Electrical Info: Articles

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

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: V_d = 2 × I × R
  
  For 3 Phase System: V_d = √3 × I × R
  
  where V_d 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.
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 × I_SC × √t (For Al conductor)
    - A = 9.41 × I_SC × √t (For Cu conductor)
  - For 22 kV System:
    - A = 16.39 × I_SC × √t (For Al conductor)
    - A = 10.98 × I_SC × √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 I_SC is the short circuit current in kA and is calculated by the formula I_SC = 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:

The cable drum should always be rolled in the direction of the arrow, if it is to be moved over a short distance.

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.

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.

Minimum bending radius should be as given below, wherever possible 25 % larger radius than the value given in the table should be used:

System Voltage	Single Core	Multi Core (Unarmoured)	Multi Core (Armoured)
Up to 11 kV	20 D	15 D	12 D
Up to 22 kV	25 D	20 D	15 D
Up to 33 kV	30 D	25 D	20 D

Where D is the overall diameter of the cable.

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:

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

Non-Flexible Cables & Bare Conductor for Fixed Wiring:

Function	Colour Identification
Earthing	Green and Yellow or Green
Neutral of single / three phase circuit	Black
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 wire	Red
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 cable	Orange

Flexible Cables & Flexible Cords:

Function Colour Identification

Live Brown

Neutral Blue

Earthing Green and Yellow

Function	Colour Identification
Live	Brown
Neutral	Blue
Earthing	Green and Yellow

COMPARISON OF ALUMINUM AND COPPER CONDUCTORS
Ratio	Criterion
	Equal Resistance	Equal current and temp. rise	Equal diameter
	Ratio of Aluminum to Copper
Area ratio	1.61	1.39	---
Diameter ratio	1.27	1.18	---
Weight Ratio	0.48	0.42	0.30
Current Carrying Capacity	---	---	0.78

CURRENT RATING OF COPPER AND ALUMINUM CONDUCTOR VULCANIZED RUBBER / PVC INSULATED CABLE (40 °C Ambient)
Standard Copper Conductor			Continuous Current Rating					Standard Al. Conductor
Nominal Conductor Area	Conventional Sizes	No. & dia of wire	In conduit/casing	Bunched in air or open trench		In air/open trench		No. & dia. of wire	Nominal Conductor Area
Nominal Conductor Area	Conventional Sizes	No. & dia of wire	2 Single core	2 Single core	3 / 4 Single core	1 / 2 core DC / AC	3 / 4 core AC	No. & dia. of wire	Nominal Conductor Area
mm²	SWG	mm	A	A	A	A	A	mm²	mm²
1.00	1/18	1/1.12	4	4	4	4	4
1.25	3/22	3/0.74	7	7	7	7	5	1/1.40	1.5
2.00	3/20	3/0.91	10	10	9	10	7	1/1.80	2.5
3.00	7/22	7/0.74	14	14	10	14	10
			14	14	12	14	10	1/2.24	4
			19	19	17	19	13	1/2.80	6
4.50	7/20	7/0.91	19	19	17	19	14
			23	23	21	23	17	1/3.55	10
6.75	7/18	7/1.12	25	25	22	25	17
10.0	7/17	7/1.32	30	30	26	30	21	7/1.70	16
14.5	7/16	7/1.62	37	37	33	37	26
			41	41	41	41	29	7/2.24	25
18.5	19/18	19/1.12	43	43	39	43	30
			48	48	43	48	33	7/2.50	35
25.0	19/17	19/1.32	51	51	46	51	36
			63	63	57	63	43	7/3.00 19/1.80	50
40.0	19/16	19/1.62		67	61	67	47
50.0	19/15	19/1.82		85	74	79	54
				92	82	81	57	19/2.24	70
				106	95	93	65	19/2.50	95
65.0	19/14	19/2.10		110	97	97	68
70.0	37/16	37/1.62		122	109	109	70
				127	117	112	79	37/2.06	120
95.0	37/15	37/1.82		141	128	124	87
				145	128	125	88	37/2.24	150
				170	149	144	101	37/2.50	185
120.0	37/14	37/2.10		173	152	150	106
				200	171	165	117	37/2.80	225
150.0	37/13	37/2.36		202	179	174	123
185.0	37/12	37/2.62		231	204	196	137
				244	208	200	139	61/2.50	300
250.0	61/13	61/2.36		293	248	235	165
				300	257		275	61/3.00	400
300.0	61/12	61/2.62		330	283

