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Three phase voltage drop

Derek Eccles

For a balanced 3 phase system the supply authority, distributes 400v at the supply transformer.

The supply cables are 150m in length, so care is required to ensure the voltage drop does not become excessive at the customers terminals

Assuming there is a votage drop of 20v, across each cable at a particular balanced load, what voltage would the customer see at his terminals ?

I presume 360v ?

Regards, Derek

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  • 0

    Err what  ???

    20v in each phase conductor certainly turns 230V to 210V, but the phase to phase voltage is only affected pro-rata -  so the 400V also  loses 20/230ths. 

    i.e that  is 8% voltage droop

    So more like 33V lost, so the 400V becomes  367v to 370V within rounding errors.

    This probably means your cable is undersized or you should look at a higher distribution voltage if the load is constant.

  • 0 in reply to mapj1

    Using Vl = sqrt(3)* Vp, I also get Vl = 367v.

    What happens when the load is imbalanced i.e. first phase loses 20v and second 10v ?

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  • 0 in reply to mapj1

    Using Vl = sqrt(3)* Vp, I also get Vl = 367v.

    What happens when the load is imbalanced i.e. first phase loses 20v and second 10v ?

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  • 0 in reply to Derek Eccles

    I hope I have found the answer.

    For a STAR connected load with a neutral connection the line (and in this case, phase) currents I1, I2 and I3 are phasors and are related to the neutral current In by :

    In = I1 + I2 + I3.

    If the load is balanced In = 0.

    If the load is unbalanced, it is possible to calculate the neutral current (using phasors) and therefore the voltage drop in the supply cable.

    This means the star/neutral point at the load will not be zero volts - not sure what this is,

    I guess the effect will be the phase voltages will not be 230v and not equal. Likewise the line voltages at the load will not be 400v and not equal.

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