With the advent of switched power supplies, the neutral current is no longer just the unbalance of the 3 phases- current on the neutral ends up being, in the worst case, essentially a square wave with an RMS value close to to the peak current on the primary conductors. So, if you've got a 100A 3-phase 60Hz circuit, you neutral could actually be seeing 173A 180Hz current with the 3rd harmonic alone. In the real world, it should never actually be this high, but it's very hard to predict, and because the neutral is unprotected from harmonic current, it's critical to err on the side of caution; I've seen 150% neutrals specced out on several installations.
The same is true of transformers- the delta primary in a delta/wye tends to trap circulating current, and thus must also be rated (or derated) to handle the harmonic current.
I would not be the least bit surprised if future revisions of the NEC require OCP on the neutral as well as the ungrounded conductors.
I'm not aware of any single-phase switched power supplies that would require an oversized nuetral.
Edit: I neglected to consider that the distorted wave on the 100A current conductors would allow higher peak current through OCP than if it were undistorted. 200% is generally regarded as the worst-case scenario, and I'm inclined to believe it! 245% is the theoretical maximum, but you're never going to see that in the real world. Neutrals are mostly impacted by odd triplen harmonics- 3, 9 and 15.
[This message has been edited by SteveFehr (edited 02-26-2007).]
[This message has been edited by SteveFehr (edited 02-26-2007).]
