How to properly select suitable cable size for three phase load/three phase line

What are the important things to know to determine the size of the cable?

1. Determining the amount of load current.

1. Determining the ambient temperature.

1. Voltage drop determination.

Suppose a three - phase meter is 5kw. We know that the value of a three - phase meter is usually in kilowatts or horsepower.

Our first task will be to convert the power value to watt value. If the horsepower is multiplied by 746 and if it is kilowatt then multiplied by 1000.

If the power of the meter is 15 kw.

So, the power of the meter is P = 15 kw

                                                   = 15*1000 watts

                                                   = 15000 watts

Adding an additional 20% of the load to the value of the power we receive will increase the safety and subsequent amount of load, so the there is no pressure on our selected wires. As a result accidents can be avoided. 

P =15000 + 20%

    =15000 + (15000*20)/100

    = 15000 + 3000

    = 18000  watts

Determining the amount of load current.

Formula for three-phase lines

P = V*I*PF*1.732

kw = (V * I * PF * 1.732)

Suppose,

Three-phase motor voltage = 380 volt

We know,

Frequency (f) = 50 Hz

Power factor (PF) = 0.85 

P = V*I*PF*1.732

I = P/(V*PF*1.732)

    = 18000 / ( 380*.8*1.732)
    = 34.22 Amp

Determining the ambient temperature.

Suppose that the place where the three-phase matter is located has an ambient temperature of 60 degrees Celsius with a correction factor of 0.71. 

We have got the load current for the motor 34.22 Amp, we can get the acceptance only if we fix the correction factors with this value.

Acceptable current: 34.22*0.71=24.29<37 Amp

Therefore , the cable size that we have chosen is correct. If this value is more than 37  then we need to change the cable size.

Voltage drop determination.

Rules: The voltage drop in act of which the cable forms part should not exceed 3-5% of the nominal voltage.

Nominal voltage: 440*3/100

                               = 13.2  volt

Suppose the distance of our motor from the main line is 50 meters.

4mm(sq) cable voltage drop =10mv*A*m

                                                    =10*10^3*34.22*50

                                                    =17.11 volt

Of course the voltage drop of the cable must be higher than the nominal voltage drop If so, you need to change the cable size and select higher voltage drop cable. Since the voltage drop of our selected cable is 4mm (sq) current cable.

https://drive.google.com/file/d/1KoYjjIbcYBKSqilhvcVkLzdHi-Of-v7t/view?usp=sharing