UPS system

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jociha

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I have 2 80kw UPS systems, powering a total load of 80kw. the load has duel power inputs so when both ups's are on line they each are running 40kw to load. If one fails, the other runs the entine load so it is outputting 80kw. is my service caculation here 80kw or 160kw? My guess is 80 if i supply both with 1 breaker sized for 80kw, and 160 if i use 2 seperate breakers to power them. Question 2- if i have a ups with a nameplate rating of 80kw and i only install 40 kw worth of batteries to fit the current load of 40kw can i treat it as if it were a 40 kw ups? john
 

ron

Senior Member
John,
It's not that straight forward.
Is there one service for both UPS', or does each one have its own service.
Since these are so small, I will assume one service for both.
You need to realize that the critical load served by the UPS may total 80kW, but the UPS itself is not 100% efficient, and you need to account for additional charging current for the batteries during full recharge at full critical load.
Let's pretend that the critical load is 80kW, the losses within the UPS is 20kW and the battery charging current at full charge is 20kW. You have a total of 80+20+20=120kW of load for one UPS load (when one carries the full load), and 20+20=40kW of load for the other UPS.
Your load toward the service calc is 160kW, but not for the reasons thought of before.

Q2: A 80kW UPS operating at 40kW of critical load, can be treated like a 40kW UPS, as long as you lookup the losses at 50% load and the battery charging currents. The client will be annoyed that in order to utilize the remaining capacity of the UPS, it would require batteries and service changes. :)
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I don't think you can install "40kw of batteries" on a 80kw UPS. Additional batteries usually give you more run time. So if you install fewer batteries, you will still have 80KW of capacity, but less run time off the batteries.

Steve
 
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