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Load Analysis - Adding Three Phase and Single Phase Loads

Simon87

Hello All, new hear so be gentle.

In the process of creating a new electrical load analysis spreadsheet, and im getting stuck when summing all the three phase and single phase loads together for the maximum demand... Wondering if anyone can help me out here.

Example:

L1

1kVA Single Phase Load = 4.3A

L2

1kVA Single Phase Load = 4.3A

L3

1kVA Single Phase Load = 4.3A

Three Phase

1kVA Three Phase Load = 1.4A

Summing the powers I get 2kVA (as i have a balanced load), but summing the currents i get 5.7A. If I calculate the current of a 2kW three phase load i get 2.9A! I’m probably doing something stupid here but cant seem to figure out why the two figures don’t align... Any help would be greatly appreciated.

 

Thanks, Simon.

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    well the example you describe is not 2kW in total, but 4kW total - one kW on each phase , and the 4th kilowatt is  split, we have to assume equally, across the 3 phases at 333 watts per phase.

    so total power 4000 W

    made of 3 phases each with

    total power per phase 1333 watts

    current per phase = power per phase/ phase to neutral voltage

    = 5.8 amps

    Oh and a 2000watt load over 3 phases is 666 watts per phase, so 2.8 amps per phase is correct to within the rounding of 230V versus 240.

    Hope that helps.

    Mike.

    PS - are you imagining loads strung phase to phase so they see 400V each ? if so you introduce the same load current in two phase wires, out of phase with the voltages that drive them, and factors of root 3 hit you. Far easier to convert all 3 phase loads to an equivalent star and then sum the currents that are in phase with the L-N voltages as I have above.

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  • 0

    well the example you describe is not 2kW in total, but 4kW total - one kW on each phase , and the 4th kilowatt is  split, we have to assume equally, across the 3 phases at 333 watts per phase.

    so total power 4000 W

    made of 3 phases each with

    total power per phase 1333 watts

    current per phase = power per phase/ phase to neutral voltage

    = 5.8 amps

    Oh and a 2000watt load over 3 phases is 666 watts per phase, so 2.8 amps per phase is correct to within the rounding of 230V versus 240.

    Hope that helps.

    Mike.

    PS - are you imagining loads strung phase to phase so they see 400V each ? if so you introduce the same load current in two phase wires, out of phase with the voltages that drive them, and factors of root 3 hit you. Far easier to convert all 3 phase loads to an equivalent star and then sum the currents that are in phase with the L-N voltages as I have above.

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