Power for server room

[kevinware]

I need some help how to calculate the feeder size to bring to a room that will house I.T. equipment. This room is existing, but it is going to be modified into a server room. I can tell everyone up front that this room will NOT be compliant with 645.4. The equipment that will be installed in this room will be as follows:

The room will be capable of holding 24 racks. Each rack will hold 15 servers. The math breakdown is this, One server = 416VA,
2A x 208V single phase = 416VA.
so a rack with 15 servers at 416VA
each = 416VA x 15 servers = 6240VA per rack. Now 24 racks will be 6240VA x 24 racks = 149,760VA. My boss says this room will never get that amount of equipment. So lets say my boss is right and I only have 14 racks and he does the math on that amount, which would be
6240VA x 14 racks = 87360VA. All of this equipment will be placed on a room UPS system and my boss wants to use this model from Powerware
Powerware UPS
The 80KVA 480V/208V/120V 3-Phase unit is what he wants to use and to my understanding there will be two of these 80KVA units, but where I am getting confused is these two units will be setup in what powerware is calling "Redundant Parallel Capacity". If there are two 80KVA setup in "Redundant Parallel Capacity" doesn't this mean my room load is split between the two UPS's 87360VA / 2 = 43680VA which the UPS should hold fine, but if one of the UPS's fail wouldn't that cause the other UPS to overload and go to bypass? Should the feeder that is brought to this room be sized on the 87360VA or the load of the two UPS's? 160KVA I hope I didn't confuse everyone to bad.

I am caculating a feeder size based on the room load of:
87360VA / 360 = 243 x 1.25 = 300Amp feeder

Please help me and correct me.......


Thanks,
Kevin

 

[steve66]

Re: Power for server room

So basically your boss is saying the load will never be more than 80KVA (so either UPS can run the load with the other UPS on the fritz.)

I would size the feeder for one UPS's max input current, plus the battery charging current of the other UPS. (After a power failure, both UPS's could be charging while one is driving the load). If you look in the appendix of the powerware installation manual (available on line), you should be able to calculate what is needed.

Oh, and don't forget there will have to be an air conditioner or something to cool the room. What kind of power will that be on?

Steve

 

[kevinware]

Re: Power for server room

Good point Steve66,

I hope they don't decide to put the A/C units on the panels that the UPS's will be feeding. There are other people making those decisions and they are much smarter then me :roll:

The UPS's are 80KVA 480V 3-phase with a power factor of 0.9, so I am doing the math like this:

UPS 1 - 80KVA x .9 = 72000W / 480V = 150Amp

UPS 2 - 80KVA x .9 = 72000W / 480V = 150Amp

I will need a feed to an external bypass switch so that will be 150Amp also. So I am going to say a 300Amp feeder with the information I have so far.

Please give your opinion on the calculations.

 

[steve66]

Re: Power for server room

hope they don't decide to put the A/C units on the panels that the UPS's will be feeding.

If the power goes out, they are going to need some way to cool the UPS's. An exhaust fan is the minimum, and AC is better.

Do these have a 3 phase input? If so, then you can divide the input current by sqrt (3).

I'm not sure why you are multiplying by a power factor. I would assume that any non-unity PF is going to make the input current higher. So I would divide 80KA by 0.9. (Maybe the power factor should actually be ignored? If so, it gives you an extra safety factor.)

I would also divide by 0.9 or something like that for efficency. Then I would multiply by 1.25 to take into account that the UPS has to drive the full 80KVA load while charging the batteries.

So for one UPS I would probably use something like this:

80000 VA * 1.25
----------------------- = 149 amps
480 * sqrt 3 *0.9 * 0.9

which is about what you had, but my calculation is for a 3 phase unit.

For the second unit, I would probably only add the battery charging current, since we are assuming only one UPS can drive the load. So the battery charging current might be something like 1/4 of the 80KVA, or 20KVA. Thats about 25 amps. So I would probably spec. a 200A feeder (again, if they are 3 phase units).

Hope that helps some:
Steve

 

[dereckbc]

Re: Power for server room

Originally posted by kevinware:
The 80KVA 480V/208V/120V 3-Phase unit is what he wants to use and to my understanding there will be two of these 80KVA units, but where I am getting confused is these two units will be setup in what powerware is calling "Redundant Parallel Capacity". If there are two 80KVA setup in "Redundant Parallel Capacity" doesn't this mean my room load is split between the two UPS's 87360VA / 2 = 43680VA which the UPS should hold fine, but if one of the UPS's fail wouldn't that cause the other UPS to overload and go to bypass?

Kevin, Parallel Redundant Configuration means the UPS are wired in the "Exclusive OR" configuration. Or in other words one UPS handles all the load at once while the other is in HOT STAND BY by waiting for the other UPS to fail.

If the site has a back up generator properly configured and with Parallel Redundant UPS system, it is almost impossible to ever have a power outage or PQ problems.

 

[ron]

Re: Power for server room

I believe the way that Powerware sets up redundant parallel capacity, is to have two 80kVA modules, paralleled on the output, each sharing half the load. If one module fails, the other carries the full load. This works out fine, as long as there isn't a fault on the paralleled output bus, which will drag down both modules.