I'm sure others can provide better insight, but heres my take:
In a balanced 3-phase, 4-wire, wye system, the neutral currents cancel each other out and the result is zero current flow on the neutral conductor. Increased neutral currents can result from equipment that draws an asymmetrical current wave, such as computer equipment or SCR driven equipment. These currents are additive as opposed to canceling out and can exceed the ampacity of standard sized neutrals. The same problem may arise with certain ballast loads due to 3rd-order harmonics, which also are additive.
The solutions can include individual neutrals for each circuit, or a single oversized neutral with adequate ampacity.
