How to calculate cable size,if we know the load and distance to the load?

Cable sizing is done based on three parameters:

1. Load current,
2. Short circuit current carrying capacity,
3. Voltage drop.

Step 1:

Calculate the load current from the load data available. Formula is,

I=Load (in kW)/(sqrt(3)*V(in kV)*p.f) (A)

The current obtained has to derated since the cable is designed to operate at a defined temperature. Derating factors will be available in cable catalog.

Get the cable size corresponding to this derated current from the catalog.

Step 2:

Next, find the cable size for the short circuit capacity of the system. Formula used is,

Cable size= (Fault current(in kA)*Fault clearing time)/K

where, K-constant depending on the conductor and insulation used. K=90 (for Al cables), K=140 (for Cu cables)

Step 2 is normally done for HT cables. For LT, if the circuit breaker used is ACB, then this step is done. For MCCB, this step is ignored.

Choose the cable size whichever is greater in step 1 or step 2.

Step 3:

Check for voltage drop with the cable size chosen.

The voltage drop from transformer to the load (lighting, motor) should not exceed 5%.

Also, if it is a motor load, the voltage drop for motor starting has to be checked. It has to be less than 15%.