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Thread: Branch Circuit Sizing for Telecommunications Load 645.5(A)

  1. #1
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    Branch Circuit Sizing for Telecommunications Load 645.5(A)

    Circuit Sizing for communications equipment with "known" load profiles relies on the definition of total connected load which I have yet to find a straight forward definition in NEC 70.

    So, is the total connected load considered peak power, average power, engineer's discretion, or something else? For IT loads, BICSI (technical/professional certification organization) https://www.bicsi.org/ implements average power, but this may not be completely correct as often the power supplies are rated disproportionately higher than average power and I want a second opinion. For instance, a 1200W peak power rated switch (appliance) can often sip power somewhere around 200W on average. So to meet the intent of the Code, prevent fires, is the circuit sizing for this load use the peak power (say 1200W)? Or average power (say 200W)? The reality is that there is often 5-10 pieces of equipment on this circuit with similar power profiles, but to keep the example simplistic.

    This is my first post, please be gentle... I did look through the threads for something similar, but came up empty. Thanks for the help!

  2. #2
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    In most data center applications, the only way to know what equipment actually draws, is to put it in a burn in configuration and measure the actual load. The nameplate is not typically the actual load, whether it be peak or average.

    Some manufacturers will publish a document such as this http://www.dell.com/downloads/global...amily-PPDS.pdf

    It offers the variety of loading methodologies of the equipment in how the IT guys can use it.

    In the example I linked, a 350W PSU, but the IT guys may have provisioned it in the minimum configuration and it will never draw more than 100W
    Ron

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    Ron, Thanks for the reply! The question then becomes how much risk are you willing to live with... since any tech can come in and reconfigure the switch to use more juice.

  4. #4
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    Quote Originally Posted by j_dobber View Post
    The question then becomes how much risk are you willing to live with... since any tech can come in and reconfigure the switch to use more juice.
    That is the case. Depending on history with the client, I often install 60A-3Pole CB's which future proofs the branch circuit to the rack.

    It is unusual for them to draw 15kW in every rack, but they can draw 15kW in any rack and average XkW that we pick a value and size the panelboard and upstream feeders and UPSs, etc to support.
    Ron

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