Cables

Selection of Cables: Selection of cable is based on the following factors:

  1. For LT Cable: 

    1. Current carrying capacity.
    2. Voltage drop.

      The voltage drop should not normally exceed 2 % of rated voltage. The voltage drop is calculated as follows:

        For DC System or AC 2 wire system: Vd = 2 × I × R

        For 3 Phase System: Vd = √3 × I × R

      where Vd is the voltage drop, I is current, R is the resistance of one conductor (not lead or return).

    3. Environmental Condition.
    4. Formation and distance between cables.
  2. For HT Cable: 

    1. Rupturing capacity.

      Once rupturing capacity is known from the supply company, the HT cable size is worked out as under:

      • For 11 kV System: 

        • A = 14.2 × ISC × √t (For Al conductor)
        • A = 9.41 × ISC × √t (For Cu conductor)
      • For 22 kV System: 

        • A = 16.39 × ISC × √t (For Al conductor)
        • A = 10.98 × ISC × √t (For Cu conductor)

       where A is the cross section area of conductor in mm², t is the fault current duration in seconds, normally taken as 0.2 seconds and ISC is the short circuit current in kA and is calculated by the formula ISC = Rupturing Capacity in MVA / (√3 × System Voltage in kV)

      After selection based on rupturing capacity, current carrying capacity should be seen and higher size of the cable satisfying both the criteria should be selected.

    2. Current carrying capacity.

IMPORTANT POINTS FOR PILC CABLE LAYING: 

  1. The cable drum should always be rolled in the direction of the arrow, if it is to be moved over a short distance.
  2. Recommended depth of the trench should be:

    Up to 1.1 kV: 46 to 76 cm plus radius of complete cable.
    3.3 to 11 kV: 91 cm plus radius of complete cable.
    22 & 33 kV:107 cm plus radius of complete cable.
  3. While pulling the cable before laying, the direction in which the cable is to be pulled should be first decided. If it is new laying of cable, no special precautions are required, but if it is to be connected with the existing cable, the sequence or core numbers at the running ends should be first ascertained and the new cable pulled in accordingly. This procedure is always better to avoid crossing of cores in the joint. The cable to be jointed should be laid with sufficient overlap for jointing.
  4. Minimum bending radius should be as given below, wherever possible 25 % larger radius than the value given in the table should be used:

    System VoltageSingle CoreMulti Core (Unarmoured)Multi Core (Armoured)
    Up to 11 kV20 D15 D12 D
    Up to 22 kV25 D20 D15 D
    Up to 33 kV30 D25 D20 D

     Where D is the overall diameter of the cable.

  5. Other general points to be observed are mentioned below:

    1. Prior to joint a proper jointing position should be selected.
    2. The joint hole should be of sufficient dimension for easy working.
    3. The depth of joint hole should be at least 31 cm below the proposed laying depth.
    4. The sides of the hole should be well covered with tarpaulin to prevent loose earth from falling.
    5. The jointing should always be carried out inside a tent.
    6. For jointing of higher voltage live cables, the cable should be made DEAD and earthed before commencement of the jointing. This should be confirmed by an instrument like induction coil, headphones.
    7. Any mistake in the identification of the proper cable will result in a fatal accident.
    8. While handling the insulation cleanliness is very essential. Any contamination of the insulation by dust or moisture is detrimental to the joint.
    9. The cable should never be exposed to atmosphere for a long time.
    10. In monsoon season before commencement of jointing special precaution should be taken.
    11. The cable seal should be examined for any damage or puncture. The paper insulation should then be tested for presence of moisture. This is done by dipping the insulation paper next to the conductor and sheath in hot cable compound of paraffin wax at a temperature between 120 to 140 °C, the presence of the moisture is indicated by formation of bubbles on the paper. Only one single strip of paper gripped by a pair or tweezers should be used. The paper removed from the cable should be handled carefully to avoid contamination. After this the insulation of cable to be jointed should be tested to check the condition of the insulation.
    12. Check the continuity and phasing out of the cables to be jointed. If necessary. The no. of cores normally represent the number of phases, but it should not be taken for granted.

JOINTING AND TERMINATION OF XLPE CABLES: 

XLPE cable is suitable for installation in ground duct or air similar to conventional cables. It is easier to handle and install. Being lighter and of smaller dimension compared to the other power cables of the same capacity, this cable requires lesser number of supports, clamps etc when installed in the air. Unlike paper insulated cable, XLPE cable can be installed in vertical runs of unlimited difference in level. The recommended minimum bending radius for the armoured XLPE cable are:

For screened cable: 12 D
For unscreened cable: 8 D

  where D is the overall diameter of the cable.

