Wiring Material For Domestic And Industries Wiring and Cable properties

TYPES OF WIRES AND CABLES:

Before considering types of wiring, suitable for any installation, the selection of cables based on its qualities in regard to material, insulation etc. is very important. Therefore different types of cables are mentioned as under:

The cable consists of three parts :

(a)   Aluminum or copper core in single, double or several cores stranded together.

(b)   Insulation portion to cover the aluminum or copper core.

(c)   The outer covering, known as protective covering, used to cover the insulation for 

        protec­tion against mechanical damage and moisture etc.

Conductor with single layer of insulation covered with tough rubber sheath is called 'Sheathed Wire'. Normally used for batten wiring

 Fig:   Stranded conductor having layer of insulation, covered with tough rubber sheath and additional outer layer of insulation is called "tough rubber sheathed stranded conductor."

Various types of insulating material used in cables are described as follows. The insulation of  cables should have high resistance, dielectric strength, capacity to withstand high temperature, mechanical strength and should be non inflammable. The insulation material commonly used for general lighting and power cables is rubber, vulcanized Indian rubber, paper, poly-vinyl chloride (P.V.C-)/ vulcanized bitumen and cotton and tape.

1. Rubber : It is most commonly used insulating material for house wiring and general purpose cables. Natural rubber is obtained from milky sap of tropical trees. Synthetic rubber is manufac­tured from alcohol or oil products. It absorbs moisture and does not withstand high temperature. It is not suitable for manufacture of cables in its pure form.

2. Vulcanized Indian Rubber (V.I.R.):  If mineral material such as sculpture zinc-oxide etc. are mixed with the rubber, its mechanical strength, heat resistance and stability towards solvents are increased. The process is called 'vulcanization' V.I.R. does not absorb moisture from the atmosphere. The V.I.R. cables are used mainly in internal wiring and other low voltage installations.

3. Paper: It is manufactured from wood pulp, rags or plant fiber and prepared by a suitable chemical process. As compared to V.I.R. insulation, it is cheap, has low capacitance, high dielectric strength and capacity to withstand high temperature. The paper insulated cables should always be sealed by providing protective covering, otherwise, it may absorb moisture and reduce its conduc­tivity. Its ends are always sealed with wax or tar. The paper insulated cables are used for high volt­age power transmission and distribution systems.

4. Poly-vinyl Chloride (P.V.C.):  It is processed with certain material. It is cheaper than rubber and resistive against alkalis and most acids. Its use is therefore preferred over V.I.R. wires. Its resis­tivity is lower than V.I.R.

5. Vulcanized Bitumen:  It is cheaper than rubber. Its resistivity against corrosion due to gases, fumes and water is good. The main disadvantage of its insulating material is that it cannot with­stand higher temperature. If used in conditions where it is subject to come under high temperature, it will become soft and cable conductor will sink down and chances of short circuit higher.

6. Cotton and Silk Insulation:  This type of insulation coverings is used for conductors required for low voltages. The conductor may have a single layer or double layer covering. It is normally used for instruments and motor winding.

CHOICE OF CABLE

The following considerations are kept in mind, while deciding a choice of conductor for a given situation. The situation may be for instance prone to mechanical injury, cable expose to atmos­phere etc

 (a) Current carrying capacity : The current carrying capacity of the conductor size is the maxi­mum current it can carry without the cable getting overloaded and overheated. It is one of the important considerations while selecting a wire or cable for a given situation.

(b) Resistance and impedance : The conductor size selected for a given situation must have low resistance per unit length so that the losses are acceptable and within prescribed lim­its. The configuration of conductors and insulators used must result into acceptable impedance.

(c) Mechanical strength : The conductor size in terms of material used in cable must provide sufficient physical strength for installation without stretching or bending.

(d)Termination : The conductor selected for a given situation is bound to commence and ter­minate at the required connector or other devices in a serviceable manner. The termination of the conductor should be so sound that it is held under screws tightly through thimbles. If the wire is to be soldered, the soldered joint should be sound enough.

(e) Flexibility: The flexibility of the conductor selected for a given situation should be suffi­cient enough to withstand the installation stresses without becoming brittle and breaking.

(f) Conductor configuration: The word configuration means, the arrangement of conductors in a transmission or distribution line i.e. the conductors are either in triangular formation or in vertical formation. The aim is to make the distribution line system most efficient.

