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Calculate expected PDU per rack

Discussion in Server Room & Data Center Design Best Practices started by Peter , 3/18/2015 9:25 PM
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  • Peter.Burns
    Peter
    New Member
    New Member
    Peter 3/18/2015 9:25 PM

    Hello - I have been looking for some guidance on rack design with regard to power usage. I have been asked to design our rack layout in terms of equipment placement and power consumption for each PDU. 

    We have both AP8861 20A (for servers) and AP8941 30A (for network equipment) PDU's and I am trying to calculate how much load the equipment in each rack will use, I can then plan where everything can go. We will have 2 PDUs per rack and use 3 phase power.

    I used the calculation W/V=A - using the Wattage rating from the device documentation, 208 Volts to get Amps. This give me a higher number than i expected (i figured it would be an average, i understand it is not the max or min)

    Am I calculating this  incorrectly? Any guidance to documentation or help would be great! 

    I found the below info which looks to be along the correct lines:

    50A 208v 3 Phase Rack PDU Load Ratings and Calculations Explained


    Issue

    50A 208v 3 Phase Rack PDU Load Ratings and Calculations Explained


    Product Line
    • 3 Phase Rack Power Distribution models
      • AP88XX
      • AP89XX
      • AP78XX
      • AP79XX
         
    Environment

    • All serial numbers
    • All firmware versions
    • 50A 3 Phrase Rack PDU

    Cause

    This article will outline how phase current ratings and load calculations are determined as well as how load calculations are reached. AP8868 will be used as an example.


    Resolution


    In regards to the AP8868, each bank references two phases:
     

    Bank 1 - Phase 1 and 2
    Bank 2 - Phase 2 and 3
    Bank 3 - Phase 1 and 3
     

    AP8868 is one of the few units where the capacity is limited by the output circuit breakers and not the input cord/plug rating. The breaker on each bank is rated for 20A (de-rated to 16A per UL). Bank 1 draws current from phases 1 and 2 simultaneously. Bank 2 draws current from phases 2 and 3. This means that phase 2 is supplying current to both banks 1 and 2. However, like the phases, the banks are 120° out of phase. This means that part of the time that current is flowing one direction on bank 1, it is flowing the same direction on bank 2 and the two currents increase the draw from phase 2 to over 20A.  However, at other times, when current is flowing one direction on bank 1, it is being returned on bank 2.  This means that you can't just add the loads on the two banks to get the load on phase 2.
     

    Here is another way to look at it. We are in essence supplying three 120V phases, but we are powering three 208V loads.  If the loads are 16A, the total power consumed is (3 banks) x (208 Volts) x (16 Amps) = 9984 Watts. The current on the input must then be (20A x 208v x 3 banks)/(three phases x 120 Volts) = 34.6 Amps. This is how the total current per phase is determined but then please note the de-rating of values, per UL, which is 9984W.

    Now that we know the load ratings, let's look at how to calculate the total power output.

    • For a single phase Rack PDU, Total Power = (Total Current) x (208V)
    • For a three phase Rack PDU, Total Power = (Total Three Phase Current) x (208V) x [sqrt(3)]


    For a PDU like AP8868 with 3 banks, we'd use the single phase calculation for each of the three banks and then add them together to get the "Total Three Phase Current." The input of the Rack PDU is actually limited by the internal circuit breakers.

     

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