As XLPE cable insulation is highly resistant to moisture, hence jointing and termination of XLPE cable is simpler to PILC cable. Up to 3.3 kV, the cable can be terminated straightway with compression glands similar to other polymeric insulated cables. A new range of jointing and termination system has been evolved to meet the need of such cables to operate at a higher temperature, the salient feature of which is given below:

  1. JOINTING OF CONDUCTORS: Since the maximum permissible temperature is 250 °C for XLPE cables, soldered type joints becomes unsuitable as solder melts at much lower temperature. Welded or crimped / compression type conductor joints are therefore adopted for XLPE cables.

  2. INSULATION BUILD-UP: The conductor joints is insulated by lapping of EPR self-amalgamating (S.A.) tape to the required thickness. Tape in universally accepted as the most reliable material because of its resistance to water, capacity to operate at higher temperature, high dielectric strength and void free amalgamating properties.

    For cables above 3.3 kV, semiconductor S.A. tape is applied on the conductor to equalize distribution of electrical stress on the conductor surface. The insulated cores are further lapped with the tape to serve as core screening. For cable up to 3.3 kV, S.A. tape is not necessary.

  3. STRAIGHT THROUGH JOINT: The joint assembled (with insulated core) is incapsulated in special resin cast compound.

  4. STRESS RELIEVED TERMINATION: The termination of cables above 3.3 kV is provided with stress cone to relieve electrical stress formed on the insulation at the point of the termination of the cable screen. S.A. tape is lapped over the insulation to the required thickness to build up the stress cone.

  5. INDOOR TYPE TERMINATION: Indoor type termination up to 3.3 kV is provided with stress cone but it does not require any compound for encapsulation. However, it must be protected by lapping of adhesive PVC for protection against external abuses.

  6. OUTDOOR TYPE TERMINATION: Outdoor type termination above 3.3 kV is provided with stress cone, but this termination is encapsulated in special resin cast compound to protect the joint from atmospheric abuses.

COLOR IDENTIFICATION OF CABLES: 

It is done by colour identification of core of rubber / PVC insulated cable or of sleeve / disc to be applied to conductor or cable core. Following is adopted:

  1. Non-Flexible Cables & Bare Conductor for Fixed Wiring: 

    FunctionColour Identification
    Earthing Green and Yellow or Green
    Neutral of single / three phase circuitBlack
    Live of single phase circuit Red
    Phases of three phase circuit Red, Yellow, Blue
    Outer (positive or negative) of DC 2 wire circuit derived from 3 wireRed
    Positive of three wire DC Red
    Middle of three wire DC Black
    Negative of three wire DC Blue
    Fifth core (special purpose) in 5-core armoured PVC cableOrange
  2. Flexible Cables & Flexible Cords: 

    FunctionColour Identification
    LiveBrown
    NeutralBlue
    EarthingGreen and Yellow
COMPARISON OF ALUMINUM AND COPPER CONDUCTORS
RatioCriterion
Equal ResistanceEqual current and temp. riseEqual diameter
Ratio of Aluminum to Copper
Area ratio 1.611.39---
Diameter ratio1.271.18---
Weight Ratio0.480.420.30
Current Carrying Capacity------0.78

CURRENT RATING OF COPPER AND ALUMINUM CONDUCTOR VULCANIZED RUBBER / PVC INSULATED CABLE (40 °C Ambient)
Standard Copper ConductorContinuous Current RatingStandard Al. Conductor
Nominal Conductor AreaConventional Sizes No. & dia of wireIn conduit/casingBunched in air or open trenchIn air/open trenchNo. & dia. of wireNominal Conductor Area
2 Single core2 Single core3 / 4 Single core1 / 2 core DC / AC3 / 4 core AC
mm²SWGmmAAAAAmm²mm²
1.001/181/1.1244444  
1.253/223/0.74777751/1.401.5
2.003/203/0.91101091071/1.802.5
3.007/227/0.741414101410  
   14141214101/2.244
   19191719131/2.806
4.507/207/0.911919171914  
   23232123171/3.5510
6.757/187/1.122525222517  
10.07/177/1.3230302630217/1.7016
14.57/167/1.623737333726  
   41414141297/2.2425
18.519/1819/1.124343394330  
   48484348337/2.5035
25.019/1719/1.325151465136  
   6363576343