(g)  The other considerations include weight and cost which also affects choice of conductor.

2.3 TYPES OF CABLES USED FOR INTERNAL WIRING

Complete specifications of cables to be used in internal wiring must be mentioned, i.e. whether it is copper of aluminum conductor, type of insulation used, voltage grading and number of cores used in the cables. The cables may be 250/500 volts or 650/1100 volts grade. The cables may be single twin or three cores.

The wires used in domestic installation of electrical wiring can be divided into the following :

(a) V-I.R. (Vulcanized  Indian Rubber) wires.

(b)  T.R.S. (Tough Rubber Sheathed) wires.

(c)  P.V.C. (Poly-vinyl chloride insulated) wires.

(d) Lead sheathed or Lead alloy sheathed wires.

(e) Weather proof cables

(f) Metal sheathed weather proof cables.

(g) Flexible cords.

 GENERAL SPECIFICATION: For Bangladesh

CLIMATIC CONDITIONS:

The climate is tropical and has marked Monsoon character with seasonal changes from humid, warm, rainy season, summers to cool and dry winters. Maximum temperature occurs during the period from April to May reaching approximately 43 deg. C (110 deg. F) with a relative air humidity of 60% to 70%.

The annual mean temperature is approximately 29 deg. C (84 deg. C). During the rainy monsoon month from June to September, the average relative air humidity is 80% and reaches extreme values up to saturation point during longer periods. The annual rainfall, most of which occurs from June to September is 2000mm to 2500mm.

OPERATING CONDITIONS:

The cable will be connected to the 11KV or 415 volts 3-phase or 220 volt single phase, 50 Hz Bus of Power Development Board/REB/DESA/Bangladesh.

STANDARDS:

All equipment and materials must be in conformity with the most recent relevant Bangladeshi laws, standard rules and regulation. Particular attention is to be paid to the Electrical Act 1910 and Electricity Rules 1937 (as amended in 1946). All equipment and materials to be supplied which required any form of approval by the Bangladesh Government or a Local Authority like PDB/DESA or REB must satisfactorily pass all inspection and tests procedures imposed by them. Otherwise, all the equipment and materials must be in conformity with the most recent international rules, regulation, standards and recommendation: IEC.

STANDARD DATA:

The following standard values for high and low voltage are standard in Bangladesh.
Distribution bus High Voltage:           11 KV,

Maximum system High Voltage:        12 KV.

Distribution bus Low Voltage     :      415/240 V

FREQUENCY:

The standard power frequency in Bangladesh will be: 50Hz.

DESIGN & CONSTRUCTION REQUIREMENTS:

All Cables are to be in accordance with ihe latest recogni/.ed rules of workmanship and

modern engineering practice and must be suitable in every respect for continuous operation at maximum output under the climatic conditions as specified above.

MATERIAL REQUIREMENTS:

Conducting materials for cables must be of 99.99% pure annealed stranded copper and insulation of thermoplastic materials based on PVC or XLPE for HT and LT Cables shall be complying with relevant IEC/VDE/BS/BDS standards.

2.5 PROPERTIES OF PVC COMPOUND:

1. Tensile Strength

Thermoplastic PVC compound	Tensile Strength
	Before Ageing (kgf) min	1 After Ageing at 80+2°C for 7 days (kgf) min	Difference after ageing % max
Insulation mixture ( LT Cables)	125	125	25
Insulation mixture (HT Cables)	150	150	25
Sheath mixture	100	100	25

2. Elongation:

Thermoplastic PVC compound	Elongation
	Before Ageing % min	After Ageing at SO-i-2°C for 7 days % min	Difference after ageing % max
Insulation mixture ( LT Cables)	125	125	25
Insulation mixture { HT Cables)	100	100	25
Sheath mixture	150	150	25

3. Thermal Stability at 200°C:

Thermoplastic PVC compound	Thermal Stability at 200°C
	Volume resistivity at 70 C Ohm. cm	Before Ageing % min	After Ageing % min	Deformation under pressure at 70°C. Depth of indentation % max	Heat Shock at 150°C	Shrinkage at!50°C % max
Insulation	10	80	80	50	No Crack	4
mixture
( IT Cables)
Insulation                10		120	120	50	No Crack	4
mixture
( HT Cables)
Sheath mixture	_	40	40	50	No Crack	_

2.6 MARKING AND CODE:

The Cables shall be marked and coded in accordance with BS, IEC, VDE or equivalent standards.

