5 key factors to the correct cable selection and application

Cable selection and application

It is essential to know cable construction, characteristics, and ratings to understand problems related to cable systems. However, to correctly select a cable system and assure its satisfactory operation, additional knowledge is required. This knowledge may consist of service conditions, type of load served, mode of operation and maintenance, and the like.

The key to the successful operation of a cable system is to select the most suitable cable for the application, make a correct installation, and perform the required maintenance.

In this technical article, discussion is based on the correct cable selection and application for power distribution and utilization.

Cable selection can be based upon the following five key factors:

1. Cable installation
2. Cable construction
3. Cable operation (voltage and current)
4. Cable size
5. Shielding requirements

1. Cable installation

Cables can be used for outdoor or indoor installations depending upon the distribution system and the load served.

A good understanding of local conditions, installation crews, and maintenance personnel is essential to assure that the selected cable system will operate satisfactorily! Many times cable insulation is damaged or weakened during installation by applying the incorrect pulling tensions.

Designs of conduit systems not only should minimize the number of conduit bends and distances between manholes but also should specify the pulling tensions.

The inspection personnel should ensure that installation crews do not exceed these values during installations. It is also important that correct bending radius be maintained in order to avoid unnecessary stress points. Once a correct installation is made, routine inspection, testing, and maintenance should be carried out on a regular basis to chart the gradual deterioration and upkeep of the cable system.

Cable systems are the arteries of the electric power distribution system and carry the energy required for the successful operation of a plant. Following is a brief discussion on cable installation and maintenance.

There are several types of cable systems available for carrying electrical energy in a given distribution system. The selection of a particular system may be influenced by local conditions, existing company policies, or past experience.

No set standards or established guidelines can be given for the selection of a particular system.

2. Cable construction

Selection and application of cable involves the type of cable construction needed for a particular installation. Cable construction involves conductors, cable arrangement, and insulation and finish covering.

2.1 Conductors

Conductor materials such as copper and aluminum should be given consideration with regard to workmanship, environmental conditions, and maintenance. The requirements for aluminum conductors with regard to these factors are more critical than for copper conductors.

Cable conductors should be selected based upon the class of stranding required for a particular installation.

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Sizing of power cables for circuit breaker controlled feeders (part 3)

3. Criteria Starting and running voltage drops in cable

This criterion is applied so that the cross sectional area of the cable is sufficient to keep the voltage drop (due to impedance of cable conductor) within the specified limit so that the equipment which is being supplied power through that cable gets at least the minimum required voltage at its power supply input terminal during starting and running condition both.

Cables shall be sized so that the maximum voltage drop between the supply source and the load when carrying the design current does not exceed that which will ensure safe and efficient operation of the associated equipment. It is a requirement that the voltage at the equipment is greater than the lowest operating voltage specified for the equipment in the relevant equipment standard.
So before starting with calculation for voltage drop let us first analyze that what is the permissible voltage drop as per relevant standards and guidelines and what is the possible logic behind selecting these values as the permissible values.

Indian standard 1255- CODE OF PRACTICE FOR INSTALLATION AND MAINTENANCE OF POWER CABLES UP TO AND INCLUDING 33 kV RATING in its clause 4.2.3.4 mentions the permissible value for different cross sectional sizes of Aluminium conductor in volts/kM/Ampere for cables from voltage grade of 1.1kV till 33kV. Since we calculate voltage drop in terms of percentage of source voltage, this clause is not very widely used in basic as well as detailed engineering fraternity.
Its complex unit requires to be multiplied by cable length and ampacity. However one can definitely check for any cable size and length, what value is obtained in terms of percentage?

IEEE standard 525 – Guide for the Design and Installation of Cable Systems in Substations in its annexure C, clause number C3 mentions that Voltage drop is commonly expressed as a percentage of the source voltage. An acceptable voltage drop is determined based on an overall knowledge of the system. Typical limits are 3% from source to load center, 3% from load center to load, and 5% total from source to load. These values are indicated diagrammatically below.

