Wire size Calculation-For pump

 While working in the sites, sometimes, we need to know the wire size for the pump and other electrical equipments which can be stressful if you don't know how to find the size of the wire. In case of single phase, we can simply use the formula and find out the Current using known voltage and power. But in case of three phase, We need:

1. Power factor=cosΦ = .8

2. Efficiency = .95

3. Voltage= 415

4. Power= known value in HP or kW

                          Thumb rule: TO FIND CURRENT flowing through each wire in 3 phase circuit

                                               For kW, I= 1.8xkW

                                                For HP, I = 1.35XHP

Rated current = (kW * 1000) / (1.732 * rated line voltage * motor efficiency * motor power factor)

How to calculate the Cable size for LT & HT Motors?

Selecting the right cable size for the motor is an important parameter for the Industry whether it’s during Installation and Commissioning or during running condition. It is a very important aspect for Safety, Cost minimization and reduction of undesired losses. An undersized cable can burn during motor operation causing risk to human life, machine, infrastructure, loss of production and replacement cost.

Cable Size Calculation of 125 Kw LT Motor

Motor KW = 125

Pf = 0.8, Efficiency = 94%

System Voltage, V1 = 415

Cable length = 200 m

Load Current = P / (1.732 x V x Pf x Eff) —> (P = √3 x Vx I CosΦ = for three phase circuits)

= 125000 / (1.732 x 415 x 0.8 x 0.94)

~ 230 A

This is the full load current cable needs to cater in Ideal condition. But in practical situation, there are several derating factors which need to be considered.

The current rating given for the cables is defined for ambient temp of 40* C. If the ambient temp is greater than that, cable current carrying capacity derates.

Suppose our cable is in Air laid on cable tray,

Cable Size Calculation for 350 KW HT Motor

In case of LV system cable can be selected on the basis of its current carrying capacity and voltage drop but in case of MV/HV system cable short circuit capacity is an important/deciding factor. So in case of HT motor, the cable short circuit capacity alone is enough to determine the cable size as rest two parameters will automatically follow.

Consider the below example:

Motor KW = 350

Pf = 0.8, Efficiency = 94%

System Voltage, V1 = 6.6 KV

Cable length = 200 m

Load Current = P / (1.732 x V x Pf x Eff)

= 350000 / (1.732 x 6600 x 0.8 x 0.94)

= 41 A

Suppose Short circuit level/Fault level for H.T. system, Ish (for duration t=1sec) = 26.2 KA

With Aluminum conductor, XLPE insulated cable =

= 278.72 http://Sq.mm

Hence nearest higher size 300 sq mm is required.

We can see from the below table also that the short circuit capacity of 300sqmm cable is 28 KA which is more than our fault level.