How to calculate cable size and its current carrying capacity? Explain it by using formulas?

Thanks for the invite. The cable size calculation depends on the type of switchgear used. Whether it is Fuse Protected OR fed from circuit breaker, will also affcet the selection criteria.

The method inclues calculating Voltage Drop and Size of Electrical cable based on load data:

•  Electrical Details Required: Electrical Load in KW, Distance between Source and Load in Meter, System Voltage in V , no of phases, Power Factor and limits for Permissible Voltage drop
• Cable Laying Detail: details of method for laying of cables i.e. underground, over ground, on trays and alongwith it the proximity with other cables. Then based on these derating factors to be derived from Manufacturer calalogue..
• Soil Details: Thermal Resistivity of Soil

Calculation:

• Consumed Load= Total Load x Demand Factor
• Consumed Load in KVA= KW/P.F
• Full Load Current= (KVAx1000) / (1.732xVoltage)
• Calculating Correction Factor of Cable from manufacturer catalog
• Soil temperature correction factor - K1
• Ground Temperature Correction Factor (K2) 
• Soil Correction Factor  K4
• Cable Depth Correction Factor K5
• Cable Distance correction Factor K6
• Total De rating Factor= k1x k2 x k3 x K4 x K5 x K6 
• Selection of Cable:
  • For selection of Proper Cable following Conditions should be satisfied
  • (1) Cable De rating Amp should be higher than Full Load Current of Load.
  • (2) Cable Voltage Drop should be less than Defined Voltage drop.
  • (3) No of Run of Cable >= (Full Load current / Cable De rating Current).
  • (4) Cable Short Circuit Capacity should be higher than System S.C Capacity at that Point

commonly thumb rule shoud not use

make it formula

calculate voltage drop

consider laying place and temprature

finaly cable size avialable