Need ball park power and cooling requirements

Robert G. Brown rgb at
Tue Oct 1 12:12:52 PDT 2002

On Tue, 1 Oct 2002, Dan Sturtevant wrote:

> > I'm setting up lab space and plan to build a small (8-16 node) cluster
> > of dual Xeon nodes (Dell 2650 or equivalent). I need to give the
> > engineers the power and cooling needed in the 6'x8' server room that
> > will house the cluster. Any rules-of-thumb for what's required? Thanks.
> I would put each set of 8 nodes on a separate 20 amp circuit.

Also, I'd strongly suggest that you ensure that you are using power
factor correcting power supplies in the nodes themselves AND make sure
that your electrical contractors are competent.  For example, if they
run multiple circuits with different phases, do not permit them to share
a common neutral.  There are companies that make harmonic mediating
transformers that can compensate for the harmonic distortion caused by
switching power supplies, if it turns out to be impossible to get PFC
supplies from Dell or whoever.

Because switching power supplies draw basically all of their current in
the middle third of each voltage half-cycle: a) cancellation that might
have occurred in shared neutrals becomes addition instead, leading to
dangerous overheating of the neutral line; b) just using RMS power
consumption and the assumption of voltage and currents in phase
significantly underestimates the peak currents drawn by the systems --
you may find your 20A breakers blowing when you are only drawing
1100-1200 watts instead of the 1500-1600 you might have expected to run
on a 2400 VA line; c) PFC supplies consume less energy on average, and
the power companies often charge you less money for what you use if you
don't need the high peak currents associated with a relatively poor
power factor.

Your cooling capacity is determined by your load estimate.  You need
enough AC to remove all the heat being consumed in the room AND keep the
air downright cold (60F is good) on the fan intakes of the nodes.  There
are roughly 3500 Watts of power removal capability per "ton" of air
conditioning, and at a guess you will need at least a ton of AC serving
the room.  Having a margin here is a very good idea, as is having a
thermal kill switch that shuts off the main breakers if the room
temperature ever exceeds (say) 80F.  Your room is small -- if AC fails
and power doesn't, your systems will die horribly in about thirty
minutes or even less.


> Dan Sturtevant
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Robert G. Brown	             
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at

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