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  1. #1

    Advice needed - Colocation power questions.

    Hello again, WHT,

    If you have read my prior thread you know that in the next few months I plan on colocationg a few machines along with some developer friends of mine. That thread is solved, I have decided on a switch.

    I have a set of new questions pertaining to power in colocation racks. I have received quotes on space for both 10 and 20 amp allocations. It is my understanding that this number is the absolute maximum you are allowed to consume, and you should always stay below 17 amps or so to accommodate for spikes, etc. I plan on investing in an APC PDU for monitoring of this.

    I am concerned with power consumption because most of our servers are aging Dell, HP and IBM. All of these were basically EOL'd because of the power that they use. I bought a kill-a-watt and used it to gather some information before actually racking these things up. My results are confusing and I thought I would ask for some insight or at the very least ahre my findings.

    Generally, each server eats about 2 amps at idle and 3 at full load on the hard drives, processors and RAM. If I plug in all 5 servers into the kill-a-watt at once through a standard power strip, the device only reads about 8 amps with all the machines at full load!

    This doesn't make sense to me. I figured I could take readings of each individual piece and add them up to estimate usage. Is there something I missed on EE day in physics class? Does combining machines like I did somehow make them use less power, or make them appear to use less power? Will a PDU with separate outlets for each device mirror my results here?

  2. #2
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    First its the 80% rule you will need to follow so its gonna be 16 amps and not 17 before you hit the breaker. And yes hopefully your results should reflect on the PDU too.
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  3. #3
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    That's strange, your kilawatt should do exactly what you expected it to. I did the same thing you were talking about with (2) 1850's (2) 1750's and (1) 2850. I was getting over 12 amps?

  4. #4
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    Rserial = Rload1 + Rload2 + RloadN
    Rparallel = 1 / (1/Rload1 + 1/Rload2 + 1/RloadN)

  5. #5
    Join Date
    Dec 2004
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    49
    Quote Originally Posted by iptelligent View Post
    Rserial = Rload1 + Rload2 + RloadN
    Rparallel = 1 / (1/Rload1 + 1/Rload2 + 1/RloadN)
    Parallel and Series resistive loads? Power supplies are not resistive loads, that is why power supplies often correct the power factor.

    The amperage of all your servers added individually should be very close to measuring them all at once. If all the servers are going to be on the same power strip I would just use your kilowatt meter and measure them all at the same time and boot the servers simultaneously and use that reading.

  6. #6
    Join Date
    Mar 2009
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    Quote Originally Posted by pentiumone133 View Post
    I am concerned with power consumption because most of our servers are aging Dell, HP and IBM. All of these were basically EOL'd because of the power that they use. I bought a kill-a-watt and used it to gather some information before actually racking these things up. My results are confusing and I thought I would ask for some insight or at the very least ahre my findings.

    Generally, each server eats about 2 amps at idle and 3 at full load on the hard drives, processors and RAM. If I plug in all 5 servers into the kill-a-watt at once through a standard power strip, the device only reads about 8 amps with all the machines at full load!

    This doesn't make sense to me. I figured I could take readings of each individual piece and add them up to estimate usage. Is there something I missed on EE day in physics class? Does combining machines like I did somehow make them use less power, or make them appear to use less power? Will a PDU with separate outlets for each device mirror my results here?
    Is the meter designed for use with switch-mode power supplies? If not, it may simply be inaccurate.

    As you add more servers to the power strip, maybe the peak current capability gets saturated. That's not a problem, the PC power supplies will simply draw current for a greater proportion of each mains cycle; but maybe the meter's accuracy depends on the proportion of each mains cycle that current is flowing.

    You should ask the data centre what the maximum permissible sustained current is, and what the maximum permissible start-up current is. If you're within the sustained limit, but over the start-up limit, consider getting a method of staging the startup.

    (I suppose in theory you could have one machine start the others using the wake-up-on-LAN feature over a separate LAN within the rack, but I've never tried it.)

  7. #7
    Join Date
    Aug 2009
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    Orlando, FL
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    I think there must be something wrong with your killawatt. I had some extra stuff laying around in my office so I just tried it.

    Here is what I have:

    (2) poweredge 1650s
    (1) poweredge 1750
    (1) poweredge 1850
    (1) poweredge 2850
    (1) Juniper SSG 5

    The juniper takes .41 amps as that was originally the only thing plugged into my killawatt. I added the other machines, not all of them has OSes installed, but they at least had drives. Most of them 3 or more SCSI drives. It's currently showing 8.7 and I did see it hit 10 when most of them were booting/spinning up drives.

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