INSULATION CO-ORDINATION:

The insulation of thermoplastic materials based on PVC or XLPE for HT and LT Cables shall be comply with relevant IEC/VDE/BS/BDS sMncUirds.

The insulation test voltage for Cables is as follows:

Rated  voltage of the cables, KV	Single      phase      test  voltage, KV	Three      phase      test voltage, KV	Direct    test    voltage, KV
0.6	4	4	12
3.5	11	11	33
5.8	17	17	51

TECHNICAL REQUIREMENTS:

The Cable specified in the following items shall withstand the impulse levels and test voltages specified by the recommendations of IHC, as stipulated before. They must be capable of carrying the short time current for three seconds and must withstand the short circuit (Peak value) current.

The cable must be designed accordingly in order to withstand the mechanical short circuit stresses. They must contain all technical particulars which are mentioned in the Schedule of Technical Data.

The Owner reserves the right to have routine tests carried out on each type of equipment at the manufacturer's factory in the presence of his representative.

The single bars shall be marked by the colors as per IEC, VDE or BS standard.

H.T. CABLE:

HT XLP Cables Construction.

2.8 CONDUCTOR:

The Conductor is Plain annealed stranded and compacted round Copper in accordance with IEC-60228. Compacting of conductor improves its current carrying capacity, reduces losses and reduces overall cable diameter.

CONDUCTROR SCREEN:

An extruded tight 'fitting layer of Semi-conducting is provided over the copper conductor as the provisions of 1EC-60502.-2.

INSULATION :

The insulation is Cross-linked polyethylene (XLI'H) applied by extrusion as per 1EC-60502-2.

INSULATION SCREEN:

An extruded tight fitting layer of Semi-conducting is provided over the XLPE insulation as the provisions of IEC-60502-2.

METALLIC SCREEN:

A metallic screen of copper is provided over insulation screen as per the provisions of IEC-60502-2.

INNER COVERING:

Inner covering of extruded or taped PVC is provided wherever applicable as per IEC-60502-2.

ARMOUR:

Armoring by Round wires or Rat wires or Double tapes. The material of armor for Single core is aluminum whereas, for multi-core cables it is Galvanized Sheet, Counter helix of Galvanized sheet tape is provided on request for round steel wire & Flat wire armored cables. The armor is applied over the inner covering as per il:C-60502O.

OVERSHETH:

Over sheath is of extruded PVC as per IEC-60502-2. Special properties for sheath can be provided on specific request, such as ERLS, anti-termite & anti-rodent, resistance to ultraviolet radiation etc.

3 Core Round/Flat Wire HT XLPE Cables Type: 2xSEYRGY/2xSEYFGY

Applicable Specification: IEC-60502-2. Voltage grade 6/10(12) KV.

Construction: Stranded copper conductor, extruded semi conducting conductor screen, XLPE insulation, extruded semi conducting insulation screen, metallic screen of copper, cores laid up with PVC fillers. Hxtruded PVC Inner covering, Galvanized steel Round/Flat wire armour with counter helix binder tape, PVC over sheathed.

2.9 Dimension & Mechanical Data

Nominal conductor cross section	No. of strands in conductor	Nominal thickness of insulation	Steel armour wire Dia.	Nominal Over Sheath thickness	Approx. Cable Diameter	Approx. weight of cable
mm2 3x50	Nos. 19	mm 3.4	mm 2.5	mm 2.7	mm 58,7	kg/km 6108
3x70	19	3.4	2.5	2.7	58.7	7089
3x95	19	3.4	2.5	2.8	52.6	8281
3x120	37	3.4	2.5.	2.9	66	9376
3x150	37	3.4	5 t-.ij	3.0	69.4	10657
3x185	37	3.4	2.5	3.2	73.1	12084
3x240	61	3.4	3.2	3.4	80.4	15322
3x300	61	3.4 ,->.4	3.2	3.5	85.1	17669

Electrical Data:

