CABLE SIZING

ELECTRICAL CABLE SIZING

This is the main thing consideration among some of others. Briefly, we can shortcut that whole lesson in order to serve reader to faster understand and clear with this subject. There are three factors to consider:

1. VOLTAGE DROP CONSIDERATION

 1.1 Voltage drop is the big factor to cause any drop voltage on the system line. To calculate this, a familiar is provided :

  Voltage drop = Ib ( Rcos@ + Xsin@) L/1000 , Single Phase System
    Where : Ib is the design current
                  R is the cable resistance, m-ohm
                  X is the cable reactant, m-ohm
                  L is the length of cable/wire, meter
    Note: The ratio of R and X are bring from the tabulate.

2. OVERLOAD CONSIDERATION

 2.1 Over load consideration is the most common that always took account to cable sizing methods. But, the correct and professional methods is required the design engineer to well understand about few main factor as:

     #1. Kind of loads ( Motor, Lighting....)
     #2. Stating operation methods ( A/T, DOL and Soft Stator, I.e Motor)
     #3. CB operation Current, called I2.
     #4. The additional standard, mean I2<= 1.45Iz , Ib <=In<=Iz or It
     #5. And starting  current Is, Is = n^2(7.5xIb or Ifl)
     Note: A/T mean Auto transformer
               DOL mean Direct On Line
               I2 mean CB operating current, i.e 1.3-1.4In
               In mean CB trip rating current
               It/Iz mean the current carrying capacity of cable (From tabulate)
               Is mean starting current or inrush current

3. SHORT CIRCUIT CONSIDERATION

 3.1 Most of Electrical engineers never realize about this methods but, it is too simply if we well know about this brief methods:

    K^2 x S^2 <= Isc^2 x t

    Where: K = 115 for PVC Cable Insulation
                 K = 143 for XLPE Cable Insulation
                 S mean the Cross Sectional Area of cable/wire, Sq.mm
                 Isc mean Fault/ Short Circuit Current, kA
                 t mean short circuit duration, S.

Above are the common methods that make reader faster understand and clear. To make more closer about this subject, later article will show about the actual example about how to official calculations methods with this kind of works.


Thanks & Regards,