If the cable meets European standards, the formula is as follows:

European Formula (mV/A/m)

VD = R x I x L / 1000
VD: The voltage drop (V).
R: The resistance value from voltage drop per ampere per meter table (mV/A/m)
supplied by the cable manufacturer.
I : The load current (A)
L: The length of conductor (m)
The value R is determined from a table provided by the cable manufacturer. See Table 1
below as an example of a table that is provided by a manufacturer.
Below is an example calculation for determining voltage drop.
Determine the voltage drop of a 380V, 3 phase circuit with a current of 100A and a
length of 150 m and a conductor size of 35 mm. This is a secondary service feed.
The voltage drop of secondary service is limited to 3% (380 x 3% = 11.4 Volts).
Volt drop = Resistance x Current x Length / 1000. Resistance is found on Table 1
below.
VD = 1.1 x 100 x 150 / 1000
= 16.5 V

The maximum voltage drop allowed is 11.4 V. To determine the size of cable that will be
required to meet the voltage drop requirement, determine the value of R that will meet the
requirement.
11.4 = R x 100 x 150 / 1000
R = 11.4 x 1000/100 x 150
= 0.76 mV/A/m
Referencing the table provided by the cable manufacturer (Table 1), the cable that has a
resistance of .76mV/A/m or less is a 70 mm cable with a resistance of 0.55 mV/A/m.
Calculating the voltage drop for the 70mm cable results in:
VD = R x I x L / 1000
= 0.55 x 100 x 150 / 1000
= 8.25 V
The percentage voltage drop is:
Percentage Voltage Drop = 8.25 x 100 / 380 = 2.17 %
Therefore, in order to transmit a 3 phase current of 100A per phase over a length of 150
m, with a total voltage drop equal to or less than the maximum 11.4 volts, a 70 mm cable
is needed.
This same procedure would be repeated for a feeder or branch circuit and the results
added. The total voltage drop should not exceed 5%


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