Nominal conductor cross section	Max. conductor DC resistance @20(1C	Current rating @30"C in ground	Current rating @30nC in air	Capacitance	Inductance
mm2 3x50	Ohm/km 0.387	Amps 202	Amps 218	micro F/km 0.26	mH/km 0.364
3x70	0.268	249	270	0.29	0.344
3x95	0.193	300	333	0.33	0.327
3x120	0.153	335	378	0.35	0.314
3x150 3x185 3x240	0.124 0.0991 0.0754	381 421 488	436 503 580	0.38 0.42 0.46	0.304 0.295 0.284
3x300	0.0601	525	617	0.50	0.275

2.10 L.T. CABLE:

Single/Multi-core low voltage cables shall be PVC/XLPE insulated, PVC sheathed armored /non-armored direct burial type, termite proof, made and tested according to relevant 1EC/VDE/BS/BDS for this type of installation rated voltage being 600/1000V. LT cable shall be used for domestic wiring, control and power wiring.

BYA Cable :

Description: Plain annealed Stranded Copper conductor of 99.99% purity, PVC insulated single core cable.

Application: Suitable for use in surface mounted or cosseted PVC conduits or trunking. Also suitable for field protected installation in and appliances up to 1000 V a.c or up to 750 V to earth d.c.

Reference Standards: IEC-60502-1, VDE-0250, 13DS-900 and BS: 6004.

BYM Cable:

Description: Plain annealed Stranded Copper conductor of 99.99% purity, PVC insulated, PVC outer sheathed single core cable.

Application: Suitable for use in fixed installations in dry or damp premises clipped direct to a surface or on a cable tray unclosed and also for use in non-metallic conduit (PVC) to be used in appliances up to 1000 V a.c or up to 750 V to earth d.c.

Reference Standards: 1EC-6Q502-1, VDE-0250, BDS-900 and BS: 6004.

BYFYE Cable:

Description: Plain annealed Stranded Copper conductor of 99.99% purity, PVC   insulated, two core flat

cable with earth continuity conductor and PVC sheathed.

Application: Suitable for use in fixed installation in dry or damp premises and for installation in walls, only boards and in channels or embedded in plaster. Not to be used in three phase 400 V circuits.

Reference Standards: IEC-60502-1, VDE-0250, BDS-900 and BS: 6004.

NYY Cables:

Description: Plain annealed Stranded Copper conductor of 99.99% purity, PVC insulated, PVC outer sheathed.

Application: Suitable for use in indoors, outdoors, underground and in water for continuous permissible service voltage of 720/1200 Volts.

Reference Standards: 1EC-60502-1, VDE-0271, BDS-900 and BS: 600

NYFGbY Cable:

Description: Plain annealed Stranded Copper conductor of 99.99% purity, PVC insulated, 4 cores laid up, PVC inner sheathed, Galvanized steel strip armoured, PVC outer sheathed.

Application: Suitable for use in indoors, outdoors, underground and in water for continuous permissible service voltage of 720/1200 Volts.

Reference Standards: lEC-h0502-l, VDE-0271, BDS-900 and BS: 6004.

2.11 INSTALLATION:

Cable in conduits:

Generally, single core cable (non sheathed) are to be installed PVC conduits. The conduit sizes shall be as specified in the drawing. It must be ensured that cables are not scratched/damaged during pulling. For long lengths, Pull boxes must be used even if not indicated in the Drawings. Cable shall not be drawn round more than two 90° bends (or their equivalent) between drawing-in-boxes and any single bend must be less than 90°

Cable bending radii:

The internal radius of every bend in a cable shall be not less than the appropriate value stated below:

Insulation	Finish	Overall diameter	Factor  to  be  applied  to overall   diameter of cable to determine    minimum internal radius of bend.
Rubber of PVC Non (circular       copper        or circular                 standard aluminum conductor	Non Armored	Not exceeding 10mm Exceeding 10mm but not exceeding 25mm Exceeding 25mm	3 4 6
	Armoured	Any	6
PVC (solid) aluminum or shaped copper non-armoured conductor.	Armoured or Any		8

Construction Joint Crossing:

At construction joint crossing, a brass expansion joint fitting as per drawing is to be installed and the cables arc to be run through such fitting.

2.12 Cable Trench :

The size of the trench shall be of minimum 2'-9" depth and l'-6" width for each cable to be laid. Where more than one cable is to be laid in the trench, the width of the trenches are to be increased by 6" for each extra cable for size below 70mm^: (3&1/2 core or 4 core) and 12" for bigger size cables.

