First thing that comes to mind is unbalanced load.
I'd second that!.
Neutrals in 3 phase wye systems carry the (usually minimal) out-of-balance current and unless there is a gross imbalance of the loads, shouldn't usually be a problem.
However, one situation where I have seen a Neutral get so hot the PVC insulation melted off it, was where an Electrician used a Neutral that was only half the size that it should have been, in a 3wire 230 system over here.
[This message has been edited by Trumpy (edited 10-11-2004).]