Cable Sizing Calculations

In electrical system design, cable sizing is one of the major activity. Lets see how to size the cable for a load.


CABLE SIZING CALCULATIONS

( This article is draft stage and is under modification..........)

In electrical system design, cable sizing is one of the major activity.
Simplified Procedure for cable sizing;
• Determine the design current
• Decide the Protective device rating
• Decide the installation method of cable
• Identify the de-rating factors
• Find the Calculated current rating of required cable
• Select the cable
• Check the voltage drop
• Check the short-circuit capacity


STEP-1 : CALCULATE THE DESIGN CURRENT
Use # phase power formula to calculate the design current

Let's assume P=50 kW, v=415 V, and pf=0.8
• Where P is 3 phase load in kW and V is the Line to Line voltage.
• pf is the power factor of the load which may be taken as 0.8 , if otherwise not known.


we would get Ib= 50/ (1.732 * 0.8 * 0.415 ) = 86.95 Ampere

If it is a motor load, we have to divide by efficiency so as to get the input current drawn.


STEP-2 : SELECT THE PROTECTIVE DEVICE RATING

Protective device ( MCB/MCCB circuit breaker) is selected to protect the cable against damage from thermal overload.
Protective device must be selected such that Nominal rating of the device is more than the design current, i.e