Why Neutral size is 200% of the Phase for UPS distribution?

What does TP+2N mean? Is the UPS output single phase 3 wire (2 Line, 1 Neutral) or three phase 4 wire (3 phase 1 Neutral)?

If it is three phase 4 wire system, then wareagle's answer is correct in the consideration that the UPS loads are non-linear which generates harmonic currents. For non-linear loads, the return currents from each of the three phases are in phase in the neutral path for 3^rd harmonic current and so the total 3^rd harmonic neutral current is the arithmetic sum of the three individual 3^rd harmonic phase currents. This is also true for odd multiples of the 3^rd harmonic (9th, 15th, 21st, etc.). Generally, a neutral conductor is sized for 150 percent of the phase current in this case. I would like you to see the previous posting http://cr4.globalspec.com/thread/43708 for details.

Otherwise, the answer may different. In this case, most probably the neutral is designed to carry the return current of the sum of the both lines.

- MS