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    ctfacilities's Avatar
    ctfacilities Posts: 1, Reputation: 1
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    #1

    Sep 18, 2007, 10:33 AM
    3 phase versus 1 phase conversion
    I have an 800 amp 277/480, three phase, 4 wire service. I want to calculate the available amperage for 120 & 208 single phase assuming a PF of 1. Would this be 1.73 times the 3 phase amperage rating (800)? If not, how do I calculate this. Sorry, but I am an ME, not an EE. Thanks.
    tkrussell's Avatar
    tkrussell Posts: 9,659, Reputation: 725
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    #2

    Sep 19, 2007, 01:56 AM
    You do realize that you will need a transformer to convert the 277/480 down to 120/208 volt?

    To calculate the 120/208 volt amperage, first you must calculate the KVA capacity of the 800 amp 277/480 volt service.

    800 amp x (480 x 1.73 = 830 Phase to Phase voltage) / 1000 = 664.32 KVA is the minimum size transfomer

    664.32 KVA / (208 x 1.73 = 360 Phase to Phase voltage) = 1845 amps

    This is less any applicable deratings.

    Not sure why you would want to convert the entire 480 volt service to 120/208, usually most is used for 480 volt equipment, and a smaller transformer sized for the minimal 120/208 volt load is installed.
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