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  1. #1
    Join Date
    Jan 2001
    Posts
    495

    UPS - How to determine

    Currently we have 27 servers at one of our datacenters, some apc masterswitches and a cisco switch + a layer 3 router. We are going to bump this number up pretty quick in the near future. We need to figure out how many VA needed per server. We currently have 2 APC 1200 XA's or something like that and it had been supporting it but now they are pissed and beep at us. (these also take a tremendous amt of rack space). We just need this thing to run for 30-45 seconds tops while the diesel generators kick in... anyone have a clue how to figure it out?

    I think the majority of the servers are 200W.
    James R. Clark II

  2. #2
    You should really talk to a sales person at APC or some other competitor. What you would need to provide would be a total number of VA. They would be able to assist you in determining which product would provide a certain amount of time at a certain VA.

    Here are some VERY rough calculations that I used to use.

    VA = Volts X Amps = Amps x 100 (or 110)
    VA = Watts X .6 (.6 is a rough estimation based on the effeciency of switching power supplies found in most computers). Most mainstream equipment manufacturers will be able to provide VA numbers for you. Dell, compaq, cisco, nortel, etc should provide this information.
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  3. #3
    Join Date
    Jun 2003
    Location
    Boca Raton, FL
    Posts
    14
    First, you should determine the exact load you are going to be running. If your servers are similar, you can measure the load on one and use that for all of them. If they are very dissimilar, I would measure all of them. This is easy to do with a simple clamp on ammeter you can get from home depot or an electronics store.

    To take a server reading, attach the meter with the server off, then turn on the server and watch the meter. Watch it until the server is completely booted. The highest reading you see during this period is the one to use. You don't want to measure a server that is already powered on because it will use more power during startup.

    One word of advice, DO NOT just use the wattage rating of the power supply or the KVA rating of the UPS.

    Power supplies are rated at maximum output, so a computer with a 350 watt power supply may actually be using only 100 watts. Our servers are rated at 300 watts and only use 90.

    UPS's on the otherhand, are all over rated, by an average of 100%. This means that a 30KVA UPS rated at 15mins backup time will only support 15KVA for 15mins, or 30KVA for 7.5mins.
    This is because the UPS manufacturers know that most hardware uses 50% or less of the maximum rating of their power supply, and they take advantage of that.

    We went through alot of tribulations while sizing our UPS's until we discovered this.

    So, I would measure the load on all equipment, estimate the load on stuff you don't want to measure, then do the following:

    For 110v equipment, add all amps together and multiply by 120 to get VA.

    For 208v equipment, add all amps together and multiply by 208 to get VA.

    If you have other voltages, do the same with them.

    Add all VA results together. This is the true VA load (the more equipment you measured, the more accurate this number is).

    Let's say you end up with 15KVA. If you go out and buy a 15KVA UPS rated at 15 minutes, when you tested it you would find it would last only about 7.5 minutes, if that.

    I hope this helps.

    Jim
    Operating Manager
    Bocacom.net

  4. #4
    Join Date
    Jan 2003
    Location
    Lake Arrowhead, CA
    Posts
    789
    I'd also say measure the true load if you want to be safe, but you can also use something like APC's UPS selector for a decent (if a bit bloated) estimation with a wide variety of equipment. You'll probably want to drop 10% or more below their lowest recommendation if you're certain you'll never need more than a minute of run time.
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  5. #5
    Join Date
    Jan 2001
    Posts
    495
    Thanks so much. I was completely boggled after I did that APC program. It was saying I needed a 5000 UVA UPS for like only 30 servers. We are running close to that on those 2000 UVA so I figured its just a sales ploy. I think I am going to get a 3000 Compaq UPS and use one of the 1000's we have now. That should be plenty for this rack and the next rack we are getting next week.
    James R. Clark II

  6. #6
    Join Date
    Dec 2002
    Location
    Miami, FL
    Posts
    1,487

    Re: UPS - How to determine

    Originally posted by jic
    We currently have 2 APC 1200 XA's or something like that and it had been supporting it but now they are pissed and beep at us.

  7. #7
    Join Date
    Jan 2001
    Posts
    495
    Yeah we just bought a new one. I guess the crappiest part is shipping on them. The actual cost to the guy *he showed it to me* is $200 =/. Thats what you get when you buy a 200 LB UPS hehe
    James R. Clark II

  8. #8
    Join Date
    Jun 2003
    Location
    Boca Raton, FL
    Posts
    14
    You should see the bill for a 5,000 pound one. hehe
    Operating Manager
    Bocacom.net

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