120/208V 3 Phase Communications

Discussion in 'General Electronics Chat' started by variable, Mar 4, 2008.

  1. variable

    Thread Starter New Member

    Mar 4, 2008
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    I am setting up a small-ish communications facility (18 racks), that initially will not be anywhere near fully loaded. I would like to make use of the 3 Phase setup the building already has. I estimated that the maximum draw of that many racks is about 90KW, or 432A @ 208V. If I split the load as evenly as possible, and I am running at max (90KW), what is the voltage and amperage that the lines will see (as in what size conductors do I have to use, I would like to use the 2/0 wire I already have, but I realize that this is probably not possible). Do I then need to have 5 wires run, 3 hot, 1 neutral, 1 ground, and do they all need to be the same size conductor?

    I somewhat understand the difference between delta and wye, but at this point I am not completely knowledgeable about any of this.

    Thanks for any help!
     
  2. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    NEC table 310-16 states that the current on 2/O conductor in a cable or raceway is 195 amps for THHN or like wire.

    The real questions are:

    What is the panel that you're feeding your racks from rated at?
    What will you be pulling per phase?

    The KW does not change from the promary to the secondary of the transformer. So in answer to your question about what kind of amps the line will see, just use the same formula for the load side. Assuming 480Volt Primary:

    Load: 90KW / 208V = 432A
    Line: 90KW / 480V = 188A

    Yes, you will need five wires for a wye, four for a delta. In a wye x-former the fourth wire taps the center of the secondary windings and acts as the neutral for your 120 volt loads. Never reduce the size of the neutral in any installation.

    The secondary of a delta has no neutral, only an equipment ground.

    Hope this helps.

    erin
     
  3. variable

    Thread Starter New Member

    Mar 4, 2008
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    0
    The building manager told us that it is a 208V 3 phase system, does that mean the lines are at 208V(432A) or 120V(750A)?

    If it is 208V then it looks that I would need 5/0 cable, that must be truly humongous (and expensive). The load is not split then across each line? So I would not just have 144A on each line but rather a full 432A on each line?

    I was going to split the power evenly across the different phases with something like:
    http://apc.com/resource/include/tec...L&page_type=displaybasic&printer_friendly=yes

    I assume that that device just lets say the first 14 plugs on phase1, second 14 on phase2, third 14 on phase3? Am I correct in this assumption?

    Where is a good place to look for 3phase panels?

    Thanks
    -Eric
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Yes. It does.;)

    You've got 208Vac between any two phases and you've got 120Vac from any one phase to neutral. 208 = 120 * \sqrt{3}

    This explains it better: http://www.allaboutcircuits.com/vol_2/chpt_10/2.html
     
  5. variable

    Thread Starter New Member

    Mar 4, 2008
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    ok, so 90kw across 120/208v 3phase would be 250A per wire requiring a quantity of 5 4/0 wires. (3 hots, 1 neutral, 1 ground).

    We need to run this across about 150ft. Would this or a splitting of the power across two 240v circuits be better? (187.5A)

    What would I use for breakers and panels for 3 phase? Would I just use a regular single phase 120v breaker for each phase? (a 250A breaker at that).
     
  6. gerty

    AAC Fanatic!

    Aug 30, 2007
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    I'm not familliar with your equipment, but that sounds like an awful lot of power... Are you sure of those calculations?
     
  7. variable

    Thread Starter New Member

    Mar 4, 2008
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    I ran the calculation as a absolute maximum;

    7 servers uses 1kw. 40 servers per rack; 14 racks = 80kw. I added 10kw for the extra racks in there (as many as 4 more) and the Air Conditioning - 90kw.
     
  8. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Ah..
    The airconditioning..
     
  9. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    Let's go back a bit. Check the name plates on the individual servers:
    What voltage will they run at each?
    How many Amps / Watts are they rated at each?

    If your math still holds true and you really do need 90KW at 208VAC, then you should probably run individual branch circuits to each rack from your service panel(s). Each branch circuit will only need wire large enough to handles it's own load.Though there are panels made to handle +400 Amps, it may be more economical to install three 200 Amp panels.

    Another consideration: If you're supplying power to servers directly from the building's electric service, do the servers have their own internal power conditioners? What about your UPS needs?
     
  10. variable

    Thread Starter New Member

    Mar 4, 2008
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    The servers are 120v, but the APC power strips I posted above take in 208v 3ph and output 120v single phase (I assume by just running each phase to neutral, and having each phase cover 1/3 of the 42 ports on the strip, so 14 ports per phase).


    This is a large-ish commercial building. There is 208v 3 phase in the basement, we are going to run 3phase to a UPS/ATS in the basement with the batteries and generator, and then run 3phase off that up to the server room itself and have a panel up there and drop off 20amp 3 phase circuits to each of the racks (20amp per phase should be 60amp @ 120v or 7.2kw)

    Does that sound like a good plan/logical?
     
  11. thingmaker3

    Retired Moderator

    May 16, 2005
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    It does sound like a good plan - IF you hire a licensed electrician to perform the work. He or she will take care of permits and scheduling the inspections in addition to performing the work safely and correctly.
     
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