I hope the 400VDC through the data center doesn't catch on.
It's already being done to convert DC to DC, right on the motherboard, for all computers these days, to derive the small voltages the CPUs need. The SMPSU only provide 12V, 5V, and 3.3V. These boards could do all this with just 12VDC alone coming in. It would be a cost shift, raising the cost of the board a little bit, and reducing the need for the PSU since it only needs to supply 12VDC in this case. This is the scenario Google has asked motherboard makers to do (with an order for on the scale of a million such boards). I don't have the details of how they plan to provide 12VDC to these boards, but it has been suggested that more than one board would be supplied from a 12VDC source. What I'm trying to envision is the possible scale of converting AC to 12VDC and distributing the 12VDC from such a common supply/converter. My gut feeling is this is practical to do on the scale of one rack of about 80 or so such boards. I certainly would not run 12VDC all around the data center (way too much wire needed, way too much I2R loss, and way too much short circuit fault current). Even 48VDC on a large data center scale bothers me (100 or more rack cabinets).
So my gut feeling is that a good solution is to run 480/277 or 416/240 around the data center, feeding AC to each rack, and converting that AC to a single DC voltage that each board can take and do the remaining conversions as it needs. I believe just one AC/DC conversion per rack can be done. But if not, then maybe it can be done on 1/2 rack or 1/3 rack basis.
It gets the SMPSU box out of the computer, which is a definite advantage.
Using 416/240 would have a slight advantage that special computers and other equipment that still takes AC directly can be suppled with the 240 L-N right from that. Routers and switches are likely limited to that (though a lot of them do have 48VDC options). It would also be a common design that can be done worldwide for companies (like Google) that put in data centers in many countries.
I do know the military makes a lot of use of 416/240 and this is apparently a choice based on wide deployment compatibility issues. Look for large military surplus gensets and you find many, if not most, are made for, or are currently wired for, 416/240 or thereabout.
416/240 would have the slight disadvantage of somewhat greater I2R losses compared to 480/277, and a special order from the utility in North America. But even with 480/277, those computers can still be supplied from AC stepped down from the 480/277 via a transformer or three, either 240 L-N from 416/240, or 240 L-L from 3 sets of 120/240, or even 208 L-L if you must. I'd light the data center with fluorescent at 277 or 240, and thus limit the total amount of 120 I need to have, minimizing the size of that extra stepdown transformer. Obviously, if there are office spaces around, you need a bunch of 120 there (because you can't be having ordinary people plugging computers and other stuff into 240 in this country).