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 Area	Continuous Current Rating			Short Circuit Rating (1 s)	DC resistance at 20 °C	Capacitance per km (at 50 Hz)	Inductive reactance per km (at 50 Hz)	3.5 Core		4 Core
Nominal Conductor Area	Ground	Duct	Air	Short Circuit Rating (1 s)	DC resistance at 20 °C	Capacitance per km (at 50 Hz)	Inductive reactance per km (at 50 Hz)	Overall Diameter	Mass	Over all Diameter	Mass
mm²	A	A	A	kA	Ω	μF	Ω	mm	kg/km	mm	kg/km
10	46	39	40	0.748	3.000	0.577	0.0888	-	-	22.0	785
16	60	50	51	1.200	1.910	0.757	0.0857	-	-	25.5	1050
25	76	63	70	2.080	1.200	1.860	0.0849	26.0	1162	27.0	1285
35	92	77	86	2.600	0.868	0.981	0.0823	28.5	1340	29.0	1425
50	110	95	105	3.660	0.641	0.997	0.0795	32.0	1702	33.5	1815
70	135	115	130	5.660	0.443	1.163	0.0769	36.0	2097	37.0	2455
95	160	140	155	7.050	0.320	1.181	0.0766	40.0	2635	42.5	2910
120	185	155	180	9.350	0.253	1.310	0.0741	44.0	3139	46.0	3465
150	200	175	205	11.070	0.206	1.282	0.0743	48.5	3667	50.5	3985
185	235	200	240	13.740	0.164	1.300	0.0742	53.5	4477	55.5	4915
225	264	225	269	17.270	0.134	1.304	0.0741	59.0	5347	61.5	5875
240	275	235	280	19.850	0.125	1.342	0.0737	60.5	5547	62.5	6475
300	305	260	315	22.630	0.100	1.365	0.0733	66.5	6673	69.0	7255
400	335	290	375	32.700	0.078	1.426	0.0729	73.5	8164	77.0	9590
500	370	320	425	38.350	0.062	1.406	0.0732	82.5	10083	87.5	10085
630	405	350	480	48.350	0.048	1.424	0.0731	94.0	-	97.5	13680

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 Area	Continuous Current Rating		DC resistance at 20 °C	Unarmored Cable		Armored Cable
No. of cores and Conductor Area	Ground	Air / Duct	DC resistance at 20 °C	Overall Diameter	Mass	Overall Diameter	Mass
mm²	A	A	Ω/km	mm	kg/km	mm	kg/km
2 x 1.5	23	20	12.10	10.6	155	14.0	418
3 x 1.5	21	17	12.10	11.0	180	14.5	452
4 x 1.5	21	17	12.10	12.0	210	15.6	496
5 x 1.5	16	14	12.10	12.6	250	16.5	566
6 x 1.5	15	13	12.10	14.0	280	17.0	627
7 x 1.5	14	13	12.10	14.0	310	17.2	637
10 x 1.5	13	11	12.10	17.0	423	19.8	695
12 x 1.5	12	10	12.10	17.6	470	20.4	723
16 x 1.5	11	9	12.10	19.7	560	21.8	885
24 x 1.5	9	8	12.10	24.3	752	25.9	1115
61 x 1.5	6	6	12.10	34.1	1705	36.4	2310
2 x 2.5	32	27	7.28	13.1	218	15.4	490
4 x 2.5	27	24	7.28	14.5	290	16.8	610
6 x 2.5	21	18	7.28	16.6	390	20.4	770
10 x 2.5	18	15	7.28	21.2	560	22.0	840
12 x 2.5	17	14	7.28	21.5	640	22.4	960
16 x 2.5	15	13	7.28	23.1	790	24.8	1160
24 x 2.5	13	11	7.28	27.7	1105	29.8	1510
61 x 2.5	9	8	7.28	40.4	2490	42.0	3200

MAXIMUM LENGTH OF CABLE (IN METER) FOR 5 % VOLTAGE DROP
Current in A	Cable cross section (mm²)
Current in A	2.5	4	6	10	16	25	35	50	70	95	120	150	185	240	300	400
4	215	383	513
5	170	263	405	707
9	98	153	235	411
16		83	128	223	329	572
23			88	153	226	393
29				122	179	311	388
40					130	226	281	393	605
53						170	212	297	456	563
66							170	238	366	453	603
80								196	302	374	497	567
95								165	254	315	419	477	575
130									186	230	306	349	420	565
190												238	287	386	417	512
270														272	293	360

CURRENT RATING OF PAPER INSULATED HT POWER CABLES
Nominal Conductor Area	Unarmored 3 Single Core			3 Core Unarmored and Served						3 Core Armored and Served
	Servings Touching			Belted Cable			Screened Cable			Belted Cable			Screened Cable
	Gnd	Duct	Air	Gnd	Duct	Air	Gnd	Duct	Air	Gnd	Duct	Air	Gnd	Duct	Air
mm²	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A
16	63	62	66	55	50	50	60	51	59	58	49	50	61	54	59
25	78	82	88	72	64	65	73	66	77	72	64	68	80	65	77
35	93	96	105	84	77	79	92	77	93	84	74	80	95	80	93
50	115	115	130	105	94	100	115	94	115	105	92	100	115	100	115
70	140	140	165	130	115	125	140	115	140	130	115	125	140	125	140
95	165	165	200	155	135	155	165	135	175	155	135	155	165	145	175
120	185	185	230	170	150	175	185	155	200	170	155	175	185	160	200
150	210	205	265	190	170	200	210	175	230	190	175	200	205	180	230
185	240	230	300	220	195	230	245	200	265	220	200	230	235	210	265
225	260	250	340	240	210	260	270	225	280	240	220	260	265	230	300
240	270	260	355	260	235	275	285	235	315	250	225	265	275	240	310
300	305	285	400	300	265	315	310	265	355	280	250	310	300	265	315
400	355	335	490	345	300	375	360	320	405	320	285	365	357	315	408
500	395	365	570	385	330	425	405	350	460	360	310	415	385	330	450
625	445	410	670	410	365	480	435	390	520	385	345	470	410	370	510
800	495	455	750
1000	550	495	820