A cushion of sand of 1^:.M. 1.5, 5" thick is to be placed over the bed of the trench over which the cables are to be laid.

After laying the cable first class brick on edge of flat are to be placed as separators in between the cables. After installation of the brick separators, sand filling is to be done upto 6" from the top of the bigger cable. After sand filling, two layers of first class brick fiats are to be placed along the length and breadth of the trench as a protection against injury and indication that a power cable is laid. The rest of the trench shall be filled with earth, watered and rammed at 6" layers. After cables are laid the original ground conditions shall be restored. But if brick pavement, drain, concrete road, or bituminous carpeting road are out across or damaged, they shall be remedied and restored to the original specification.

The cable route shall be as direct as possible and shall received the consultant's approval before excavation.

All cable bends shall have a radius of not less than 2 times the diameter of the cable drum , or 20 times the diameter of the cable whichever is greater.

G.I. pipe shall be provided for all road and drain crossing. These pipes shall be laid direct in the ground without any sand bed, sand layer, brick or cable covers.

Cables shall always be laid out or laid into the ground through G.I. pipe of suitable size as decided by the Engineer-in-charge / Consultant/Consultant the length of the pipes over the ground shall not be less than 4'-0". No extra cost shall be paid for such pipes. The exposed end of the pipes shall be sealed using PVC or wooden plugs.

The Contractor shall exercise great care in handling the cable and avoid forming 'KINKS'. The cable drums shall preferably by conveyed on wheeled cable drum carried and unrolled and laid directly from the drum carrier. Carriage by trailer or truck can be allowed only if proper care is taken during unloading the drum, and unrolling is done after placing the drum or drum jacks and spindle. The cables shall be unrolled in the directions indicated on the drum by the manufacturer.

G.I. cable marker is to be supplied and installed at every turning point of the trench.

After the cable is laid, it shall be tested by the indicated placed by the Contractor at his own cost. No extra charge shall be allowed for this.

Any damage done to any other services by the Contractor for cable laying operations, shall be made good by the Contractor.

All chasing and passages necessary for laying of cable indoor shall be done by the Contractor and the same shall be made good to the satisfaction of the Consultant by the Contractor without any extra charge to the Owner.

Whenever trenches are left open at overnight , and where road is to be cut, the Contractor shall exhibit suitable danger signal such as banners, red flags and red lamps at his own cost. Temporary arrangement by placing wooden sleepers/sheet steel etc. across the road cutting for vehicular traffic are also be made by the Contractor at no extra cost. The Contractor shall be wholly responsible for any accident which may occur due to the negligence of the Contractor.

All road excavations shall be filled up in layers with power earth and suitably watered and rammed in such _ti manner that after completion of the work there is no land subsidence. The road lop shall be reconstructed to match the existing road pavement.

No trench shall be dug until all cables meant for laying have been procured and brought at site store. Cost of any decent ring or shuttering and showing of trench required to be done shall be borne by the Contractor.

2.13 Single Core Cables :

Single core cables on ground/trench/non-metallic floor shall be laid in trefoil formation, wherever so required by the Engineer-in-charge / Consultant. Where more than one single core cable shall be used in parallel for the same system, these shall be separate, if necessary, from the relevant distribution board(s).

In these matter the instruction of the consultant shall be binding and final, no matter whether these are shown or not in the drawing or schedule. Rate for these shall be included into the cable laying rate. In cases where this type of system (i.e. more than one single core cable is parallel for the same system) are to be laid in the trefoil formation, they shall be found after regular suitable internal as per direction of the Consultant or Engineer-in-charge / Consultant.

Insulation Test:

Insulation test of the whole installation shall be carried out using Mugger, in presence of authorized representative of the Consultant, and result submitted to the Consultant for approval.

The Contractor shall conduct the following tests with the help of the concerned department/authority and the costs of performing the tests should be included in the quoted rates:

2.14 ADVANTAGES AND DISADVANTAGES OF WIRING SYSTEM

Conduit wiring system has the following advantages and disadvantages :

Advantages:

1.   It provides complete protection against fire due to short circuits etc.

2.   It provide protection against mechanical injury to the cables.

3.   It provides protection against moisture of atmosphere as conduits can be made water tight.

4.   Replacement of defective wire is easy.

5.   It has a fairly long life if properly executed.

6.   It looks beautiful.

Disadvantages:

1.   It is very costly system of wiring.

2.   Highly skilled workmanship is required.

3.   Its erection is not easy and requires more time.

It is recommended for following locations :

(a)   It is most suitable in damp situations.

