How to select cables?


In electrical systems, the cables are used to transport electrical currents. Most of the time the core of these cables is made of copper or aluminium to conduct the current with a minimum voltage drop. Most cables have a protective insulation to protect the cable and also to protect living beings from dangerous voltages. Types if the cables differ according to:

  1. Current go through (cable size)
  2. Purpose they are used
  3. Place (indoor or outdoor)
  4. Protection level required

Selection criterias for Cables:

Voltage drop:

Voltage drop is the reduction in voltage in an electrical circuit between the source and load. When a cable is being selected for taking current for a specified machine, as per IEEE regulations it is required to have the voltage drop of the cable less than 4% of the nominal supply.

All series drops must be included in the total voltage drop. The maximum permissible voltage drop for a cable should be 4%. The factors affecting for the voltage drop are:

  • Resistance of the cable for 1m length (Voltage drop for 1m- [v/Am]) - Vc
  • Rated current of the cable (or carrying current) - I
  • Length of the cable - L

So Voltage drop of the cable (Vd) can be calculated as,

Vd= ILVc

Derating Factors:

All cables in the market are marked by a current that can carry in standard conditions. But always these standard conditions cannot be practically maintained, in a construction. Then, if a cable is selected according to the (current) requirements according to our assembly method, there may be current variations. The factors that affect the previous variations are called power reduction factors. They are,

  • Ambient temperature
  • Ground temperature
  • Depth of lying
  • Soil Thermal resistivity

Current ratings of cables:

The current classifications for wires differ from one manufacturer to another, although they are almost similar. The approximate current ratings for wire sizes given under standard conditions are shown below.

Calculating the Cable for a given load:

For calculating the proper cable we have to know the power consumption Known input voltage. And also the distance from power supply to load (L) is provided.

Then steps include:

  • Using the given data, calculate the load current I.
  • Select a wire that is a little bigger to carry I (Iwire > I)
  • Then multiply the rated current of selected wire with all the derating factors.
  • Find whether, Iwire x derating factors < I
  • If so select next bigger wire size. If not select that wire
  • Then calculate the voltage drop of wire & nominal voltage drop & see whether it is ok.

Cable sizing calculation: Click here