7/3.00

19/1.80

50
40.019/1619/1.62 67616747  
50.019/1519/1.82 85747954  
    9282815719/2.2470
    10695936519/2.5095
65.019/1419/2.10 110979768  
70.037/1637/1.62 12210910970  
    1271171127937/2.06120
95.037/1537/1.82 14112812487  
    1451281258837/2.24150
    17014914410137/2.50185
120.037/1437/2.10 173152150106  
    20017116511737/2.80225
150.037/1337/2.36 202179174123  
185.037/1237/2.62 231204196137  
    24420820013961/2.50300
250.061/1361/2.36 293248235165  
    300257 27561/3.00400
300.061/1261/2.62 330283   
ELECTRICAL AND PHYSICAL DATA ON PVC INSULATED PVC SHEATHED ARMORED 3.5 & 4 CORE ALUMINUM CONDUCTOR 1.1 KV GRADE CABLE (40 °C Ambient) (IS 1554)
Nominal Conductor AreaContinuous Current RatingShort Circuit Rating (1 s)DC resistance at 20 °CCapacitance per km (at 50 Hz)Inductive reactance per km (at 50 Hz)3.5 Core4 Core
GroundDuctAirOverall DiameterMassOver all Diameter Mass
mm²AAAkAΩμFΩmmkg/kmmmkg/km
104639400.7483.0000.5770.0888--22.0785
166050511.2001.9100.7570.0857--25.51050
257663702.0801.2001.8600.084926.0116227.01285
359277862.6000.8680.9810.082328.5134029.01425
50110951053.6600.6410.9970.079532.0170233.51815
701351151305.6600.4431.1630.076936.0209737.02455
951601401557.0500.3201.1810.076640.0263542.52910
1201851551809.3500.2531.3100.074144.0313946.03465
15020017520511.0700.2061.2820.074348.5366750.53985
18523520024013.7400.1641.3000.074253.5447755.54915
22526422526917.2700.1341.3040.074159.0534761.55875
24027523528019.8500.1251.3420.073760.5554762.56475
30030526031522.6300.1001.3650.073366.5667369.07255
40033529037532.7000.0781.4260.072973.5816477.09590
50037032042538.3500.0621.4060.073282.51008387.510085
63040535048048.3500.0481.4240.073194.0-97.513680

ELECTRICAL AND PHYSICAL DATA ON PVC INSULATED ARMORED AND UNARMORED COPPER CONDUCTOR 1.1 kV GRADE CONTROL CABLE (40 °C Ambient)
No. of cores and  Conductor AreaContinuous Current RatingDC resistance at 20 °CUnarmored CableArmored Cable
Ground Air / DuctOverall DiameterMassOverall Diameter Mass
mm²AAΩ/kmmmkg/kmmmkg/km
2 x 1.5232012.1010.615514.0418
3 x 1.5211712.1011.018014.5452
4 x 1.5211712.1012.021015.6496
5 x 1.5161412.1012.625016.5566
6 x 1.5151312.1014.028017.0627
7 x 1.5141312.1014.031017.2637
10 x 1.5131112.1017.042319.8695
12 x 1.5121012.1017.647020.4723
16 x 1.511912.1019.756021.8885
24 x 1.59812.1024.375225.91115
61 x 1.56612.1034.1170536.42310
2 x 2.532277.2813.121815.4490
4 x 2.527247.2814.529016.8610
6 x 2.521187.2816.639020.4770
10 x 2.518157.2821.256022.0840
12 x 2.517147.2821.564022.4960
16 x 2.515137.2823.179024.81160
24 x 2.513117.2827.7110529.81510
61 x 2.5987.2840.4249042.03200

MAXIMUM LENGTH OF CABLE (IN METER) FOR 5 % VOLTAGE DROP
Current in ACable cross section (mm²)
2.54610162535507095120150185240300400
4215383513             
5170263405707            
998153235411            
16 83128223329572          
23  88153226393          
29   122179311388         
40    130226281393605       
53     170212297456563      
66      170238366453603     
80       196302374497567    
95       165254315419477575   
130        186230306349420565  
190           238287386417512
270             272293360

