UPS sizing

C

cppoly

Senior Member
Location
New York
Anyone have any ideas what the benefit is to having a 40 kVA UPS when it is fed from a 30 kVA transformer??

You're not getting more than 30 kVA out of the transformer so why have a 40 kVA UPS??
 
The UPS output is supplied by its batteries. The input charges the batteries.
I don’t know if UPSs are designed to be used where the input supply is less than the output capability, but it’s certainly possible.
You obviously couldn’t use full output indefinitely due to battery capacity.
 
Many users of UPSs have a safe loading factor or a general fear of utilizing the nameplate rating of equipment (UPSs are theoretically able to run at nameplate for ever), so they may have taken that into account when sizing the incoming feeder.
IMHO, not a good plan, but that might be the reason.
 
I was thinking more along the lines of the transformer's kVA rating being pretty "soft", in that it could sustain a nominal overload for a significant period without overheating or damage (depending on the thermal environment it's installed in). While the UPS's kVA rating is likely much sharper, in that its output circuitry could only briefly support an overload. So if you want to match the two graphs of "sustainable power level" vs "duration of sustained demand" as closely as possible, you'll end up with a UPS with a higher kVA rating than the transformer.

But perhaps I was just overthinking it. : - )

Cheers, Wayne
 
Then again oversizing the UPS means less drain on the batteries and longer run time versus increasing a battery pack and charger for a smaller UPS.
 
cppoly said:
Anyone have any ideas what the benefit is to having a 40 kVA UPS when it is fed from a 30 kVA transformer??
Click to expand...
By "nameplate" there's no point. But (at least in a former life when I worked for a major UPS vendor) it was common for UPS systems of that size to have a non-unity output power factor, so your 40kVa UPS system might only be able to provide 32kW of useful power. Throw in a little overhead for comfort, and there you are.

retirede said:
The UPS output is supplied by its batteries. The input charges the batteries.
I don’t know if UPSs are designed to be used where the input supply is less than the output capability, but it’s certainly possible.
You obviously couldn’t use full output indefinitely due to battery capacity.
Click to expand...
No, the batteries are just a gas tank. The output is either connected directly to the input (in the case of offline, line-interactive, or eco-bypass systems) or a rectifier-energized DC bus and inverter (in double-conversion systems). LI and DC class systems are able to run outside their nominal voltage ranges either through the use of a buck-boost transfomer (LI) or the AC/DC/AC process (DC).

jim dungar said:
Then again oversizing the UPS means less drain on the batteries and longer run time versus increasing a battery pack and charger for a smaller UPS.
Click to expand...
No, oversizing the UPS means nothing to your battery runtime. The amount of lead (or in larger systems, lithium) determines your runtime. How big your engine is makes no difference (this was something very difficult to drill into the sales reps and VARs).
 
Ahh, also forgot the most obvious possibility... At that range, the sizes often run in increments of 20kVA... and if a 20kVA isn't big enough, well, here, have this 40. Just because the capacity is there doesn't mean it'll ever get used.
 
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