[Selection Guide] How to choose the most economic size and type of cable?

​Cable selection is about choosing the appropriate type of conductor and selecting a suitable size/cross-section Area/ Diameter of the conductor according to the application. First, there is a need to understand the significance of cable sizing and selection. Then selection criteria will be discussed keeping in mind all the derating factors that might reduce cable ampacity. A law named Kelvin’s law plays a vital role in the economic sizing of conductors, so it will also be explained here. Other than the size of conductor, different types of the conductor will be studied. Also, the cable shielding and insulation will be discussed at the end.

Cable sizes are usually defined in terms of cross-sectional area, Kcmil (Kilo circular mils) or AWG (American Wire Gauge).

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Standards available for the cable selection and size are:

⦁ IEC (International Electrotechnical Commission)
⦁ NEC (National Electrical Code)
⦁ BS (British Standards)

Significance of selecting the right cable size and type:

​Selecting the right cable size and type is significant because of the following reasons:

⦁ If cable size is very small, when the current exceeds the cable ampacity, the cable will heat up and get damaged. So, there is a need to choose the cable size for which it is capable enough to withstand the full load current and the short circuit current that could flow through the cable.

⦁ Increasing the cross-section area of the cable will require more material to be used in its construction, causing it to become expensive. Therefore, it will be hard to maintain a good balance between the cable cost and its usage requirements. So, the cable’s diameter is to be sized as per the requirements.

⦁ It must be ensured to provide a load with a suitable voltage i.e. with minimum voltage drop. Cable with a small diameter will have a higher resistance. Also, it will cause more voltage drop across the cable. That’s why there is a need to select such cable a which causes no or less voltage drop.

⦁ There is a need to choose the best cable type according to the requirement of application as every type of conductor has its own resistance, thermal conductivity, etc.

Selection criteria of cables:

​Cable size is determined based on these factors:

Current carrying capacity: It is determined by evaluating the amount of current to be drawn by the equipment or load connected at the receiving end of the cable. The safety margin for overload current is also provided in it.

Voltage drop: Due to resistance of the cable, power losses occur, causing the voltage to be dropped by some magnitude. In addition to it, voltage drop also varies with respect to temperature as temperature affects the resistance. If we know the values of resistance of cable and current flowing from cable, then we can determine the voltage drop across that cable by using the formula V=I*R.

Short circuit rating: It is the capability of a cable to withstand short circuit current for the specific duration of the fault before it has been cleared without any damage.

Derating factors:

There are some external disturbances that affect the current rating of a cable i.e. cable ampacity. In such scenarios, current ratings are to be improved by applying some suitable factors known as derating factors. As we have more than one type of derating factor, so values of all of the derating factors are multiplied to get an average value. The following are the main derating factors that should be kept in consideration when selecting cable size.

  • Temperature derating factor (CT):Temperature derating factor (CT): Cables should be arranged in such a way so they get a minimum space to dissipate heat in their surroundings. This factor is used in calculations of cable sizing so as to consider the arrangement of cable for minimizing heat losses thus improving cable ampacity.

Kelvin’s law for economic cable sizing:

There are several factors which needs to be considered in selecting the suitable cable for electrical distribution network. Form the application perspective ampacity, voltage drop and short-circuit rating is essential. In addition, the derating factors such as temperature, thermal resistivity of the soil, burial depth and conductor grouping factor do play a vital role in investing on the right type of cable. Most economic conductor size can be chosen by applying the Kelvin’s Law, which is thoroughly explained with a real-life example. Have a look HERE.

Limitations:

  • Accurate interest and depreciation on capital cost can’t be determined.

Types of conductor:

Based on physical structure, conductors are either stranded (multiple thin wires) or solid (solid metal wire). Cable types (conductors) which are used in transmission lines are:

  • ACSR (Aluminum Conductor, Steel Reinforced): It consists of steel strands surrounded by aluminum strands. It is the most recommended conductor for transmission lines and used for longer spans.
⁘ IACS (International Annealed Copper Standard) is a standard introduced by US. 
It is a standard against which conductivity of any conductor is compared.
It is value of conductivity of commercially available copper.

Cable shielding and insulation:

There are different layers of various materials which are to be laid over conductor so as to provide cable insulation and shield for the purpose of protection of conductor. Each of the layer has its own specific function and its requirement depends on the application of cables. For example, for overhead lines we don’t need to have any insulation or shielding as bare conductors are used there, but for underground cables they must be insulated and shielded.

  • Insulation: Cable insulation is done by means of any dielectric like PVC so as to prevent leakage of current from a conductor.

Summing-Up:

The electrical power transmission system can be made efficient and economical by following the proper methodology of cable sizing and selection. Selection criteria, derating factors, type of conductor, proper insulation and shielding, etc. Must be in our mind at the time of cable installation. This is how we can achieve efficient, safe and cost-efficient transmission of electricity.

Let us know if you have any queries regarding this topic and do provide us with your feedback in the comments.

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