6.6kV substation layout

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Sizing of power cables for circuit breaker controlled feeders (part 2)

2. Criteria-2 Continuous current capacity (Ampacity)

This criterion is applied so that cross section of the cable can carry the required load current continuously at the designed ambient temperature and laying condition. Ampacity is defined as the current in amperes a conductor can carry continuously under the conditions of surrounding medium in which the cables are installed. An ampacity study is the calculation of the temperature rise of the conductor in a cable system under steady-state conditions.
Cable ampacity, if required to be calculated than it is calculated as per the following equation givenin IEEE -399, section 13.

This equation is based on Neher-McGrath method where,

• Tc’ – allowable conductor temperature (ºC)
• Ta’ – ambient temperature (either soil or air) (ºC)
• ∆Td – temperature rise of conductor due to dielectric heating (ºC)
• ∆Tint – temperature rise of the conductor due to interference heating from adjacent cables (ºC)
• Rac – electrical ac resistance of conductor including skin effect, proximity and temperature effects (µ_/ft)
• R’ca – effective total thermal resistance of path between conductor and surrounding ambient to include the effects of load factor, shield/sheath losses, metallic conduit losses, effects of multiple conductors in the same duct etc (thermal- Ωft, ºC-cm/W).

From the above equation it is clear that the rated current carrying capacity of a conductor is dependent on the following factors:

1. Ambient temperature (air or ground)
2. Grouping and proximity to other loaded cables, heat sources etc.
3. Method of installation (above ground or below ground)
4. Thermal conductivity of the medium in which the cable is installed
5. Thermal conductivity of the cable constituents

However please note that while sizing a power cable we never calculate the ampacity. The above equation is used to analyze the cable ampacities of unique installations. Standard ampacity tables are available for a variety of cable types and cable installation methods and can be used for determining the current carrying capacity of a cable for a particular application.

These standards provide tabulated ampacity data in manufacturers catalog for cables installed in air, in duct bank,  directly buried or in trays for a particular set of conditions clearly defined.
It is because of this reason that we need to give the reference of manufacturers catalog from where the ampacity  values are picked up.

Now once the current carrying capacity of a cable is found from standard catalog; we convert that rated capacity (Ampacity) into actual laying condition. The standard current ratings for cables are modified by the application of suitable multiplying factors to account for the actual installation conditions. Hence we define one more term here called ampacity deration factor.

Ampacity duration factor is defined as the product of various factors which accounts for the fraction decrease in the ampacity of the conductor. Those factors and physical condition deriving them are as follows:

1. K1= Variation in ambient air temperature for cables laid in air / ground temperature for cables laid underground.
2. K2 = Cable laying arrangement.
3. K3 = Depth of laying for cables laid direct in ground.
4. K4 = Variation in thermal resistivity of soil.

Ampacity Deration factor = Product of applicable multiplying factors among 1 to 4 listed above.

K = K1 x K2 x K3 x K4

Now from where do we get these multiplying factors to find the overall ampacity deration factor? Againwe get these values from manufacturers catalog because manufacturer of the cable is in best position to conduct thepractical experiments and test on the cables and find the percentage/fractional decrease in current carrying capacity of the cable in various conditions.

For better understanding of the ampacity deration factor the following pictorial representation is provided below.

Table for ampacity deration factor along with pictorial representation is provided below.

However readers to note that ampacity deration factor table provided in this article is to verified from the manufacturers catalog which is intended to be used for project.

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Sizing of power cables for circuit breaker controlled feeders (part 1)

The following three criteria apply for the sizing of cables for circuit breaker controlled feeders:

I. Short circuit current withstand capacity

 This criteria is applied to determine the minimum cross section area of the cable, so that cable can withstand the short circuit current.

Failure to check the conductor size for short-circuit heating could result in permanent damage to the cable insulation and could also result into fire. In addition to the thermal stresses, the cable may also be subjected to significant mechanical stresses.

II. Continuous current carrying capacity

 This criteria is applied so that cross section of the cable can carry the required load current continuously at the designed ambient temperature and laying condition.

