# need help with UPS sizing

I am purchasing a Liebert Series 300 UPS. Input 480V, Output 208V. And I believe the output is 208Y, so three hot 120V legs, 208 phase-phase.
Two questions.
First, from a sizing standpoint, my data is all 120V single phase. I dont really know how that correlates to the input rating of the unit which is done, 30KVA, 65KVA, etc.,.
What throughs me off is if that is per phase, or across all three phase of the 480 input, etc.,.
To put things in perspective with a practical example...
I have a total of 500 amps of 120V equipment. So with phase balancing, figure 165 amps on phase A, 165 amps on phase B, and 165 amps on phase C of my UPS's 208Y output.
To support that, what size KVA UPS do I need? Using my ubdoubtly incorrect knowlegde, I was figuring something around 60KVA (500amps x 120V / 1000).
The second question, when buying a UPS, everyone says I should get one where my real usage is close to the full capacity of the UPS. If I were to get a 60KVA UPS and only have 10KVA of load, I've been told it will waste alot of power. I sthat true? If so, how do you get around the inevitable situation where you will grow into the unit, and have to start small? Or do you simply have to deal with inefficiencies at low usage?
Thanks, John
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You have calculated correctly. However, do not assume you will be able to balance the load that well.

Most large UPS are rated for a VA at a power factor. Usually it is something like 60kVA at .8pf. This means the UPS can ONLY supply 48kW of load. Even if you are at unity power factor, you will only get the kW that is rated for. This often confuses people. Next, UPS don't really like to run at their max rating. They often are more sensitive at full load and may drop the protected load as a result of minor power disturbances. We have tested many UPS from a few hundred VA to 150kVA and this seems to apply across the board.
Underloading the UPS does make the efficiency go down. It is due to the fact that for the most part, the UPS consumes its share of power regardless of the loading. This can be quite large, 1kW or more in some cases. So you are correct, the lower your load as a percentage of the UPS rating, the lower the efficiency. However, the drop in efficiency is a small price to pay compared to overloading the UPS and having it not work at all. Shoot for 75-80% loading. Keep in mind the fact that a 60kVA UPS with NOT support 60kW even at unity power factor.

Charles Perry P.E.
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wrote:

Thanks. If I run at only 25% load, the efficiency will drop down, but how significant do you think it will be from a price standpoint? I dont mind spending an extra 5% for power for a UPS with 25% load (which will eventually grow to 75%). You said it could cost me up to 1KW of wasted power, is that all together?
1KW of extra power being consumed for me to run at 25% load or similiar doesnt sound like alot of money. 1KW 24x7 for the month is only a \$100.
The vendors I am talking too make it sound like it will cost a fortune to run at such a low load, say 10KW on a 65KVA UPS. But I really want to buy big and grow into it. Besides, the larger units are priced much better. For 10% increase in cost I can get a UPS twice the size.
-John
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essenz wrote:

You need to get detailed specifications, including load graphs, of models that you are interested in.
Some large units actually use parallel inverter modules that only start up when the load demands it - thus giving relatively high efficiency over a wide range of loads. With some, you can simply buy and add additional modules later, if your load increases.
Some actually do deliver the rated kVA as kW at unity pf, eg: http://www.apcpower.co.uk/silcon.html
Some have far, far better battery management and can work well with mixtures of battery types and ages.
etc. etc.
I'm not familiar with 120v units, so can't say what is available for you. Most of the sites that I am familiar with are concerned about efficiency but more because it gets dumped as heat for the air con to remove (tricky during a prolonged power fail) than the effect on the electric bill. But their main concerns are reliability, redundancy and maintainability..
.. Things may change as "Green" is rapidly becoming the new must have..
--
Sue