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Thread: 3 phase load calculation

  1. #1
    Join Date
    Nov 2007
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    3 phase load calculation

    I have a 3 phase panel with 7 different 208V/1 phase loads. I calculated the load for each 2 pole circuit by multiplying the load amps by 208. The VA totals for each phase are as follows ...

    A = 18,242 VA
    B = 18,242 VA
    C = 16,224 VA

    How do I convert this into a total connected load figure for this panel and would that (along with 25% of my largest motor) be the basis for sizing my feeder and main breaker?

    The system is 120/208, 3 phase, 4 wire.

  2. #2
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    That is a nicely balanced load.
    Bob O.84,Pa.15330

  3. #3
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    Quote Originally Posted by KHall View Post
    I have a 3 phase panel with 7 different 208V/1 phase loads. I calculated the load for each 2 pole circuit by multiplying the load amps by 208. The VA totals for each phase are as follows ...

    A = 18,242 VA
    B = 18,242 VA
    C = 16,224 VA

    How do I convert this into a total connected load figure for this panel and would that (along with 25% of my largest motor) be the basis for sizing my feeder and main breaker?

    The system is 120/208, 3 phase, 4 wire.
    [Minimum] sizing of main breakers and feeders are done through Article 220 calculations under Patrs III, IV, and/or V. There are quite a few factors which impact the minimum sizing, including continuous and non-continuous loads. The calculations are the only NEC-compliant method.

    In determining size for imbalance, your loads, as distributed on the phases, will have to be converted to line current and weighed against the minimum size as calculated to determine if larger sizing is required.
    I'll never get there. No matter where I go, I'm always here.

  4. #4
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    Nov 2007
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    more help

    I am reasonably familiar with the NEC rules on sizing feeders and equipment. It's the 3 phase calculations that I don't work with very much. I need help to understand how to take the A,B, & C volt-amp totals and convert that to a single amperage figure for calculating my feeder and equipment sizes. The loads are all HVAC equipment that will be continuous loads.

    Thanks!

  5. #5
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    Quote Originally Posted by KHall View Post
    I have a 3 phase panel with 7 different 208V/1 phase loads. I calculated the load for each 2 pole circuit by multiplying the load amps by 208. The VA totals for each phase are as follows ...

    A = 18,242 VA
    B = 18,242 VA
    C = 16,224 VA

    How do I convert this into a total connected load figure for this panel and would that (along with 25% of my largest motor) be the basis for sizing my feeder and main breaker?

    The system is 120/208, 3 phase, 4 wire.
    What is the individual VA ratings of the 7 individual circuits. If you calculated this wrong you could easily have high totals. I see you as reporting a total of about 52.7 KVA. I could also see you actually having a total load of about 26 KVA depending on how you came up with the numbers you posted.

    Find out what the total load is @ 208 volt single phase then work on dividing it up across the three phases. If all seven loads are equal you can easily come up with a balanced per phase load for six of the circuits and then add the unbalanced portion of the seventh to two of the phases.

    First thing is to know the VA ratings of the individual circuits.

  6. #6
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    Quote Originally Posted by KHall View Post
    A = 18,242 VA
    B = 18,242 VA
    C = 16,224 VA

    How do I convert this into a total connected load figure for this panel and would that (along with 25% of my largest motor) be the basis for sizing my feeder and main breaker?

    The system is 120/208, 3 phase, 4 wire.
    Simply, the highest load must be accommodated, so you need to use the largest number to figure the load current. In this case, you have two lines with 18.242kva loading.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  7. #7
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    I would also like to see this calculation. I see it as simply 18242va/ 208 but I am not sure that is correct. Then what do you do with 3 different currents all single phase 208V

  8. #8
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    A = 18,242 VA
    B = 18,242 VA
    C = 16,224 VA

    A 18.2kva/.120 = 152 amps

    B 18.2kva/.120 = 152 amps

    C 16.2 kva/.120 = 135 amps

    Combining all the load 18.2 + 18.2 + 16.2 = 52.6
    52.6 kva/0.208 x 1.73 = 146 amps
    If you assume the load was balanced 3 x 18.2 = 54.6
    amps = 54.6 /.208 x 1.73 = 152 amps

  9. #9
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    Quote Originally Posted by bob View Post
    If you assume the load was balanced 3 x 18.2 = 54.6
    amps = 54.6 /.208 x 1.73 = 152 amps
    You have to for designing the service and distribution. The highest line's loads must be accounted for.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  10. #10
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    California
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    Quote Originally Posted by KHall View Post
    I have a 3 phase panel with 7 different 208V/1 phase loads. I calculated the load for each 2 pole circuit by multiplying the load amps by 208. The VA totals for each phase are as follows ...

    A = 18,242 VA
    B = 18,242 VA
    C = 16,224 VA

    How do I convert this into a total connected load figure for this panel and would that (along with 25% of my largest motor) be the basis for sizing my feeder and main breaker?

    The system is 120/208, 3 phase, 4 wire.
    Your VA load does equal 52,708VA if this were the total VA result of an Art 220 calc you’d have an 87A calculated load but to arrive at this there is a method to follow something like below:
    10,000 (first 10kva at 100%)
    21,354 (remainder at 50%)
    31,354 (total non-continuous load - not likely all non-continuous)
    0 total (continuous load - not likely zero)
    0 (largest motor at 25% - probably have this too)
    31,354 (total VA)
    87 (total Amperes VA/208/1.732)

    I suggest using an example in the annex D (D3) to calculate your load and be compliant.

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