One question I have is in planning. One day we received a shipment of
about 5 or 6 Intel systems from out Toronto office. We ordered a rack,
switch, and a rack KVM. We initially plugged everything in to a wall
outlet. ran fine until it tripped a breaker, and the breaker box was not
in the computer room, but somewhere else on the floor where we had to
call building management. All I knew that the wall outlets were NEMA
5-20. The solution at that time was to take the two 6U monsters and plug
them in to separate outlets in the ceiling. This worked for quite a
while until my boss brought in a system he had at home (another 4U Intel
whitebox). At that time I had a rack power strip, and the power strip
popped a breaker, but the wall circuit was fine. I then bought another
strip to split the load. before all that I estimated our power usage by
adding up the wattage on the power supplies (each was about 700W). Each
wall outlet also went to a separate breaker. It was at this point when
we were getting the HP ESX box and IT somewhat dictated that we get 2
240V outlets). Right now I am pulling about 15A (7 on one, 8 on the
other). But, the critical factor is at takeoff, or when starting all the
systems, such as after a power fail. You've got all your systems
spinning up drives and fans. This is what we need to plan. So, I would
need a rule of thumb that I can take the wattage of each power supply
and figure out my maximum amps. Had I performed that calculation
initially, I would have had fewer outages. I can't help when a truck,
bus, or tree takes out the entire Riverside T station and us :-)
On 10/13/2011 12:29 PM, Tom Metro wrote:
Edward Ned Harvey wrote:
Hold it. P=VI is a DC rule. Power is more complex in AC.
What's the difference between VA and W?
If you have inefficient power supplies, you might be overpaying 30%
for power.
You're referring to power factor:
http://en.wikipedia.org/wiki/Power_factor
The power factor of an AC electric power system is defined as the
ratio of the real power flowing to the load over the apparent power in
the circuit,[1][2] and is a dimensionless number between 0 and 1
(frequently expressed as a percentage, e.g. 0.5 pf = 50% pf).
[...]
Circuits containing purely resistive [loads] have a power factor of
1.0. Circuits containing inductive or capacitive elements (electric
motors, solenoid valves, lamp ballasts, and others ) often have a
power factor below 1.0.
So when PF=1.0, VA==Watts. The better the quality of your power supply,
the closer its PF will be to 1.0. In the last decade it has become
common for name brand computer power supplies to specify a PF as a
selling point.
See also:
http://en.wikipedia.org/wiki/Switching_regulator#Power_factor
for discussion of PF with respect to computer power supplies.
When you're talking about 208, you're talking 3-phase.
You can attach single phase loads to a multi-phase supply, as long as
they are balanced:
http://en.wikipedia.org/wiki/Three-phase_electric_power#Single-phase_loads
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...
Old power supplies used to have a 120V/240V mechanical switch. Most
modern switching supplies will work fine with any input voltage from
like 90V up to 250V (check your supply specifications). The ability to
handle a wide input range is a byproduct of the switching regulator design:
http://en.wikipedia.org/wiki/Switching_regulator
-Tom
--
Jerry Feldman<[email protected]>
Boston Linux and Unix
PGP key id:3BC1EB90
PGP Key fingerprint: 49E2 C52A FC5A A31F 8D66 C0AF 7CEA 30FC 3BC1 EB90
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