On Monday 01 October 2007, Les wrote: > On Mon, 2007-10-01 at 12:54 -0600, Karl Larsen wrote: > > The 12 volts can not be moved as 12 volts DC very far. The length of > > a car is about it. So Google must have a lot of racks in close proximity. > And while moving this single voltage around has lots of advantages for > racks, it is not such a > clear advantage for single systems. I guess only time will tell us that > story. > > What Karl has seen in working on Amatuer Radio, and my experience on a > boat tells us the story on high current busing which is a very complex, > very hazardous proposition. I have spent 17 years as a broadcast engineer (the last four of which just consulting); RF is one of my things. So is DC power. The really nice thing about DC power is the complete lack of skin effect. That is, once you are talking lots and lots of current, and needing the lots and lots of copper to carry it, DC will use 100% of your copper, where even 60Hz AC will not. The ampacity for, say, 750kcmil copper wire, RHHW or XHHW preferably, at DC is much much higher than at AC. 12VDC is too low of a voltage to take advantage of this; 48VDC, on the other hand, is just about the sweet spot, as long as you are in the 200A or greater club. We are running 48VDC distribution for switches and servers here, and it's pretty interesting that a server that will take a 15A circuit at AC due to power factor correction things and inrush currents will take a smaller DC circuit, and the same wire size at 48VDC. The larger switches and routers that I have (Cisco 12012, 8540, and 7507) are a little different; but I can use not that much larger wire to run them (10 or 8AWG x2 instead of 12 or 10 x3, typically). The conversion efficiency is not the driver for me; the driver is the centralized UPS and single large (450Ah@ 48VDC nominal) battery that runs everything. UPS inverter losses are not insignificant in a data center; 12VDC is one of the three really useful voltages to use (24 and 48 being the other two) where battery backup really helps. AC distribution to modern power supplies, where a UPS isn't an issue, isn't any less efficient, as the typical switching power supply's input circuit is a simple rectifier to get 160VDC, which is then DC-DC converted to the various voltages. -- Lamar Owen Chief Information Officer Pisgah Astronomical Research Institute 1 PARI Drive Rosman, NC 28772 (828)862-5554 www.pari.edu