(b)   It is most suitable for workshops and factories for providing wiring to lighting

        system and electric motors.

(c)   It is installed in the private houses, offices and hotels etc. where economy is not the       

        main consideration.

(d) It is most suitable where there is fire hazard such as varnish and paint factories.

Precautions to be observed

1.   Junction boxes, 'T' joints and inspection bends etc, should be used at all bends for  

      inspec­tion purposes and easiness in pulling wires.

2.   After every cutting of the conduit. The edges should be filled before joining the

      conduit so that the wires are not damaged from mechanical injury at the time of  

      laying the wires.

3.   Wooden, ebonite or rubber must be used at the ends where the conduits terminate so

      as told mechanical damage to cables by its sharp edges.

4.  Over crowding of cables should be avoided in conduits.

5. The conduit threads should be coated with aluminum paint to maintain its continuity

     elec­trically. The conduit should be earthed at some suitable point.

6.  There should be no dampness in conduits before laying.

7.   As far as possible, one size of conduit throughout the run should be used even it may

      have to carry less number of cables for certain length.

8.   Bending of pipe should be make by pipe bending machine.

Table

Table showing Number of cables that can be accommodated in the conduit of size as shown against each for aluminum conductor cables

Size of cable no and dia Size in mm      mm2	Conduit 20 mm 250V-   660V	Conduit 25mm 250 V  -  660 V	Conduit 30mm 250 V  -  660 V	Conduit 35mm 250 V  -  660 V	Conduit 50mm 250V- 660 t
1/1.40        1.5^	6            4	10           9	14           10
1/1.80        2.5	5            3	10           6	14             8
1/2.24        4.0	4            2	6           5	10             7
1/2.80        6.0	4	6           4	10            6	7
1/3.55      10.0	2	4            3	5             5	6
7/1,70      16.0		2            2	4             3	5            3	7           7
7/2.24      25.0			2	3            2	6           5
7/2.50      35.0			2	2	5           5
7/3.00      50.0				2	3           3

CONDUCTOR SIZE CALCULATIONS

2.15 Specification of Cables

Before calculating the size of conductor for a given load, the specification of cables are always taken into account.

Cables having a number of strands of a given wire gauge are designated as for example, a cable having 3 strands each of a gauge 20 S. W. G., it may be referred as 3/20 s. The other method is giving size of cable in term of strands and diameter of each strand in mm. A cable having 3 strands and each strand of diameter 0-736 mm. may be termed as 3/0.736 mm The numerator indicates the number of strands in the cable and the denominator indicates the diameter of each strand.

As regards specification of cable, it is most desirable to mention all possible details regarding that particular type of cable in terms of its voltage grading, size in metric system, type of conductor (aluminium or copper), core and type of insulation (P.V.C., V.I.R. etc).

•   Size of conductor: The following three points must be taken into account determining the size of the conductor for internal wiring for a given circuit.

(a)  Minimum Size of the conductor for mechanical reasons.

(b)   Current carrying capacity. -

(c)   Voltage Drop.

Minimum Size of Conductor:

(d) Wire : The minimum size of copper conductor in use is 1/1.2 mm based on permissible wattage in the sub circuit i.e. 800 watts. The aluminium conductor of size 1.5 or 1 /1.40 mm (Number and diameter) will be used for a sub-circuit in house wiring and minimum size of conductor for power wiring is 4 mm² or 1/2.24 mm (No. and diameter) may be used.

(b) Underground Cable: The area of conductor for two core cable should not be less than 6sqmm. and for three and four core cables it should not be less than 25 sq mm. The cross sectional a rta of conductor for three and half core cables should be 50 sq. mm or more.

(e) ACSR (Aluminium Conductor Steel Re-inforced Conductor): The size of ACSR conductor should not be less than 6/.083 inch or 6/1 * .211 mm having total area of cross-section as 20.71 Sq. mm.

