UPS calculation help

[XSUBS]

Hello,
I'm looking to properly size a UPS system for use in a new data center (NY, USA).
It will house gear from 3 existing facilities, so current loads are available.
I have put together a spreadsheet, but am not sure I understand the proper calculations. Specifically, is amperage on each leg ADDED or AVERAGED?
I've been adding them together, but I question the output.
Any help would be greatly appreciated.

I'll use one sub-panel as an example...
Service = 208V/3-phase
Amperage on leg1 = 120A
Amperage on leg2 = 124A
Amperage on leg3 = 122A

I'm using the following formula to find kVA:
208*366*1.73 = 131701.44
131701.44/1000 = 131.70 kVA

Does that sound right?
It's my first post here and I did try to search, but came up empty... thanks!

 

[barclayd]

While not rigorously correct, taking the average current value will give you an acceptable result - 44kVA.
That said, I would strongly recommend that you contract with a reputable Engineering firm experienced in Data Center Design.
You have a chance to start with a clean sheet of paper and do it right.
The size of the UPS is only one small part of the equation.
good luck.
db

 

[XSUBS]

Thanks db... I'm with you 1000%!

I am currently working with an engineering firm, and was asked to provide a baseline to work from. I wanted to show some rough estimates to begin the discussions, and just wanted to make sure that the calculations were valid.
My calculations were not, but somehow I knew that!
It's been a very long time since Nuclear Power School!

 

[adavey]

Ideally, you want to obtain peak electrical demand readings of the ups ??????? Maybe the ups has monitoring software and recorded data for you to use

 

[dbuckley]

Whilst agreeing with DB above that "starting from a clean sheet of paper" is the best approach, in dealing with loads you need to be able to handle the worst case of either the total load, and the maximum unbalanced load. As a trivial example, if you have 15,15,30 on the three phases, the total load is 60, suggesting a 60KVA UPS would do, but it won't as a 60KVA UPS is 20KVA per phase, and one phase requires 30KVA.

 

[XSUBS]

Whilst agreeing with DB above that "starting from a clean sheet of paper" is the best approach, in dealing with loads you need to be able to handle the worst case of either the total load, and the maximum unbalanced load. As a trivial example, if you have 15,15,30 on the three phases, the total load is 60, suggesting a 60KVA UPS would do, but it won't as a 60KVA UPS is 20KVA per phase, and one phase requires 30KVA.

I'm not sure I get you... by what you're saying the amperage is added. That is the same as my original calculation:
Service = 208V/3-phase
Amperage on leg1 = 120A
Amperage on leg2 = 124A
Amperage on leg3 = 122A

I'm using the following formula to find kVA:
208*366*1.73 = 131701.44
131701.44/1000 = 131.70 kVA

db wrote that the amperage is averaged, as in this calculation:
366/3 = 122
122*208*1.73 = 43900.48
43900.48/1000 = 43.90 kVA

 

[barclayd]

It looks like dbuckley's 15,15, & 30 are kVA.
You would add kVA's (but not the currents)
We're both right.
Sizing, redundancy, maintainability, Power quality, back-up power, cooling, future growth - Just a few of the items that need to be considered.

I was on Destroyers - you guys were our targets.
db (REAL 'Old Navy')

 

[weressl]

I'm not sure I get you... by what you're saying the amperage is added. That is the same as my original calculation:
Service = 208V/3-phase
Amperage on leg1 = 120A
Amperage on leg2 = 124A
Amperage on leg3 = 122A

I'm using the following formula to find kVA:
208*366*1.73 = 131701.44
131701.44/1000 = 131.70 kVA

db wrote that the amperage is averaged, as in this calculation:
366/3 = 122
122*208*1.73 = 43900.48
43900.48/1000 = 43.90 kVA

Three single phase loads would be

P=(120V*120A)+(120V*124A)+(120V*122A)=14400VA+14880VA+14640VA=43920va or 43.92kVA

The same loads on a three phase system would be

P=((120A+124A+122A)/3)*SQRT3(1.73)*208V= 43952.52VA or 43.95252kVA

The slight difference is that 120V multiplied by SQRT(3) is not 208V but 207.8461

 

[dtiller]

Whilst agreeing with DB above that "starting from a clean sheet of paper" is the best approach, in dealing with loads you need to be able to handle the worst case of either the total load, and the maximum unbalanced load. As a trivial example, if you have 15,15,30 on the three phases, the total load is 60, suggesting a 60KVA UPS would do, but it won't as a 60KVA UPS is 20KVA per phase, and one phase requires 30KVA.

This is often misunderstood - dbuckley is right. Unless you move loads around so that the load is equal in each leg, you'd need a 90KVA UPS for the example above. Generally, the minimum UPS size would be 3 * max(P1 Load, P2 Load, P3 Load) since the largest leg's load will determine the capacity of the inverters.