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Thread: total connected current of a commercial building.

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    total connected current of a commercial building.

    i'm doing an electrical plan of a commercial building. 230volts, 3 phase, 3pole.
    the total on the schedule of loads are:
    AB = 22.6 Amperes
    BC = 14.34 A
    CA = 22.6 A
    3phase = 9.6 A (which is a 3hp pump motor)

    this is the first time i'm working on a 3phase.

    How do i compute the total connected current for the main feeder?

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    Quote Originally Posted by Ellie View Post
    i'm doing an electrical plan of a commercial building. 230volts, 3 phase, 3pole.
    the total on the schedule of loads are:
    AB = 22.6 Amperes
    BC = 14.34 A
    CA = 22.6 A
    3phase = 9.6 A (which is a 3hp pump motor)

    this is the first time i'm working on a 3phase.

    How do i compute the total connected current for the main feeder?
    It looks like you already have in the above schedule, If you mean to calculate the demand load see art 220
    John,

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    Quote Originally Posted by Ellie View Post
    ...

    How do i compute the total connected current for the main feeder?
    Panel schedule totals seldom reflect proper feeder load calculation results... but they do help when considering if the feeder size determined through proper load calculations is too small due to imbalance.

    As mentioned, see Article 220...
    I will have achieved my life's goal if I die with a smile on my face.

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    First of all, welcome to the forum.

    Secondly, it is not clear to me whether this is an existing building or something new. I am wondering, because of the degree of precision you are using for the currents. In a practical sense, for the sake of normal design processes, current cannot be accurately measured down to two significant digits. I suspect this is a homework assignment, with numbers made up by the instructor, as opposed to an actual installation. Am I right?

    If so, then article 220 will not help you. That article has to calculate the loads on a building, based on knowledge of such things as square footage, type of occupancy, intended heating and cooling loads, and a host of others. If you are simply being asked to add up a bunch of numbers, then that process wonít get you the right answer.

    Next, the one thing I believe you need to get accustomed to right at the beginning of your career is that loads should be added in terms of power, not current. So if you are given current values, then first convert to power, do the addition, and convert back to current.

    In your case, not information was given. Absent any values of power factor, I will proceed with the assumption that the single phase loads are purely resistive loads.
    22.6 amps times 230 volts equals 5198 VA.
    14.34 amps times 230 volts equals 3298 VA.
    22.6 amps times 230 volts equals 5198 VA.
    9.6 amps times 230 volts times the square root of three equals 3824 VA.
    Total load is 17,518 VA
    Divide 17,518 VA by 230 volts, and divide the result by the square root of 3, and you get a total current 44 amps.

    Please note that this process treats the three phase currents as though they were balanced (i.e., equal). They wonít be. From a design perspective, we treat them as though they were. If you need to know the three individual currents, then you need to use a far more complex mathematical method, and I donít have time to go into that.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    It looks like someone already calculated load in amperes and it looks like the load is small. Where did the ratings come from ? Based on that a 100 amp three phase service will do. 240 X 1.732 X 100 = 39.8KW Is it a small store or something like that ?

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