Current Carrying Capacity

The current when passes through a wire, a certain amount of heat may be produced which re­sults increase in temperature of conductor. The wires are therefore selected keeping into considera­tion that the size of the wire (conductor) is sufficient to carry the maximum circuit current. Over­heating for marginal increase of temperature is permitted but it should not exceed a certain Voltage Drop Before deciding a proper size of cable to be used in a circuit, due consideration must be given to the voltage drop.

Fall of pressure or voltage in an electric circuit is called voltage drop. The permissible voltage drop from supply terminals or any point on the wiring system should not exceed 2% + 1 volt for light loads and 7,5% for declared supply voltage of power load.

Calculations regarding size of wire can be determined from the following examples.

2.16. Problem

Example 2.16.1. Calculate the size of the conductor for power and lighting circuit from

     meter to main distribution board having two light/fan circuits of 800 watts    

     each and two 15 Amp. circuits of 1000 watts each. The distance falls

      within permissible no voltage drop.

Solution : Load wattage in two light/fan Sub circuit     = 1600 watts.

Load wattage in two socket circuit.                              = 2000 watts.

Total load current                                                         = 3600 watts.

Assuming 0.8 as power factor for the supply

Current in amperes  = 3600/230x0.8 = 19.5 amp say 20 amp

Current under short circuit = 20 x 1.5 = 30 Amps.

Referring to the table for aluminium conductor wire, the size of conductor comes to 6 sq. mm or 1/3.55 mm, rated to carry a load of 34 amperes.

But in general practice, the state electricity boards use next higher size from safety and possible future extension point of view. The cable of size 7/1.70 mm. will be strong enough to carry the load. The size of sub main and service line conductors will be same as given above.

Example 2.16.2. A room is to be wired for single phase a.c. supply directly taken from mains which is at a distance of 30 metres. The wiring is to be at 230 V   single phase supply. If the wire is to carry 5 Amps,, determine the size of the conductor.

Solution: Permissible voltage drop     = 1+2 % of 200 volts

= 1+ 4 = 5 volts

Referring to the table for selection of wire of aluminium conductor, the minimum size of wire, 1.5 mm² or 1/1,40 mm should be in a position to carry 5 amperes safely.

Taking voltage drop into consideration there will be a voltage drop of 1 volt after every 2.3 me­tres for 10 Amperes loading.

Voltage drop at 10 amps  = 30/2.3

Voltage drop at 5 amperes = 30/2.3x5/10 = 6.52 volts

As permissible voltage drop is 5 volts and the voltage drop will be 6.52 volts Hence this will not be suitable. Now considering the next higher size 2.5 sq. mm or 1/1.80 mm and consulting the same table.

Voltage drop at 15 amps = 30/2.3

Voltage drop at 5 amps = 30/2.3x5/15 = 4 volts

Which is within permissible limit.

It is therefore suggested that a wire of size 2.5 sq. mm or 1/1.80 mm is suitable.

Example 2.16.3. Calculate the size of the aluminum conductor cable and of copper conductor cable used in hostel wiring between meter and main D B.suitable to carry a load of 30 Amps. The dis­tance between meter and main switch is 35 meters. The supply voltage is 230 V single phase.

Solution: Total current = 30 Amps, (given), short circuit current = 30 * 1.5 = 45 Amp. The permissible voltage drop = 1 + 2% of 230 V i.e. 1 +4.6 = 5.6 volts.

Total ampere metres in circuit = Maximum current in the circuit x  length of cable (Phase + Neutral)  = 45 * 70 = 3150 Ampere meters.

     Total ampere meters in the circuit

Ampere meter per volt of drop = Permissible voltage drop

3150/5.6 = 56.3

As per table, for AT conductor cable of size 7/2.50 mm will be required as per load of 45 Amps. The conductor of this size has 495.0 ampere meters per voltage drop which is nearly equal to per­missible voltage drop.

The copper conductor cable of size 7/1.625 mm is selected as it has 820 amp. meters per voltage. The voltage drop will be 3150/820 - 3.8 volts which is less than the permissible voltage drop of 5:6 volts.

Example 2.16.4. Calculate the size of the conductor to be used for wiring of a 10 B. H. P.  400 v. 3 phase 50 c/s induction motor. The distance between main switch  and motor falls within permis­sible voltage drop limits.

Solution: Line voltage = 400 volts (given) Assume efficiency of the motor to be 85% and power factor 0.8.

