[Beowulf] UPS system for Linux cluster

Lux, James P james.p.lux at jpl.nasa.gov
Wed Apr 29 09:19:51 PDT 2009

> -----Original Message-----
> From: beowulf-bounces at beowulf.org 
> [mailto:beowulf-bounces at beowulf.org] On Behalf Of 
> vlad at geociencias.unam.mx
> Sent: Wednesday, April 29, 2009 4:46 AM
> To: beowulf at beowulf.org
> Subject: [Beowulf] UPS system for Linux cluster
> Hi,
> I have a linux cluster and need to purchase a UPS system.
> The total expected current consumption will be 240 amps. At 
> 120 V this would require a 30 kVA UPS system.
> However, I see that 30 kVA systems have a limitation for the 
> input current. For example, this 30 kVA system
> (http://www.provantage.com/tripp-lite-su30k3-3xr5~7TRPL1Q9.htm
) allows only 90 amps as input current. I am a bit confused > here, and I need some advice on what UPS system I should look for.
> Thanks,
> Vlad
Here's Tripp Lite's data sheet

Their sheet is a bit better arranged to make clear line(input) vs load(output)

That unit is 30kVA/24kW, allowing for a reasonable 0.8 power factor. At 120V out, that's 250A rms across the three phases, but if your load has good PF, you might only get 200A total (i.e. 66A per phase)

So, is your RMS current consumption for the cluster 240A?   

The 90A is the "per phase" line current (that's how three phase systems are specified) (i.e. 270A total across all three phases)

You need to know both the current draw AND the power factor for your cluster to properly specify something like this, because it has two different limits: an actual power (watts) and a apparent power (VA).  The former has to do with power handling capability, the latter with current handling. 

 But, taking your 240A @ 120V as a starting point, you have about 29kVA (and perhaps 29kW, too).  The sizing should be at least 20% over the load, so you should be looking at 35kVA units.  (i.e. in the U.S., anyway, the electrical code requires that circuits and equipment be sized so that the expected load is <80% of capacity)

Jim Lux

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