ticking time bomb

In our example in Voltage Calculation - Part 2, we have only considered a single cable and checked only the voltage drop at starting. In actual practice, we shall be comparing several cable sizes, selecting the cable that provides optimum design consideration.

In this example, we shall be using several cables from 35 mm2 up to 120 mm2. Likewise, we shall be considering, not only the starting condition but the running condition of the motor as well. You might be asking, why we selected a motor in our example. The reason, a motor is a dynamic load. A motor circuit load varies from the starting to the normal running condition, thus there will be multiple considerations when selecting the cable for a motor circuit.

In Table 2 Cable Selection, we have considered 5 cables sizes, all cables except one satisfy our design consideration which are the following:
Voltage drop during starting < 15%
Voltage drop during running < 5%

Again we will be using values from our previous example which are the following:
Voltagesending end = 400 volts
Motor Running:
Irunning = 180 A
PFrunning = cos Ørunning = 0.87
sin Ørunning = 0.493

Motor Starting Istarting = 6 * 180 = 1080 A
PFstarting = cos Østarting = 0.25
sin Østarting = 0.968

Using the formula

Table 2. Cable Selection
Size r50 x50 R X %Vdrunning %Vdstarting
35 0.674 0.0867 0.0809 0.0104 5.9% 14.2%
50 0.499 0.0858 0.0599 0.0103 4.5% 11.7%
70 0.344 0.0850 0.0413 0.0102 3.2% 9.4%
95 0.271 0.0825 0.0325 0.0099 2.6% 8.3%
120 0.214 0.0808 0.0257 0.0097 2.1% 7.4%

In Table 2. Cable Selection, the 35 mm2 cable passes the motor starting voltage drop but fails the motor running voltage drop which makes this cable to be eliminated from our selection list.

The 70 mm2, 90 mm2 and 120 mm2 cables satisfy all our design conditions, however, selecting any of these cables will make the installation more expensive than required. The 50 mm2 cable satisfy all conditions and cost of the project.

Cable selection considerations does not stop here, there are more that need to be considered such as operating temperature and short-circuit withstand. In our next article, we shall be discussing about cable operating temperatures.