UPS Determination??


Thread Starter

C. Harvell

I would appreciate your help. I need to provide a UPS for an OEM I'm designing. Problem is, I don't know if I need a UPS with the exact KVA rating of the OEM. Provided I want to have sufficient power to maintain the system during a power failure, etc., do I need to have one fully rated?

If I'm not being very clear, my design is approximately 30KVA. Supposing I need to maintain my system for 15 minutes (just guessing), do I need a UPS rated for 30KVA?

Any ideas, thoughts, suggestions would be appreciated. I've never spec'd in a UPS before, so I'm obviously lost. Also, suggested (and reputable) manufacturers would be helpful as well.

Thanks in advance.
Try APC, Powerware, MGE Systems... I believe you will need a 30KVA UPS, however the batteries will depend on you backup time. The vendor should be able to help you.


Johan Bengtsson

An UPS is rated for the maximum load it can handle, so yes
you would need 30kVA (or perhaps a little more since you
said "approximately"...) This decides how much power the
output circuits need to be able to handle before they
burn up (or - since most (if not all) UPS have a current
limit - turn themself off because of overload or lower the
output voltage).

Besides the power it can output you have to say a time it's going to last with that power, This decides the battery capacity needed. Of course if you actualy use less power it will last longer.

If you operate the UPS at half its rated load you will get
approximately twice the time before the batteries are empty and so on.

You may also need to say how long recharging the battieries are allowed to take once the power has returned, typically this takes quite some time (yes the load can be connected and running - but you don't have full protection time from the UPS while the batteries are charging - of course)

One thing you normally try to do is to shut down the UPS
as soon as a safe shutdown of the equipment is done, this
results in battereis not completely empty and will of course result in fully recharged batterieries sooner.

Then there is a question about technology, if it is allowed a very brief switchover time or if the output needs to be absolutely clean all the time. In the former case is the output circuit turned off and power feed stright thru the UPS via a relay and when power failure occurs the relay is opened and the output circuit turned on. In the later case is the input power rectified and feed to the batteries all the time and the output stage operating all the time - power failure need no switchover - of course the later solution is more expensive.

/Johan Bengtsson

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Robert Dusza

Dear C. Harvell,

Check with a manufacturer. Usually you want a UPS that is rated higher than the load depending on the time you need to maintain power before a controlled shut down. Check the Powerware site ( They have UPS selectors for different sizes. We installed a couple for the Water SCADA System here and I used their info for sizing of the system.


Bob Peterson

A UPS of that size has a lot of other factors you need to consider. Best bet is to contact several manufacturers and chat with them about what they suggest for your application. One thing you may find is that many of them are not rated for the temperatures you typically encounter in many industrsial applications, so you either have to cool them, or derate them if you are forced to put them in a warm area. You will also find that battery maint might be an issue. Some of them use batteries that are quite large and will require a forklift to get the batteries out, meaning you will need to leave room in your floor plan for theis.

Bob Peterson
Being a UPS engineer i would install a UPS for your system with a 25% room for expansion. The typical batteries associated with this size of UPS are easily man handled. As for the enviroment the UPS can handle temperatures of upto 40 degress C, The limiting factor is that for optimum battery discharge the ambient temperature should be 20 - 25 degrees C. if this temperature is exceded the life expecancy of the battery will be reduced.

I hope this is helpful.