Motor output = √3 EI cos x n-

Therefore current I =.10 x 735.5 / √3 x400x0.85x0.8 = 15.6 Amps.

At the time of starting, the induction motor will take minimum of 2 times the full load current, Starting current = 15.6 ^x 2 = 31.2 Amps, say 32 Amp. As the maximum starting current is 32 Amps, therefore a single core PVC insulated 660 V grade 1 /3.55 mm aluminium conductor cable having current carrying capacity of 34 amperes will be used or a copper conductor cable of size 7/1.12 mm having current carrying capacity of 36 amps may be used.

Examples2.16.5.  Calculate the size of the 3 core conductor for giving a three phase 3-vvire     connection to a premises in which an electric motor of 50 HP is to be installed. The distance between motor and the main switch is 40 meters.   The available voltage is 400 volts.

Solution: Assumingp.fas 0.8 Current drawn by the motor = 50x746/√3 x400x0.8 = 67.30 amps.

Referring to the table for 3 core aluminium condrctor cable, it will be seen that 70 sq. mm (19/ 2.24) PVC cable will be in a position to carry the motor current safely including the starting current.

The permissible voltage drop  = 400x5/100 = 20volts

Voltage drop at 82 Amps = 40/14.7

Voltage drop at 67.30 Amps = 40/14.7x67.30/82

= 2.21 volts.

As the drop is within permissible limits, hence 3 core PVC cable of size 50 sq, mm orl9/1.8Q mm diameter is suitable.

Table

Current rating and voltage drop of vulcanized rubber, P.V.C. or Polythene insulated or tough rubber PVC lead sheathed, twin three or four core aluminum wire or cables

Size of conductor		One twin core D.C. or single phase A.C.			One 3 core or 4 core cable balanced three phase
Nominal area in Sq. mm	Number and dia. of wire in mm	Current rating in Amperes	Approx. length of run for 1 volt, drop		Current rating in Ampres	App. length of run for one volt drop in metres
			D.C. metres	A.C. metres
1.5	1/1.40	10	2.3	2.3	7	3.7
2.5	1/1.80	15	2.5	2.5	11	3.9
4.0	1/2.24	20	2.9	2.9	14	4.8
6.0	1/2.28	27	.3.4	3.4	19	5.5
10.0	1/3.55	34	4.2	4.2	24	6.8
16.0	7/1.70	43	5.3	5.3	30	8.7
25.0	7/2.24	59	6.6	6.6	42	10.8
35.0	7/2.50	69	7.1	7.1	48	11.7
50.00	7/3.0	91	7.7	7.7	62	13.1
70.0	19/2.24	118	9.0	8.8	82	14.7
95.0	19/2.50	135	9.8	9.5	94	15.7
120.0	37/2.06	165	10.8	10.3	114	16.8
150.0	37/2.24	181	11.4	10.7	127	17.5
185.0	37/2.50	209	12.3	11.2	146	18.6
225,0	37/2.80	240	13.5	11. 7--	169	19.1

Table

Current Rating of Aluminum conductor Single core cables for VIR, PVC or Polythene insulated including tough rubber, PVC or lead sheathed.

Size of conductor		2 cables d.c. or		3 or 4 cables or		Four cables d.c. or
		single phase a.c.		balanced three phase		single phase a.c.
Nominal area in Sq.mm	No .and diameter of wire in mm.	Current rating in Amperes	Approx. length of run for one volt. (M.)	Current rating in Amps.	Approx. run for one volt drop in '(Mr.)	Current rating (amps.)	Approx. run for one volt drop in Mts.
1.5	1/1.40	10	2.3	9	2.9	9	2.5
2.5	1/1.80	15	2.5	12	3.6	11	3.4
4.0	1/2.24	20	2.9	17	3.9	15	4.4
6.0	1/2.80	27	3.4	24	4.3	21	4.3
10.0	1/3.55	34	4.3	31	5.4	27	5.4
16.0	7/1.70	43	5.4	38	7.0	35	6.8
25.0	7/2.24	59	6.8	54	8.5	48	8.5
35.0	7/2.50	69	7.2	62	9.3	55	9.0
50.0	77/3.50	91	7.9	82	10.1	69	10.0
	19/1.80
70.0	19/2.24	134	8.0	131	9.5
95.0	19/2.50	153	8.8	152	10.1