Understanding PDU's

[Anonymous user]

Hi Guys,

I'm trying to get a better understanding of how the power is measured across PDU's and phases, so that we can better plan our kit installs. We run a data centre that is kind of a halfway house between Colo and owned in that we rent a DC room, but we have a free hand to use it as we see fit. We currently have 3 "pairs" of main PDU's, all are rated at 250A. How does that equate to what's loaded on the 3 phases. ie if each phase were to register an average current of say 80A, would that indicate that we're running close to the 250A limit ?? or does it not work like that ?? What would be sensible limit to put on each phase ??

Thanks,
Mark.

[Erasmus_apc]

Hello Mark ,

I agree that a 250 amp feed is sizeable, but not out side the range of common 3-Phase equipment. As a standard, when dealing with 3-phase equipment, current limits are listed per phase. Could you take a look at the breaker that feeds any of these PDU's. If the breaker says 250 amps, then it would be 250 amps per phase.
to relate this to some APC equipment:
a 40K PDU requires 175 input protection (208V 3-Phase 175 amp {per phase} Breaker)
60K 225 amp Breaker
80K 350 amp Breaker

Do you have any specific information on the PDU(s) in question. Are they APC or MGE equipment? Perhaps you could locate a model or serial number on one of the PDU's. Also, do you see any KW or KVA ratings? Unlike current ratings, power ratings in kilowatts are always listed as a total, covering all 3 phases.

[Anonymous user]

No quite true. As I said there are 3 "pairs", each pair having an A feed, and a B feed for resilliency ( but separate PDU's ). The fact that there are 3 pairs is maybe a little misleading, and I think, just pure coincidence. Each PDU of each pair is rated at 250A. Now to me, 250A on each phase seem pretty hefty. Which is why I 'm questioning it. Is there anything I can look for that will tell me whether its per phase or overall ??

[upsguy_apc]

Here is a owners manual for the MGE brand PDU's the outputs of these PDU's can be as high as ~800A per phase for the 300kva pdu's.

http://www.apcmedia.com/salestools/MBPN-7SPHE4_R0_EN.pdf

[Erasmus_apc · Accepted solution]

Hi,

Knowing that your (3) PDU's are each rated at 250amps implies that you actually have 250 amps on each phase. So if you are running ~80 amps on each phase, you would only be using ~1/3 of your total available power. As far as a sensible limit, 80% is always a nice rule of thumb, so 200 amps per phase. You certainly can take things much closer to the 250 amp limit, but once you get above 80% you should start to consider whether you need to expand. Also, for any 3-Phase power, try and keep the load on your 3 phases as balanced as possible (because if any one leg went over 250 amps, it would trip the entire circuit).