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Distribute my power needs...

Posted on 2006-07-24
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Last Modified: 2010-04-26
I am installing a new rack of servers and need to pull circuits to power the servers. How can I determine how much power I need?
For instance, I currently have 15A. 125 Volt 6 foot power bars on each circuit. Then I have some un-organized number of servers plugged into each power bar. How many servers is too many for the power bar/circuit?
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Question by:RPIIT
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davbouchard earned 1200 total points
ID: 17170583
Watts = Volts X Amps



125 X 15 = 1875 Watts.

Check your power supplies on your servers and add them up.  If all of them have a 500 Watt powersupply than you should only put 3 of them on the power bar.
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by:Lee W, MVP
Lee W, MVP earned 800 total points
ID: 17170725
There is no simple answer anyone here can provide - we don't know what your servers are, how many hard drives, how many processors, how much RAM, how many power supplies per server, etc.  I can tell you that an AMD Athlon X2 3800 system I use with 5 hard drives and 2.25 GB RAM (4 sticks), plus two other much older servers, a 16 port gigE switch, cable modem, 4 port switch, Snap 1000 drive, and wireless router draw continuously 330 watts.  That's (Amps X Volts = Watts, assuming 120 volt, that's 2.75 amps).  Point being, while systems can add up, they don't use THAT much power... I would say 10 basic systems per strip.

Note: the ratings on the power supply are MAXIMUMS.  Continuous power is never that much.  If you have a start up and shut down procedure, 10 systems should be fine, as long as you don't start them all at once (there is usually a spike in draw as they initially power up, but within seconds it drops significantly).

For reference use, I strongly recommend getting a Kill-A-Watt meter - $25-$40 on ebay and maybe available in stores.  It's a handy little device that can measure overall power draw.  Reference:
http://www.the-gadgeteer.com/review/kill_a_watt_electric_usage_monitor_review
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by:davbouchard
ID: 17170828
Since you are pulling circuits to the server room, I'd still go with the total power of the power supply you have.  This way you'll be able to replace with bigger servers when the time comes and keep in mind that depending on the price per circuit, cost might be good compared to a possible power issue if you overload a circuit and you get a strike or something like that.

My thumb rule is to load a 1500 UPS with 3 to 4 servers and put it alone on a circuit.  You can't be safer, and since the elect. guy is coming anyways...
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Author Comment

by:RPIIT
ID: 17170842
I understand and agree with your point leew but for planning purposes I need to be able to document to the boss why I need to spend my company's money have more power pulled into my server room. This method gives me a great way to do that.

Ok, so I'm looking at
3 HP DL380's and 13 HP DL320's.
2 KVM Terminals
2 KVM's

DL380's have 1 400 Watt Power Supply
DL320's have 1 180 Watt Power Supply
KVM Terminal 50 Watts
KVM 50

so...

(400 * 3) + (180 * 13) + (50 * 2) +(50 * 2) = 1200+2340+100+100 = 3740 Total Watts. So 2 circuits should just cover it.
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by:davbouchard
ID: 17171204
It doesn't that much more to pull an extra circuit and you'd be amazed of how quickly you'll grow into that.  Safer is better, especially in the server room.
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Expert Comment

by:Lee W, MVP
ID: 17171635
If you're under time pressure, do it that way... but if you've got a week, get yourself a Kill-A-Watt meter and measure it.  Then add 50% to be safe.

Now the more circuits you have, the better.  What you put there today could change in 3 years and do you really want to disrupt things 3 years from now if you can do it all once, now?

You might even consider putting in a 220V line - in case you get hardware that might otherwise require it.  I'm not sure about the new models, but the older IBM Blade Centers required 220V lines.
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Author Comment

by:RPIIT
ID: 17171734
Under time pressue would be one way to put it. I found out about this Friday, and the power techinician will be here tomorrow.

Thanks for the help.
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