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calculating amps * volts on dell poweredge 2950

Posted on 2009-06-26
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Hello guys and as always thanks for your time and expertise.
We have a Dell Poweredge 2950 and I need to calculate the power requirements for a ups.  We're going to connect five of these servers into a ups.
On the power supply for the server - here's the info listed:
Input 100-240v
50/60 H
Max AC Current 9.3AMP
Output +12v 62.4amp
+3.3usb 3.8amp
Total output power not to exceed 750 watts
- Now I haven't done this in a while but I thought the calculation was amps * volts.  With that being said, am I using the 9.3 amps in my calculation.  I would appreciate any help on this.  Thanks.
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Question by:pendal1
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by:chronosyu
chronosyu earned 165 total points
ID: 24725890
Hi, i thionk this page will hlep you a lot, take a look. let me know

http://bugclub.org/beginners/miscellaneous/upscalc.html
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by:willsorrell
willsorrell earned 165 total points
ID: 24726400
You are not going to get a true picture of how much power your servers draw by looking at the tag on the power supply.  You've got a couple of better options:

1 - Go to a UPS manufacturer's website (like www.apc.com) and use their selector tool.  This will ask you for input on the model, processor, number and type of drives, etc.
2 - Purchase a tool like the "Kill-a-Watt" from p3International and measure the power draw directly.  I bought mine at Harbor Freight Tools for $40.  This is really simple - you power down the equipment, unplug it from the wall, plu the "Kill-a-watt" device in the wall, and then plug your equipment into the Kill-a-watt.  Depending on the total draw (check the rating) you could just plug a power strip into the Kill-a-watt and then put all your equipment on the power strip and measure it all at once.

What I found is that everyone, especially the folks trying to sell you a UPS unit, will dramatically overstate the power draw of your equipment.  Who knows why (liability, greed, whatver).

The formula for Watts in a standard electrical device (light bulb, hair dryer, etc.) is Volts times Amps, but electronic equipment like servers have what is known as a Power Factor, which actually reduces the total draw.  So for a hairdryer, 1850VA = 1850 watts, but for a server 1850VA might only be 1200watts (and that would be one hell of a big server, illustration purposes only).  That's why you should always consider the watts you need, not the volt amps.  Manufacturers like APC and Tripplite always use VA as their "marketing measurement" but when you dig into the details, you'll see that a 1500VA UPS might only be rated for 1200 watts, or even 1050.

So the simple way to do it is use APC or similar to estimate the watt draw of your equipment and size the UPS accordingly.  The other, more accurate (and fun) way, is to buy a tool and watch the draw in real time.

There are other considerations too, like "line-interactive" vs. "double online conversion" UPS. but that's a little esoteric and probably not something to worry too much about unless you are going to get over 5KVA.  I'm sure APC and Tripplite would love to argue the relative merits of the technologies, but all you really want is battery-backup for your two servers.

Another consideration is extended run time.  Vendors usually sell two different models of same size units - one capable of daisy-chaining additional battery and one not.  If you want to have any kind of extended run time (more than just graceful shutdown time) you should look into this as well.

Hope that helps.
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dnilson earned 170 total points
ID: 24726617
WHile I agree that an actual measurement or a UPS manufacturesr calculator is a better way to go, you question was to formula.

9.3 amps time 120 volts = 1116 VA.  This doesnt have to be discounted by the sine factor (0.707) because current was already defined as AC current.

750 / 1116 = .67 (67%) a respectable and believable conversion factor for a switching power supply.

The UPS Will have a MAX impress load of (approx) 1116 VA, and an average dependant upon installed hardware, speed, ambient tenperature, etc.

===============================
ANother thing to take into consideration is the effect of the dual redundnat power supplies.  In my measurements, having BOTH supplies powered draws 1-1.5 amps LESS than having either one powered, or in worst case terms, apower supply failure generates a n approxc 1.5 amp step function increase on the opposite bus.  Both continous capacity (the VA ratign o nthe box) and the intantaneous surege capacity (a function of the UPS to supply constant voltage depite astep load) which is a spec ofeten more deeply buried.


lots of little peices to look at ......

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Author Closing Comment

by:pendal1
ID: 31597072
Guys - thanks for the great input.  Very helpful and informative.  Much appreciated.
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