What is the equation using to calculate voltage drop ?

VOLTAGE DROP= X  COPPER WIRE LENGTH IN FEET X CURRENT IN AMPERES

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                                                              CIRCULAR MILLS OF COPPER WIRE

IF IT IS ALUMINUM WIRE THEN CHANGE FACTOR ABOVE TO.

3 phase Current

Voltege Drop = (Current x 1.732 x  L x ( R * cos phy + X * Sin Phy ) ) / N

Single phase Volltage Drop = ( 2 x Current  x  L x ( R * cos phy + X * Sin Phy ) ) / N

DC current

VD = (2 x Current x R x L ) / N

where 

R is cable resistance per meter 

X is cable Reactance per meter

L is cable length

N is number of Run.

Please note that , Cable vendor provide voltage drop rating (mV/A/m) in cable catalog.

hence VD  = (mV/A/m) Rating * L * Current.