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Single Phase Circuit
Vd = 2 I L ( R cos Ø + X sin Ø )
Three Phase Circuit
Vd = √3 I L ( R cos Ø + X sin Ø )
Volts Dropped = (2 x 0.866) x L x R x Amps/1000
L=lenght
R=resistance
Many modified versions of that equation are used in different fields, I assume you are interested in 3-phase ac, it will be helpful to have the cable datasheet nearby, the equation is:
Voltage Drop of Cable =(1.732 · Current · (RcosǾ + jXsinǾ) · Cable length · 100) / (Line voltage · No of run · 1000)
Where:
current: load required current.
R,X: resistance, reactance per unit length of the cable, which can be aquired from the cable's datasheet/catalogue.
Cable length: the distance from feeding end to terminating end at load, along the desired path, don't combine the length of each core, just the overall length of the cable.
line voltage: supply voltage
Number of runs: in case you have multiple runs of the same cable to share the current, along the same path, from the same source, if you are using only one cable, substitute 1 in the equation.
you will get the result in %, the maximum allowed voltage drop is constrained to the applicable code of cable installation area/country, but it's advisable to stay below 5% in industrial installations, 3% in residential applications.
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