help understanding 3 phase UPS input load numbers

[r7k]

Can someone help me make accurate sense of the following:

• I have a Liebert 80 kva UPS at work supplying a computer server room.
  • it is 480v 3-phase input, and the same output
  • On the UPS load side is a 480/208 transformer and distribution panel that wires some 20+ L6-30R receptacles in the server room
  • There is a 60A main input breaker on the UPS input side
  • is doing 480 x 60 = 28.8 kva accurate in determining the amount of power that could be drawn? if not please elaborate.
• I cannot make sense of the INPUT and LOAD numbers displayed on the UPS lcd panel...
  • Input shows 44,44,45 when equipment on and mostly running
  • Input will show 55,55,55 when running stress.exe on all computers to make them draw max power.
  • Load numbers will show 19,25,33 when not at 100%, and around 25,31,40 when running stress.exe.
  • I had been told the seemingly significant difference between the input and load numbers on the UPS panel was the result of reactive power because of current having to run around in the UPS capacitor banks.
    • this UPS has 40 car size batteries in it and is 3 cabinets each the size of a large refrigerator to give you an idea.
    • it is a double-conversion type... AC in, rectified to DC to charge batteries then inverted back to AC to clean/pure sine wave to the load.
  • can someone elaborate on the input and load current numbers I am seeing on the UPS lcd panel and how to correctly use them to show total power being drawn? I don't get the 3 legs (3-phase) and how to correctly deal with that? Why are the input numbers almost always equal but the load numbers wildly different (i know why) but how do I add up or interpret the load current numbers to get a total power on the load side, to then compare to the input side? I see ~44 on the input and 19,25,33 on the load, all at 480v, that a significant difference no?

for reference... the APC rack PDU's in the computer rack have little LCD screens that shows "kw" and adding all those up for the computers I have in operating comes to ~ 31. My 480/208 distribution panel has an LCD panel also and will read 33.8 "kva" when all my computers are at 100% via stress.exe however this 33.8 number includes some minimal but not insignificant other equipement (servers, network switches, disk storage, etc). I realize the 60A input breaker is way below meeting the capacity of an 80kva UPS can draw, that was known way back and was a limitation due to various other factors at the time... 60A was better than 0A.

what is the best/easiest way to determine what "reactive" current there is on the input side of the UPS at that 60A input breaker, to know how much we're dealing with? The UPS manual says this model UPS has a power factor rating of greater than 0.8, and LAGGING, and an optional input filter can be purchased to increase pf to 0.9 or 0.93 which we do not have and are not sure if is worth looking into.

 

[ron]

You have a lot going on. To know how much power that can be drawn, you need to know the kW rating of the unit. Newer units will be 80kW, but older units will be 72kW or 64kW. The rating is the amount you can draw on the output. Most recent IT loads are relative close to unity power factor, so kW is often the rating to keep an eye on. Whichever rating you hit first, kW or kVA, is the one to stop at (typically the kW rating on the output first).

The 60A main input has to serve the output load plus UPS losses and battery recharge, so 60A seems undersized when the load itself would draw more, nevermind the losses and battery recharge.

Since it is a double conversion UPS, the input and output are decoupled, that is why the input can be nicely balanced when the output is imbalanced.

 

[r7k]

Since it is a double conversion UPS, the input and output are decoupled, that is why the input can be nicely balanced when the output is imbalanced.

thanks.

I tried to include all pertinent information above, I'm still mostly interested in knowing how to interpret the UPS panel showing output current numbers... for example when it shows 19, 25, 33 for the 3 legs because of "3-phase", and each is broken out on the load side distribution panel where there's a 480->208 transformer to then provide 208v in the server room, how do I make sense of the 19, 25, 33 ? I know these three numbers vary because how I have differing quantities to equipment connecting to the wall receptacles and not balancing out across the three legs, but if I do:

19 x 480 = 9120VA 25 x 480 = 12000VA 31 x 480 = 14880VA those 3 numbers added is 36000VA, which does match a kva meter on my distribution panel. It also doesn't match the kva/kw % value on the ups display which says something like 35% when this is an 80kva unit, so 35% of 80000 is 28000 and 8000 difference.... for example I'm going from memory with these numbers a little bit

In working on updating our setup, what's the {best} way to measure or have capability to see current draw and power factor is at my input breaker panel (where I have my existing 60A 480v breaker that we need to upgrade)? I would like to have an LCD type display similar to what's on my 208v distribution panel but provides everything that you could possibly want to know, is such a thing available at 480v?

