Department of EEE, SJBIT Page 1
VTU Question Solution
Unit-1
1.Define the estimating and mention the purpose of estimating and costing. (Jan-2016) Estimating is an art of assessment of quantities of different items and cost thereof to plan the
amount required for executing a work before actually carrying out the work. OR
Estimating means to determine the quantities of various items required to execute a job and to assets the cost of execution.
2.Write a short note on comparative statement. (Jan-2016) Bidding goods and services is important for several reasons. The bidding process:
• allows "comparison shopping" for the best pricing and service
• allows for an informed and objective choice among potential suppliers
• encourages competition among suppliers • provides a standard for comparing price, quality, and service
• provides a list of qualified suppliers for future bids • provides access to University business for suppliers
The bid process begins with the development of a set of specifications or objectives. The Contract Administrator (CA) in conjunction with the requester must define the requirements exactly. Colleagues, technical personnel, trade manuals, and suppliers may be consulted for assistance in developing specifications. The requirements are then communicated to the selected suppliers by a Request for Quotation (RFQ) or a Request for Proposal (RFP).
3.Briefly explain the modes of tendering. (Jan-2016)
Prepare Reasonable Cost Estimates Project cost estimating is not an exact science; however, estimators are expected to prepare reasonable project cost estimates that represent the cost to complete the project. These costs include those required not only for the contractor to construct
Department of EEE, SJBIT Page 2 the project but, also includes the costs for the purchase of right of way, mitigation of environmental issues and any other costs that will be incurred to complete the project.
Project alternatives and their associated cost estimates must be thoroughly compiled by diligently using all of the available data, modifying that data with good judgment and using past cost estimating experience so that the cost estimates can be used with confidence. Coordination between the project planning cost estimates, the project design cost estimates, and the specifications and policies that will be in place during the construction of the project is required. Cost Estimates are Not Static Cost estimates, in a sense, are never completed. They are not static, but have to be reviewed continually to keep them current. The Project Manager (PM) is responsible for keeping the project cost estimate up-to-date throughout the project development process, reviewing all project cost estimates and ensuring that the current project cost estimates are entered into the Project Management data base and a hard copy is in the project file.
4. What is estimating and what are the importance of the estimating and costing(Jun -2015) Electrical estimating is a process used by electricians, construction managers and engineers to determine the amount and cost of electricity required for a specific location or process. There are two general methods of creating accurate electrical estimates: computer software or manual calculations. Both methods have value, benefits and risks. Original electrical estimating software options were quite clumsy to use, but recent enhancements have vastly improved this tool for electrical estimation.
Electrical estimating computer software has increased in popularity as it has improved in quality and performance. This tool is designed for use by electricians, architects and electrical engineers. There are different versions available for residential, commercial or prototype development. An estimating technique is an approach to predicting cost or revenue. Using a consistent methodology is important to achieve reliable and comparable results. Firms may have specific policies their personnel have to follow when making estimates to ensure that the approach will be similar no matter who prepares the estimate. This can help reduce problems associated with variances in methodology, like an offer from one mechanic in a shop of a very low price for service while another indicates the cost of a job will be much higher.
Department of EEE, SJBIT Page 3 When preparing estimates, people can broadly divide them into detailed and approximate types. Approximate estimates offer a rough guess of the cost, based on similar projects, experience, and quick research. They can be helpful for getting a general idea of expenses before proceeding with a more detailed estimate. For people soliciting estimates, they can't be quoted as firm bids, but may provide a frame of reference. A homeowner looking for a new roof, for example, could ask for an approximate estimate from several contractors to learn more about the range of possible prices.
5.Explain the following:1)Electrical schedules 2)Catalogues 3)Purchase system 4)Market
survey . (Jun -2015)
Electrical Schedule:
The electrical load schedule is an estimate of the instantaneous electrical loads operating in a facility, in terms of active, reactive and apparent power (measured in kW, kVAR and kVA respectively). The load schedule is usually categorised by switchboard or occasionally by sub-facility / area.
Catalogues:
The main objective of a catalogue is to promote the products and services offered by your company. A catalogue layout properly designed must show your company's products or services arranged neatly, so that they can be easily recognized; and, at the same time, it must look attractive to improve your sales. In addition, the catalogue layout must be strategically arranged in order to give more importance to certain items or to make the catalogue look more eye-catching. Finally, the visual coherence on which a company's corporate Image is supported must be kept. A catalogue may promote products within promotional packages or lithe known products; it may inform the audience about the new comfort and convenience of a service or it can simply contain small businesses' month offers.
Market Survey and source selection:
Market research is a continuous process for gathering data on product characteristics, suppliers' capabilities and the business practices that surround them—plus the analysis of that data to make acquisition decisions. This requires one to collect and analyze information about the market that subsequently can be used to determine whether the need can be met by products or services
Department of EEE, SJBIT Page 4 available in the commercial market; whether commercial practices regaiding customizing, modifying products or tailoring services are available to meet customer needs; what are the customary terms and conditions, including warranty, buyer financing, and discounts under which commercial sales are made; and whether the distribution and logistics support capabilities of potential suppliers are sufficient to meet the needs of the government. Marret research information can be used to shape the acquisition strategy, to determine the type and content of the product description or statement of work, to develop the support strategy, the terms and conditions included in the contract, and the evaluation factors used for source selection.
