On Fri, Dec 3, 2010 at 07:54, Chuck Anderson <c...@wpi.edu> wrote: .... > On another note, how do you calculate N+1 power feeds in your racks? > If you have 2 PDUs fed from two different branch circuits/UPSes/etc. > do you just set your PDU load alarm thresholds at 50% of the max > rating of each PDU and never load them beyond that point, so that if > you lose one PDU/branch circuit/UPS and the dual-power servers > transfer their load over to the other side, it doesn't get overloaded?
That would be around 40%, not 50% (80% of 50%). Note that there are some caveats. Some power supplies are more or less efficient at different (low vs. high) utilizations, and depending on the design, you are running (with 2 power supplies) either each at (around) 50% of load, or 1 at 100% and the other at 0%. It is *possible* to be able to run near 60% on two UPS circuits if the power supplies are inefficient at 50%. But this requires a lot more design and evaluation work than the (easy to calculate) 40% target. Also note that *your* electrical engineer may de-rate the circuits capacity due to the fact that switching power supplies generate numerous artifacts on the lines. These are all advanced (electrical) engineering topics. Gary
