[Discuss] Server Room Power

[Edward Ned Harvey]

> From: discuss-bounces+blu=nedharvey.com at blu.org [mailto:discuss-
> bounces+blu=nedharvey.com at blu.org] On Behalf Of Matthew Kowalski
> 
> Does anyone have any experience with running server equipment (Dell
> Power
> Edges, HP Proliant DL G7s & HP c7000 Enclosures) at 208 VAC instead of 120
> VAC?  Right now we're running 20 Amp circuits at 120 VAC but I've been
> reading some on the web about better efficiency at 208V along with
> additional power available.  The main issue we're having is getting enough
> power to the racks to run the power dense equipment.
> 
> Given one form of Ohm's as Power (Watts) = Current (Amps) x Voltage
> (Volts):

Hold it.  P=VI is a DC rule.  Power is more complex in AC.  (Literally, you
have to use complex imaginary mathematics.)  

What's the difference between VA and W?  The difference is imaginary, but
that doesn't mean it's not real.  There is a reason why UPS'es and so forth
are rated for VA and W separately.

VA is always >= W.  Unfortunately W is your real power consumption, and VA
is what the power company bills you for.  If you have inefficient power
supplies, you might be overpaying 30% for power.  I find with any decent
name brand server (Dell, HP, etc that you mentioned) they're pretty close.
VA might only be 5% higher than W.  The imaginary component comes from
storage elements such as capacitors and inductors, which are necessary
whenever you're converting AC to DC.

When you're talking about 208, you're talking 3-phase.  Imagine pedaling
your bicycle.  You only press down on the downstroke.  That's single phase
120.  Now you put on special shoes, which allow you to pull up on the
upstroke.  Now you're talking 2-phase 240.  But if you sprout a third leg,
and arrange each of your legs symmetrically around the center of the pedals
(you're no longer human obviously) then while one leg is still pushing, the
next one starts to push, an when that one is still pushing, the 3rd one
starts to push, and it all goes around in a circle.

On 3-phase power, you're able to get a more efficient usage of your power,
because the applied force is more steady.  You don't get the idle period
that you have in 1-phase or 2-phase power (when your foot is at the bottom
or top of the pedal stroke, you're idle momentarily.)  3-phase is smoother
power.  Which means you don't need so many storage elements inside your AC
to DC power supply.  This means your power supplies in your servers  are
able to more closely match VA = W, and waste less. 


> The max power out of a single phase 20A 120VAC circuit is 2400 Watts.
> The max power out of a single phase 20A 208VAC circuit is 4160 Watts.

Yikes.  No.  Drop it at that.

If you want to use 2-phase 240, you probably can.  This uses the same power
supply in the server as the 120, provided that the server power supply is
rated up to 240V.  Most computer power supplies are in fact rated up to 240.
Then all you have to do is change the tip on your power cord, so you can
physically get it to plug into the 240 outlet.  Never, never, never do this
on any power supply unless it's rated for 240.  The consequence of changing
the tip of a 120 power supply and connecting to a 240 outlet would be
explosion and fire.  Just look at the power supply to see it says 240 on it.
Then you're good to go.

If you want to use 3-phase 208, you need a special power supply in the
server.  Generally you don't have such a thing, unless it's a weird special
server that specifically requires 3-phase.  You probably would never buy one
of those without already knowing you did that.