Filipino Engineer

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:
Voltage_{sending end} = 400 volts
Motor Running:
I_running = 180 A
PF_running = cos Ø_running = 0.87
sin Ø_running = 0.493

Motor Starting I_starting = 6 * 180 = 1080 A
PF_starting = cos Ø_starting = 0.25
sin Ø_starting = 0.968

Using the formula

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.