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Thread: SINGLE PHASE AND 3 PHASE LOAD CALCULATION

  1. #1
    Join Date
    Feb 2005
    Posts
    6

    SINGLE PHASE AND 3 PHASE LOAD CALCULATION

    My company is operating on a 3phase 4 wire systems
    ie(415v b/w phases, 240v between phase and neutral wire).
    1 have the following load calculation to make

    (A)SINGLE PHASE LOADS

    (1)flourescent fitings 40W 240v 100 units
    (2)electric motor 1kw 240v 30 units
    (3)air conditinal 2kw 240v 10 units

    (B)3PHASE LOADS

    (1)water pump 4kw 415v 6 units
    (2)5tonnes airconditioners 12kw 415v 10 units
    (3)packaging m/cs 4kw 415v 100units

    what I did was to add all the KW of the single phase loads(A)which gave 54KW and divide the value by 3 to get the 3phase Equivalent of the single phase loads.(54/3=18KW)
    this was then added to the total kws of the 3phase loads.

    which gave=18kw +544kw
    =562kw
    as the combined load rating for the factory

    can someone out there confirm if this calculation is correct.
    also,I would like to know the size of generator that would be adequate to supply power to this loads.
    :confused:

  2. #2
    Join Date
    Nov 2004
    Location
    Las Cruces N.M.
    Posts
    1,090

    Re: SINGLE PHASE AND 3 PHASE LOAD CALCULATION

    You were close, except you don't divide single phase loads by 3 to get a three phase equivalent. You just add the single phase kw to the 3 phase kw and work with that total. In your case, 54 kw + 544 kw = 598 kw.
    Sizing the generator is a slightly different story. It will depend on how the equipment is loaded, the altitude you are at, your need for growth, and other factors.
    Many members here are more informed about generator sizing than I am, you could also contact a generator supplier. They often give away software to help size gen sets.
    Jim T

  3. #3
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    17,160

    Re: SINGLE PHASE AND 3 PHASE LOAD CALCULATION

    Jim T. is correct about the way you add the loads. However, I am not completely confident in the way you derived the loads. You gave the numbers in terms of “kilowatts” (KW), not in terms of “kilovolt amperes” (KVA). The two are never the same. They have the same numerical value for resistive loads. But the value of KVA, for a given component’s load, is higher than the value of its KW, if the load is a motor. So I would ask how you derived the values you are giving us.

    For example, to get the motor load, did you take a value off the nameplate? For the purposes of NEC load calculations, you cannot use the manufacturer’s amp or watt rating. Instead, for single phase motors, you find the horsepower, look up the amps in Table 430.148, and multiply by the appropriate voltage rating, taking that value of voltage from the Table. That gives you KVA for that motor. Similarly, for three phase motors, you find the horsepower, look up the amps in Table 430.150, and multiply by the appropriate voltage rating, taking that value of voltage from the Table, and multiply that result by 1.732 (i.e., the square root of 3). That gives you KVA for that motor.

    To get the required generator rating, you need to know if the owner plans on connecting all of the loads, or just a portion. Generators are rated in terms of “KW, at a power factor of (some value).” Divide that KW by that power factor, and you get the generator’s rating in terms of KVA. You need a generator that can handle the intended load (in KVA).
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  4. #4
    Join Date
    Feb 2005
    Posts
    6

    Re: SINGLE PHASE AND 3 PHASE LOAD CALCULATION

    Thanks gents for your reply.
    I quite agree with your maths,
    though the kw listed in the calculation is based on the name plate ratings of the equipment , but I can handle the necessary correction. also I would be glad if I can get the address of a generator supplier that would give the free soft ware for rating calculations.

    please advice if the maths for the problem below is okay or not.

    (A)single phase loads
    (1) floor lights 250 va(240-v) 21units

    (B) 3phase loads

    (1) Water pump 1.6 kva (415v) 1 unit

    The above loads are to be connected on a 3phase generator by connecting 7unit of the flood light on each phase and neutral. while the electric motor is connected across the 3 phases.
    therefore,
    each phase of the generator will carry a
    total load of =(250va x 7) +1.6kva
    =1.75+1.6kva
    =3.35kva
    meaning that 3.35kva 3phase generator would be adequate for this loads.

    alternatively do I have to add all the kva to have the total load ratings

    i.e (250va x 21) +1.6kva
    =6.8 5kva
    meaning that 6.85kva 3phase generator will be required for this loads.

    hoping to have your advice thanks.

    lukuluku.

  5. #5
    Join Date
    Nov 2004
    Location
    Las Cruces N.M.
    Posts
    1,090

    Re: SINGLE PHASE AND 3 PHASE LOAD CALCULATION

    alternatively do I have to add all the kva to have the total load ratings

    i.e (250va x 21) +1.6kva
    =6.8 5kva
    meaning that 6.85kva 3phase generator will be required for this loads.

    This way is correct to calculate load, however you still may have to make adjustments for the generator to operate.
    Jim T

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