UPS sizing


Thread Starter

Mark Riche

Although this thread has probably already been discussed on the list I am looking for a good rule of thumb for sizing UPSs vs load size in the 5 - 10 kva range. The application I am looking at will not have any more future load. Does one usually use match the load exactly or use 25 %, 50 % greater than the existing load ?


Mark Riche
Electrical Engineer
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Curt Wuollet

Usually one would consider load and time together. A little surplus capacity will translate to a longer runtime at reduced load. And I have never seen a system that didn't grow loads as time goes on. And of course, having to upgrade the UPS to run another display or
peripheral makes people unhappy. In the 5-10 kva range, 125% of load might be reasonable at 10kva and a little more at the bottom of that range. Connecting a PC or such for testing or diagnostics shouldn't max you out. You may want to do something like that if it looks like the power isn't returning :^) And maintenance might plug in a drill and and.......



R R Stephens


One thing I learned the hard way is to be sure and consider the max current a UPS can provide instanteously, generally about 2.5 times rating. This fact may have implications for the loads you are considering using, i.e. lots of coils (like on relays) with high inrush va could shut down the UPS. Also keep in mind a short circuit on the output of the UPS will shut it down even if
you are feeding loads through an emergency service panel with breakers. The UPS senses the overload much quicker than breakers.