Why choose 120 volts?

Leo Bicknell bicknell at ufp.org
Tue May 26 17:05:51 CDT 2009


In a message written on Tue, May 26, 2009 at 12:39:10PM -0700, Seth Mattinen wrote:
> I have a pure curiosity question for the NANOG crowd here. If you run
> your facility/datacenter/cage/rack on 120 volts, why?

If folks are making their own choices, mainly for historical and
convenience reasons.  If folks are building data centers for others,
it's that customers demand 120V power in many instances, for some
good, and many bad reasons.

However, for all the talk of power loss that's not the real issue.
The loss due to wire or amperage is a very small part of the equation.
While this paper is very much vendor produced, it's a good high
level summary none the less:

http://www.apcmedia.com/salestools/NRAN-6CN8PK_R0_EN.pdf

Note that in a 600Kw installation power loss is reduced from 8,894
W to 845 W, a savings of 1.3%.  Note that they have included the
savings from additional cooling in that figure.  Even at 1.3%, if
you looked at the cost of rewiring an existing data center based on
that figure you'd be nutty; return on investment would be forever.

But what you'll find in the paper is that the change allows you to
re-architect the power plant in a way that saves you money on PDU's,
transformers, and other stuff.  Thus this makes the most sense to
consider in a green field deployment.

Thus, to reframe your question, in your existing, already built out data
center is it worth replacing 120V circuits with 208V/230V ones to save
power?  No.  Savings is likely well under 1% in that situation, and time
you add in the capital cost to do the work it makes no sense.  In your
green field, new data center, does it make sense to look at power from an
entirely new point of view?  Quite possibly.

-- 
       Leo Bicknell - bicknell at ufp.org - CCIE 3440
        PGP keys at http://www.ufp.org/~bicknell/
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