Re: Want to move to all 208V for server racks

[Gary Buhrmaster <gary.buhrmaster () gmail com>]

On Fri, Dec 3, 2010 at 07:54, Chuck Anderson <cra () 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