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I currently have a 2200 VA UPS unit (APC Smart-UPS 2200XL) in my rack and may need a 3000 VA unit to be able to load additional servers. Under my assessment, based upon whitepapers for each server, the current can achieve a max of 1335 VA yet adding additional servers would push the limit to 2344 VA.

Should I consider testing the load under the old UPS or should I just utilize the 3000 VA UPS (APC Smart-UPS 3000XL)?

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3 Answers 3

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I don't like to go any higher than 50% of the UPS rating, so for a 3kVA UPS I don't want to place more than a 1.5kVA load on it. Unless you have additional battery packs the run time is just far too short to be useful when loaded too heavily.

Despite the forgoing, although your servers "can achieve a max of 1335 VA", that doesn't mean that's what they normally run at. If in doubt you should measure current draw under normal working conditions. Talk to an electrician if you don't have the gear to do that yourself.

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Can you please explain why you run your UPS's at half-capacity? I've generally heard that the rule of thumb is 90%. –  jamieb Feb 15 '10 at 20:48
    
See the second sentence. –  John Gardeniers Feb 15 '10 at 20:57
    
Specing a UPS runtime of >15 mins is rather useless. Statistically speaking, if your mains is out for more than 15 minutes, it's going to be out for a while (certainly longer than your battery capacity). If you need a greater runtime in the event of power failure, then you should be looking at a generator. –  jamieb Feb 15 '10 at 23:08
    
Maybe this sort of thing varies from place to place. We experience many outages of up to 20 or 30 minutes. Beyond that it's likely to be several hours or more. Therefore, it makes sense for us to have a run time of 35 to 40 minutes, as that carries us through about 90+% of power outages. –  John Gardeniers Feb 16 '10 at 0:11

Your existing UPS is rated at 2200 VA, but you think you'll be drawing 2344 VA. So you're wondering whether you ought to test the 2200 @ 2344 VA, or just buy a new UPS? I'd definitely recommend that you do some testing before laying out the cash. As Scotty taught us, good engineers are always overly conservative on paper. :-) In practice, you might be fine with the existing set up.

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I could be wrong but I believe the maximum capacity of the 2200 is 1980 Watts / 2200 VA. If you loaded it up to 2344 VA I'm pretty sure it would be in a constant error state, may trip the output breakers, and would have practically 0 run time if it handled the load at all.

I'm not even sure the 3000 VA unit will fit your needs as it's rated at 2700 Watts / 3000 VA and your proposed load of 2344 VA is around 87 - 88% of the capacity of the 3000 VA model. Everything I've ever heard or read says that you should keep your loads below 80%.

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