ELECTRICAL AND PHYSICAL DATA ON PAPER INSULATED LEAD COVERED SCREENED ARMORED 3 CORE ALUMINUM CONDUCTOR 11 kV GRADE CABLE
Nominal Conductor Area	Continuous Current Rating			Short Circuit Rating (1 s)	DC resistance at 20 °C	Capacitance (at 50 Hz)	Inductive reactance (at 50 Hz)	Overall Diameter	Mass
Nominal Conductor Area	Ground	Duct	Air	Short Circuit Rating (1 s)	DC resistance at 20 °C	Capacitance (at 50 Hz)	Inductive reactance (at 50 Hz)	Overall Diameter	Mass
mm²	A	A	A	kA	Ω/km	μF/km	Ω/km	mm	kg/km
50	115	100	115	3.62	0.641	0.370	0.0973	49.0	5855
70	140	125	140	5.63	0.443	0.440	0.0918	53.5	6855
95	165	145	175	7.00	0.320	0.480	0.0893	57.5	7495
120	185	160	200	9.29	0.253	0.538	0.0853	60.0	8180
150	205	180	230	10.98	0.206	0.580	0.0837	64.5	9390
185	235	210	265	13.64	0.164	0.635	0.0817	68.0	10200
225	265	230	300	17.14	0.134	0.702	0.0798	72.0	11565
240	275	240	310	19.70	0.125	0.745	0.0787	73.5	12080
300	300	265	315	22.47	0.100	0.789	0.0776	73.5	13900
400	357	315	408	32.45	0.078	0.931	0.0751	85.5	16880

CURRENT RATING OF COPPER CONDUCTOR VULCANIZED RUBBER/PVC INSULATED FLEXIBLE CORDS (IS 434 & IS 694)
Nominal Cross-sectional Area	No. and dia. of wires	650 V / 1.1 kV Grade
Nominal Cross-sectional Area	No. and dia. of wires	Twin Core	Three Core	Four Core
mm²	mm	A	A	A
0.5	16 / 0.200	11	9	9
0.75	24 / 0.200	14	12	11
1.00	32 / 0.200	15	13	12
1.50	48 / 0.200	21	18	16
2.50	80 / 0.200	28	24	22
4.00	128 / 0.200	37	32	29

CURRENT RATING OF COPPER CONDUCTOR VULCANIZED RUBBER INSULATED FLEXIBLE CABLES (IS 434)
Nominal Cross-sectional Area	No. and Dia of wires	650 V / 1.1 kV Grade
Nominal Cross-sectional Area	No. and Dia of wires	Single Core	Twin Core	Three Core	Four Core
mm²	mm	A	A	A	A
6	85 / 0.300	55	45	38	36
10	62 / 0.450	75	60	52	47
16	101 / 0.450	100	80	70	64
25	157 / 0.450	130	105	92	83
35	220 / 0.450	160	130	120	100
50	314 / 0.450	205	160	140	125
70	158 / 0.750	250	200	175	155
95	215 / 0.750	300	240	205	190
120	272 / 0.750	345	270	240	220
150	340 / 0.750	415	320	285	260
185	419 / 0.750	465	360	315	295
240	543 / 0.750	540	420	365	340
300	679 / 0.750	640	495	435	400
400	905 / 0.750	770

SINGLE CORE AUTO CABLE WITH COPPER CONDUCTOR
Cable Size (mm)	Current Rating (A)	Cable Size (mm)	Current Rating (A)
9/0.300	5	28/0.300	15
14/0.300	9	35/0.300	18
16/0.300	10	44/0.300	20
19/0.300	11	65/0.300	25

COPPER CONDUCTOR VULCANISED RUBBER INSULATED SHEATHED WELDING CABLE (IS 434)
Nominal Cross-sectional Area	No. and dia. of wires	Current Rating
mm²	mm	A
16	509 / 0.200	100
25	796 / 0.200	150
35	1114 / 0.200	230
50	1591 / 0.200	400
70	2228 / 0.200	600
95	1344 / 0.300	Used when voltage drop with smaller cable becomes excessive.

CAPACITIES OF STEEL TRUNKING FOR CABLES
Cable Size in mm²	50 mm × 50 mm	75 mm × 75 mm	100 mm × 100 mm
1.0	65	140	200
1.5	45	100	180
2.5	40	90	160
4.0	35	80	140
6.0	29	65	116
10.0	24	54	96
16.0	14	35	56
25.0	12	27	48
35.0	10	21	40
50.0	8	18	32
70.0	5	10	20
95.0	4	9	16
120.0	3	5	12
150.0		4	7

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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