What happens when a UPS is undersized?

[Snowjob]

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?

 

[weressl]

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.

(Really reassuring to know that you work for the FAA.:grin: Especially before the Thanksgiving flying frenzy and the recent 'computer glitch'.)

 

[qcroanoke]

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.

I agree with Laszlo.
Did I say that out loud?

 

[winnie]

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

 

[iMuse97]

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

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." ??

p.s. I realized you're saying that the load calc. errs on the side of safety, and agree with you. I'd make your first bet, too.

Still, this is gov't we're dealing with, and if they're recalculating their requirements, they will need to upsize their system, or redefine their critical loads.

 

[weressl]

I agree with Laszlo.
Did I say that out loud?

Igor always agrees with Master.

My family IS from Transylvania.

 

[winnie]

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." ??

That would really depend: is it your plane in the air, or _my_ plane in the air.
*grin*

Me: I'd rather be safe than cheap, but I cringe ant excessive oversizing in the name of safety because it means money being wasted that could pay for real safety improvements elsewhere.

-Jon

 

[augie47]

strange timing ... two days after air traffic went to heck die to an FAA computer glitch wonder if the glitch occurred while ammeter readings were in process

 

[steve66]

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??

That said, UPS's do normally have an overload ratings for short time periods. For a powerware 9355 (just for example), its 150% for 5 sec. and 125% for 1 minute. But that won't help much if your load is continuous.

If it's a 1990's UPS, I would also worry about weather the batteries have ever been replaced. If not, you probably have about 10 milliseconds of backup:grin:

Steve

 

[dbuckley]

I'd check the front panel LCD doesn't say "running on bypass"...

 

[winnie]

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??

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

 

[zbang]

2) the fact that the UPS is equipment which can be more easily updated as loads increase,

While I generally agree with Jon, I'll take some issue with the above. Yes, replacing a UPS is probable easier than running new conduit and pulling larger wire, but updating/replacing a 200kva UPS is not to be taken lightly unless it's modular with empty inverter slots, but then you're not really updating it, just adding more output.

 

[MDOC7]

What's the point of doing a NEC load calculation if you ignore the result??

Steve

LOL... Hard to ignore a cataclysmic overload condition if that happens. New term: risk management.

 

[steve66]

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

Jon-

I think you have some good points. But I think we also need to consider that loads supplied by a UPS are usually the same loads you don't want to undersize the supply for. So at the least, I think there should be some ballance between realizing that Article 220 calcs are very conservative, and realizing that UPS loads are usually of a critical nature.

To be honest, I haven't looked in the code to see if UPS's are supposed to be calculated using 220 or not. I was basically assuming that was the correct way to do it, but you may be right about a 220 calc. not applying.

For hospital applications (article 517), we are allowed to size generators for the "maximum expected load" or something like that. We don't have to size them per the 220 calc. The handbook states this is allowed to prevent the need to grossly oversize generators. So I guess I shouldn't really be supprised if a similar thing applied to UPS's.

Steve

 

[K8MHZ]

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?

If the UPS has it's own OCPD it will trip if overloaded regardless if there is mains power to it or not. That being said, if it has been in use without being overloaded I see no reason for it to be overloaded when the mains drop.

I own several small ones and I know the above is true from experience.

The condition of the batteries should be checked as has been mentioned. Most places will rotate the batteries before they show signs of wear. Sometimes I get lucky and get the 'pulls' which almost always have life in them, they just need some conditioning.