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When we know the nominal current of a motor, we can select the cable's section either from the tables of " IEC364-5-52 " or from the tables of the manufactures, but these values are in the standard conditions, so, we should integrate the Correction factors accordingly to : the kind of cable, Method of installation, Ambient temperature, and the no. of cables in the same conduit.
Ex:
P(kW) = HP x0.74
I = P *10^3 / (1,73*400*0,8)
P=5,6 kW => I=10, A ; wire2,5 mm2
P=7,5 kW => I=13,5 A ; wire2,5 mm2
P=11 kW => I=19,9 A ; wire4 mm2
P=15 kW => I=27 A ; wire6 mm2
P=18,6 kW => I=33,6; ; wire6 mm2 and so on>>
Depends of watts of your machine and be sure you know about the electrical standards and also electrical code
Step1: Calculate full load amps of the machine. (or from nameplate)
I = P *10^3 / (1,73*V*pf)
Step2: Select cable size after apllying 1.2 to 1.25 times the above current value (to account for short time overload) and applying derating factors depending on grouping,type of instalation,grouping etc. from the chart
Step3: Calculate the recommended size using short circuit current value
S= k^2 * I^2*t (k-constant ; I-fault current, t- clearing time; s-size)
Step4: Calculate voltage drop using the above selected cable's parameters and ensure it is within allowable limits.
Vd = {1.732 * I * L * (RcosФ + XsinФ) x 100 } / n * V * 1000
L = Length of cable in meters.
I = Current through cable in amps
R = Resistance of Cable in Ω/km.
X = Reactance of Cable in Ω/km.
Vd = Actual voltage drop in % of system voltage.
V = System Voltage in volts.
n= No. of run per phase
