I was doing a calculation on our critical loads when I noticed that our UPS is rated at 100 KVA or 278A at 208V.
Our total Critical Loads are 424 A.
This UPS is a circa 1990's model.
What is the exact scenario of events that take place when a UPS is undersized?
I presume that this is an existing installation that has been added to since the initial installation.
There are two factors affecting actual load.
1./ Utilization factor; where not all equipment is on line all the time.
2./ Load factor: where the equipment does not draw the current that is stamped on the maneplate.( Equipment seldom do, especially electronics do significantly less current than what their nameplate states.)
So hook up a recording ammeter to each phase and see where you actually are with your load profile. UPS's are very load limited, eg. they are not capable of supporting overloads. Most go on bypass and alarms the heck out of the system. Having sufficiently large overload, they will eventually shut down.
Other side of the coin: if you do an article 220 load calculation, and the number that you get is 50% larger than what the UPS is rated for, I'd bet that the UPS will do just fine.
-Jon
I agree with Laszlo.
Did I say that out loud?
Would you also be willing to bet that if you lost your first bet (ie. the UPS failed on overload), it would happen at a convenient time when "we really didn't need to run that equipment right now, anyway." ??
Other side of the coin: if you do an article 220 load calculation, and the number that you get is 50% larger than what the UPS is rated for, I'd bet that the UPS will do just fine.
-Jon
In a way I agree, the UPS would probably be fine. But you could "probably" also run a 20A circuit with #14 wire and never have a problem either.
What's the point of doing a NEC load calculation if you ignore the result??
2) the fact that the UPS is equipment which can be more easily updated as loads increase,
LOL... Hard to ignore a cataclysmic overload condition if that happens. New term: risk management.What's the point of doing a NEC load calculation if you ignore the result??
Steve
IMHO the point is that NEC load calculations are applicable to some but not all of an electrical installation.
NEC load calculations are know to be very conservative. I believe that they are appropriately conservative when used to size branch circuits, feeders, panelboards, etc. In part this overbuilding is necessary because of the tolerances in apparatus (for example, the fact that breakers are not very precise devices, and their trip curves span quite a different range of values at any given duration), and in part this overbuilding is necessary because the installations are buried in the walls and not easily updated as loads increase.
NEC load calculations are known to be excessively conservative for other applications, eg. sizing utility transformers.
I contend that given:
1) the cost of UPS hardware,
2) the fact that the UPS is equipment which can be more easily updated as loads increase, and
3) the fact that the UPS probably has very accurate load measurement and overload protection,
that the NEC load calculations are excessively conservative for sizing UPS hardware.
I agree that the UPS should be engineered to serve the connected load, but I believe that the calculations in article 220 are not the right way to do this engineering.
-Jon
I was doing a calculation on our critical loads when I noticed that our UPS is rated at 100 KVA or 278A at 208V.
Our total Critical Loads are 424 A.
This UPS is a circa 1990's model.
What is the exact scenario of events that take place when a UPS is undersized?