Can I Know how to Calculate the cable from panel to motor / what is the Formula?

IL ≤ Cb ≤ Iz               eq. ------------- 1.

IL       is load current ( ampare )

Cb      is circuit breaker size

Iz       is cable rating

now we calculate load current

IL = ( hp x 746) / (√3 x Volt x PF x Efficiency) 

and this the breaker size from eq. -----1. ( rounded off to nearest value of breaker available in market eg.:- load corrent is 118 amps we can use 150 amp breaker as per available in market )

and from here we choose a cable size from catalogue of the selected approved companies 

Let me introduce this simple method:

1. Calculate the ampere capacity of the circuit, based on the power, power factor, number of phases, short circuit current level  and voltage.

2. Estimate the total lenght of the distance.

3. Calculate the drop voltage.

4. Define the kind oc cable required.

5. Go to the tables of the National Electric Code of USA, they use or work based on ANSI/IEEE.

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%.

take the GA of PLANT you are estimating and calculate the length of cables first and then go for other calculations.

I  think it's base on Panel Type and the daimentions of the panel

but still you can use two formula as Muhammad Arshad mention:

IL ≤ Cb ≤ Iz

IL = ( hp x 746) / (√3 x Volt x PF x Efficiency) 

after that you can check your answer if it's as you expect or not