugrading the neutrals, wants me to bill the outstate electrician who built them. That company will not pay for that. The tenants are not going to pay for it either. Can't fix the problem without repulling the feeders. It a no win situation. It may go to a lawsuit. With more and more electronic items going into houses,apartments, and other places this problem will just get worse. Downsizing a neutral is not worth the hassle now when you justify to the inspector, or later when problems occur.
Sam; We ran into this problem with an air-tool manufactureer, in Cleveland. We wound up installing a special shielded transformer, re-pulled every circuit on the computer network, 208/120, 3-phase oversized the neutral in all cases had a special panelboard made just for the problem and even installed insulated/isolated gronding receptacles. And for all our trouble we eliminated about 95% of the problem. Which was a huge improvement. It is true to correct a problem like this it is costly and with DC/AC drives there is a similar problem but different order harmonics. However I have never seen this problem in a residence.
While I agree that down sizing the neutral is not worth it in most cases I would have to say that you have bigger problems than an undersized neutral in your apartment complex. Third order harmonics, the type generated by computer power supplies, do not multiply in 240/120V single-phase systems as they do in 208/120V systems. Also, oversized neutrals mitigate heating problems. They do not cancel harmonics in the system. They merely create the capacity to handle them.
Warren, The harmonics caused by non-linear loads and or switching power supplies do not cause problems with neutrals on single phase systems. These harmonic currents only add on three phase systems. Don(resqcapt19)