[Discuss] Server Room Power

[gaf at blu.org (Jerry Feldman)]

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<gaf at blu.org>
Boston Linux and Unix
PGP key id:3BC1EB90
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