III. Starting and running voltage drops in cable

This criteria is applied to make sure that the cross sectional area of the cable is sufficient to keep the voltage drop (due to impedance of cable conductor) within the specified limit so that the equipment which is being supplied power through that cable gets at least the minimum required voltage at its power supply input terminal during starting and running condition both.

1. Criteria-1 Short circuit capacity

The maximum temperature reached under short circuit depends on both the magnitude and duration of the short circuit current. The quantity I2t represents the energy input by a fault that acts to heat up the cable conductor. This can be related to conductor size by the formula:

A = Minimum required cross section area in mm2
t = Operating time of disconnecting device in seconds
Isc = RMS Short Circuit current Value in Ampere
C = Constant equal to 0.0297 for copper & 0.0125 for aluminum
T2 = Final temp. ° C (max. short circuit temperature)
T1 = Initial temp. ° C (max. cable operating temperature – normal conditions)
T0 = 234.5° C for copper and 228.1° C for aluminum

Equation-1 can be simplified to obtain the expression for minimum conductor size as given below in equation-2:

 


Now K can be defined as a Constant whose value depends upon the conductor material, its insulation and boundary conditions of initial and final temperature because during short circuit conditions, the temperature of the conductor rises rapidly. The short circuit capacity is limited by the maximum temperature capability of the insulation. The value of K hence is as given in Table 2.
Boundary conditions of initial and final temperature for different insulation is as given under in Table 1 below.

Table 1

Insulation material	Final temperature T2	Initial temperature T1
PVC	160° C	70° C
Butyl Rubber	220° C	85° C
XLPE / EPR	250° C	90° C

 

Table 2

Material →	Copper			Aluminum
Insulation →	PVC	Butyl Rubber	XLPE / EPR	PVC	Butyl Rubber	XLPE / EPR
(K) 1 Second Current Rating in Amp/mm2	115	134	143	76	89	94
(K) 3 Second Current Rating in Amp/mm2	66	77	83	44	51	54

In the final equation-2 we have determined the value of constant K. Now the value of t is to be determined. The fault current (ISC) in the above equation varies with time. However, calculating the exact value of the fault current and sizing the power cable based on that can be complicated. To simplify the process the cable can be sized based on the interrupting capability of the circuit breakers/fuses that protect them.

This approach assumes that the available fault current is the maximum capability of the breaker/fuse and also accounts for the cable impedances in reducing the fault levels.

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5 key factors to the correct cable selection and application

Cable selection and application

It is essential to know cable construction, characteristics, and ratings to understand problems related to cable systems. However, to correctly select a cable system and assure its satisfactory

operation, additional knowledge is required. This knowledge may consist of service conditions, type of load served, mode of operation and maintenance, and the like.

The key to the successful operation of a cable system is to select the most suitable cable for the application, make a correct installation, and perform the required maintenance.

In this technical article, discussion is based on the correct cable selection and application for power distribution and utilization.
Cable selection can be based upon the following five key factors:

1. Cable installation
2. Cable construction
3. Cable operation (voltage and current)
4. Cable size
5. Shielding requirements

1. Cable installation

Cables can be used for outdoor or indoor installations depending upon the distribution system and the load served.
A good understanding of local conditions, installation crews, and maintenance personnel is essential to assure that the selected cable system will operate satisfactorily! Many times cable insulation is damaged or weakened during installation by applying the incorrect pulling tensions.
Designs of conduit systems not only should minimize the number of conduit bends and distances between manholes but also should specify the pulling tensions.


The inspection personnel should ensure that installation crews do not exceed these values during installations. It is also important that correct bending radius be maintained in order to avoid unnecessary stress points. Once a correct installation is made, routine inspection, testing, and maintenance should be carried out on a regular basis to chart the gradual deterioration and upkeep of the cable system.

Cable systems are the arteries of the electric power distribution system and carry the energy required for the successful operation of a plant. Following is a brief discussion on cable installation and maintenance.

 

There are several types of cable systems available for carrying electrical energy in a given distribution system. The selection of a particular system may be influenced by local conditions, existing company policies, or past experience.
No set standards or established guidelines can be given for the selection of a particular system.

Click here to access the full article