Dear Ahmad ;
It will be better to define the Standard that your are using in your country, if you use the ” IEC… “, please find below the method to define the Cable's Section.
1- Calculate the Load's Current that we called ” Ib “.
2- Accordingly to :
– The methode oh installation of cable ” Barried or Unbarried ” and the type of installation for each one.
– The number of the circuits laid togother.
– The ambiant temperature.
we will define the ” Correction Factors – k1, k2, … ” from the table of ” IEC 60364-5-52 “
3- We calculate the Total Correction Factor ” k ” by multiplying all correction factors ” k = k1 x k2 x … “
4- we calculate the nominal current for the cable in the working condition ” I’z ” by the formula : I’z = Ib / k “
5- From the ” IEC 60364-5-52 ” tables that mentioned to the Nominal Current of each section in the Standard Conditions ” Iz ” depending on :
– The ” methode oh installation “
– The kind of cable ” Cu or Alu ; Single Core or Multi Cores ; PVC or XLPE “
we will define the suitable cable's section where ” Iz ≥ I’z “.
5- Then we calculate the Voltage Drop by the Formula : ” Δ V = 1.73 x I ( R . Cosφ + X . Sinφ ) “, where : R = ρ . L / S ; X = λ . L
6- This value of the Voltage Drop should be smaller or equal than the value defined by ” IEC… ” ( 3% for Lighting ; 5% for Machines ), if it's not the case, then the Cable's Section should be oversized.
7- In the 1st & 2nd levels of the network where the values of the short circuit currents are important, it will be better to be sure that the cable can withstand the short circuit current during the tripping time of the protection device, where we use the formula : ” I² . t ≤ K² . S² “,
if it's not the case, then the section of cable should be oversized.