New wording added in the 2008 NEC [Sec. 310.4(D)] requires that you count each ungrounded conductor of a parallel set of conductors as a current-carrying conductor and adjust their ampacities in accordance with Table 310.15(B). You may have to consider the neutral conductor as current-carrying in the presence of triplen harmonics (3rd, 9th, 15th, etc. harmonic currents) or in unbalanced load conditions in 3-phase, 4-wire systems.