CURRENT RATING OF PAPER INSULATED HT POWER CABLES
Nominal Conductor AreaUnarmored 3 Single Core3 Core Unarmored and Served3 Core Armored and Served
Servings TouchingBelted CableScreened CableBelted CableScreened Cable
GndDuctAirGndDuctAirGndDuctAirGndDuctAirGndDuctAir
mm²AAAAAAAAAAAAAAA
16636266555050605159584950615459
25788288726465736677726468806577
359396105847779927793847480958093
50115115130105941001159411510592100115100115
70140140165130115125140115140130115125140125140
95165165200155135155165135175155135155165145175
120185185230170150175185155200170155175185160200
150210205265190170200210175230190175200205180230
185240230300220195230245200265220200230235210265
225260250340240210260270225280240220260265230300
240270260355260235275285235315250225265275240310
300305285400300265315310265355280250310300265315
400355335490345300375360320405320285365357315408
500395365570385330425405350460360310415385330450
625445410670410365480435390520385345470410370510
800495455750            
1000550495820            

ELECTRICAL AND PHYSICAL DATA ON PAPER INSULATED LEAD COVERED SCREENED ARMORED 3 CORE ALUMINUM CONDUCTOR 11 kV GRADE CABLE
Nominal Conductor AreaContinuous Current RatingShort Circuit Rating (1 s)DC resistance at 20 °CCapacitance (at 50 Hz)Inductive reactance (at 50 Hz)Overall DiameterMass
GroundDuctAir
mm²AAAkAΩ/kmμF/kmΩ/kmmmkg/km
501151001153.620.6410.3700.097349.05855
701401251405.630.4430.4400.091853.56855
951651451757.000.3200.4800.089357.57495
1201851602009.290.2530.5380.085360.08180
15020518023010.980.2060.5800.083764.59390
18523521026513.640.1640.6350.081768.010200
22526523030017.140.1340.7020.079872.011565
24027524031019.700.1250.7450.078773.512080
30030026531522.470.1000.7890.077673.513900
40035731540832.450.0780.9310.075185.516880

CURRENT RATING OF COPPER CONDUCTOR VULCANIZED RUBBER/PVC INSULATED FLEXIBLE CORDS (IS 434 & IS 694)
Nominal Cross-sectional AreaNo. and dia. of wires650 V / 1.1 kV Grade
Twin CoreThree CoreFour Core
mm²mmAAA
0.516 / 0.2001199
0.7524 / 0.200141211
1.0032 / 0.200151312
1.5048 / 0.200211816
2.5080 / 0.200282422
4.00128 / 0.200373229

CURRENT RATING OF COPPER CONDUCTOR VULCANIZED RUBBER INSULATED FLEXIBLE CABLES (IS 434)
Nominal Cross-sectional AreaNo. and Dia of wires650 V / 1.1 kV Grade
Single CoreTwin CoreThree CoreFour Core
mm²mmAAAA
685 / 0.30055453836
1062 / 0.45075605247
16101 / 0.450100807064
25157 / 0.4501301059283
35220 / 0.450160130120100
50314 / 0.450205160140125
70158 / 0.750250200175155
95215 / 0.750300240205190
120272 / 0.750345270240220
150340 / 0.750415320285260
185419 / 0.750465360315295
240543 / 0.750540420365340
300679 / 0.750640495435400
400905 / 0.750770   

SINGLE CORE AUTO CABLE WITH COPPER CONDUCTOR
Cable Size (mm)Current Rating (A)Cable Size (mm)Current Rating (A)
9/0.300528/0.30015
14/0.300935/0.30018
16/0.3001044/0.30020
19/0.3001165/0.30025

COPPER CONDUCTOR VULCANISED RUBBER INSULATED SHEATHED WELDING CABLE (IS 434)
Nominal Cross-sectional AreaNo. and dia. of wiresCurrent Rating
mm²mmA
16509 / 0.200100
25796 / 0.200150
351114 / 0.200230
501591 / 0.200400
702228 / 0.200600
951344 / 0.300Used when voltage drop with smaller cable becomes excessive.

CAPACITIES OF STEEL TRUNKING FOR CABLES
Cable Size in mm²50 mm × 50 mm75 mm × 75 mm100 mm × 100 mm
1.065140200
1.545100180
2.54090160
4.03580140
6.02965116
10.0245496
16.0143556
25.0122748
35.0102140
50.081832
70.051020
95.04916
120.03512
150.0 47

Note

Number of cables is based on a space factor of 45 %. Suitable allowance should be made for voltage drop and temperature rise of bunched cables.

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