 

[r7k]

ignore my previous post regarding 19, 25, 31. Time ran out I can't edit it that above post.
I just got "official" numbers off the UPS LCD panel for what it is reading now, as well as the nameplate info :

Nameplate: Input Rating = 72 kva / 69 kw Output Rating = 80 kva / 64 kw Existing display numbers: Input : 39 39 39 Output: L-L 480 480 480 L-N 277 277 277 I 25 26 33 KVA 7 7 9 KW 7 7 9 KVA = 30% KW = 36%

how do I make sense of the 25, 26, 33 current numbers here?

on the output side, 30% of 80kva is 24kva, and 36% of 64kw is 23kw.

39 input amps at 480v = 18.7kva, 39 x 3 = 117, 117 x 480 = 56.1kva; which way is correct here?

for my 60A 3-phase input breaker, is that 60 x 480 = 28.8 max power available? or because of 3-phase do I do a x3 which is 86.4 kva/kw max power available?

can someone elaborate on these displayed numbers, and let me know how much

• current and power is being drawn on the input side of the UPS, which is coming through my 60A main input breaker?
• is there a way to tell power factor and amount of reactive current happening on the input side of the UPS?
• how much power is being used by the UPS for it to simply run, based on input vs output numbers?
• the batteries show 100% capacity and there should be no battery charge current happening
• thanks

 

[jim dungar]

You need to include the 1.732x multiplier (actually the square roof of 3) for converting your input amps/phase to input kVA 3phase.

You can determine your input power factor is simply your input kW/kVA, but the only input value your display gives you is amps, so you probably cannot calculate your input PF.

The internal power usage and losses can be calculated by comparing your input kVA 3 phase to your output kVA 3 phase.

The output currents of 25, 26, and 33 indicate that your output loading is not balanced on the output of the 208y/120V transformer feeding your single phase loads. This is not a big deal but it makes the amps to KVA or kW calculations difficult so it is better to trust your display. For the most part you can simply add the three single kW or kVA values to get the 3 phase output.

 

[augie47]

Even taking the 3 phase factor into account (1.732),a 60 amp breaker seems small for 80kva UPS unless that rating is a possible output due to operation on battery.

 

[topgone]

ignore my previous post regarding 19, 25, 31. Time ran out I can't edit it that above post.
I just got "official" numbers off the UPS LCD panel for what it is reading now, as well as the nameplate info :

Nameplate: Input Rating = 72 kva / 69 kw Output Rating = 80 kva / 64 kw Existing display numbers: Input : 39 39 39 Output: L-L 480 480 480 L-N 277 277 277 I 25 26 33 KVA 7 7 9 KW 7 7 9 KVA = 30% KW = 36%

how do I make sense of the 25, 26, 33 current numbers here?

on the output side, 30% of 80kva is 24kva, and 36% of 64kw is 23kw.

39 input amps at 480v = 18.7kva, 39 x 3 = 117, 117 x 480 = 56.1kva; which way is correct here?

for my 60A 3-phase input breaker, is that 60 x 480 = 28.8 max power available? or because of 3-phase do I do a x3 which is 86.4 kva/kw max power available?

can someone elaborate on these displayed numbers, and let me know how much

• current and power is being drawn on the input side of the UPS, which is coming through my 60A main input breaker?
• is there a way to tell power factor and amount of reactive current happening on the input side of the UPS?
• how much power is being used by the UPS for it to simply run, based on input vs output numbers?
• the batteries show 100% capacity and there should be no battery charge current happening
• thanks

Your load amps mean they are not balanced. The 277V loads are
Phase AN = 6,928 VA
Phase BN = 7,205 VA
Phase CN = 9,145 VA
The total load is 23.3 kVA which is roughly 29% (~30%) of 80 kVA.