Selection of Neutral Size for Different Load Applications

These threads/links may help:

http://cr4.globalspec.com/thread/43708

http://cr4.globalspec.com/thread/52474

- MS

Not to ignore the level of unbalance in the single phase load distribution and utilisation amongst the three phases.

mostly as you said third harmonic is the major cause of heating of the neutral.......and then the unbalance single phase loading.

Earlier half size neutrals were there because the installing electrician cut costs. Consider what would happen to the half size neutral if you had a fault on the "full size" conductor and the half size had to carry that fault current?

The neutral cable should be at least the size of the conductors. However, if you do have a system saturated with harmonic currents, you should calculate the neutral size (Some installers double the size of the neutral as a rule of thumb).