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Thread: Breaker calculation when using 3 wires, 3 phase circuit

  1. #1

    Breaker calculation when using 3 wires, 3 phase circuit

    Looking for thought process clarification from any ETAP program users:

    I am currentlly using etap to review current load flow, perform short circuit and arc flash analysis.

    There are no single phase load and the circuit from service to branch is 3phase with no neutral or return back to the secondary center tap of the customer 6 power transformers (Delta/Y). The transformers are distributed among 3 substations. They are used to reduce the voltage from 20.8KV to 480V phase to phase.

    The center tap secondary power transformers are tied to the Ufer building safety ground.

    There are quite a load arrangement distributed among 12 hi power molded case circuit breaker panels (MCCBs) out of the substations to where it will be used in the building (motor control centers and branch protective circuit panels).

    Circuit scenario as follows:
    Equipment load is given as a 3 phase load and the current requirement is given as 3 phase current. Breakers data in ETAP software program is entered as single phase data.

    Let me follow with one example load calculation:
    There is one piece of equipment which call for 150KVA at 480V with .9 pf.
    Because of the pf, I calculated the actual load as 150KVA/.9which will yield about 167KVA. Because ETAP rquires the per phase load, the per phase load will be 167KVA/SQRT of 3. About 96.32 KVA. ETAP calculate the current based on the per phase voltage, that is 96.32KVA/277V about 348 amps.

    If you calculate the current based on 3 phase only, the current will be 167000/ 480X SQRTof 3 about 200 amps

    The question:
    Since there is no single phase load and therefore no phase to neutral fault Do I need to adjust the KVA load to yield 200 amps or do I go with ETAP suggested current of 348 amps?

    The approach I use will determine the size of the breaker per phase AT.

    Any suggestion will be appreciated.
    Juan A. Araujo, PE

  2. #2

    Issue solved

    ETAP technical support provided me with a solution. If anyone who uses ETAP and have similar issues could send me a private email.

    Thanks,
    Juan A. Araujo, PE

  3. #3
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    16,969
    I am guessing that the answer you got was to use 200 amps. My reasoning is that you have an error in your first method. If the total 3-phase KVA is 167, then the per phase KVA is one third of that value, and not, as you show, the value divided by the square root of three. So the per phase KVA is 167/3, or about 55.67 KVA. When you divide that value by 277 volts, you are back to the same 200 amps or so.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

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