Various locational difficulties are described: 1. Remoteness
2. Confined sites 3. Labor availability 4. Weather
5. Design considerations (related to location). 6. Vandalism and site security
6. List out guidelines for inviting tenders. (Jan -2015)
Bidding goods and services is important for several reasons. The bidding process: • allows "comparison shopping" for the best pricing and service
• allows for an informed and objective choice among potential suppliers
• encourages competition among suppliers • provides a standard for comparing price, quality, and service
• provides a list of qualified suppliers for future bids • provides access to University business for suppliers
The bid process begins with the development of a set of specifications or objectives. The Contract Administrator (CA) in conjunction with the requester must define the requirements exactly. Colleagues, technical personnel, trade manuals, and suppliers may be consulted for
Department of EEE, SJBIT Page 5 assistance in developing specifications. The requirements are then communicated to the selected suppliers by a Request for Quotation (RFQ) or a Request for Proposal (RFP).
7. Write the necessity of estimating and costing. (Jan-2015) Electrical estimating is a process used by electricians, construction managers and engineers to determine the amount and cost of electricity required for a specific location or process. There are two general methods of creating accurate electrical estimates: computer software or manual calculations. Both methods have value, benefits and risks. Original electrical estimating software options were quite clumsy to use, but recent enhancements have vastly improved this tool for electrical estimation.
Electrical estimating computer software has increased in popularity as it has improved in quality and performance. This tool is designed for use by electricians, architects and electrical engineers. There are different versions available for residential, commercial or prototype development. An estimating technique is an approach to predicting cost or revenue. Using a consistent methodology is important to achieve reliable and comparable results
8. Explain by giving examples for the following terms.
i) Contingencies ii) Overhead charges iii) Profit (June-2014) Estimating the cost of labor for electrical construction can vary greatly from project to project, depending on the installation crew’s experience and the complexity of the project. Charging an hourly installation rate is common for electrical contractors until installation data (number of hours per installer for job completion) can be collected and projects can be estimated based on the amount of work.
Electrical contractors are responsible for installing, repairing, and maintaining electrical systems in homes and commercial buildings. Due to the differences in skills and costs between home systems and commercial systems, most companies will focus solely on either residential or commercial work. Fortunately, the process of pricing an electrical job is similar no matter what
Department of EEE, SJBIT Page 6 type of building is involved. For those with a basic understanding of construction and electricity, it is fairly easy to price an electrical job and develop an appropriate estimate.
9. Explain activities of Purchase department. (June-2014)
FUNCTION/ROLE OF A PURCHASING DEPARTMENT
To buy at the right time, right price and right terms Ensuring the continuity of supply
Selection and evaluation of suppliers/vendors Aware of long-term and short term effects Preserving and enhancing reputation of company Aware of all supply options
Maintain stock level
MAJOR PURCHASING ACTIVITIES
Obtaining and analyzing quotations of vendors/suppliers Interview representatives and correspondence
Deciding best buying terms and conditions Negotiating and checking contracts
Scheduling orders and following up
Work with finance department to obtain discount, matching invoices, verify receipt, purchase journal entry,passing of invoices for payment and settlement of accounts
Disposing of surpluses
Other activities like assisting with preparation of material expenditure/purchasing budget.
Unit – 2
1. Briefly explain the factors to be considered while deciding choice of wiring sys(Jan-2016) General rules for interior wiring:
1) No lighting circuit should contain more than 10 points of lights, fans and socket outlets or a total load of 800 W.
Department of EEE, SJBIT Page 7 2) Switches must always be placed on the live wires only.
3) Each circuit should be provided with a separate cut-out in the distribution boards for their live wires.
4) When the total load exceeds 800 W for lighting only, 3-ph supply is to be taken and the load is to be distributed equally among the three phases.
5) No power circuit should contain more than 3000 W load and in no case, more than two socket outlets are allowed in one power circuit.
6) The main switch board must be fixed within 15 cms from the meter board (MB) so that it is easily accessible to disconnect in case of emergency.
7) The meter board, main switch and distribution board are to be installed at a minimum height of 1.5 m from the floor.
8) All sub-circuits (lighting or power circuit) should have its own continuous earth wire (each sub-circuit is earthed separately).
9) The wall plug socket should be of 3-pin type and the third (big) terminal is always connected to the earth. Adequate numbers of socket outlets are to be provided at suitable places in all rooms so as to avoid use of long lengths of flexible cords. Only 3-pin, 5A socket outlets are to be used in all light and fan circuits and only 3-pin, 5A socket outlets are to be used in all power circuits. All socket outlets are to be mounted at a minimum height of 1.3 m above floor and are to be controlled by individual switches which are to be located immediately adjacent to it. For 5A socket outlets installed at a height of 25 cm above the floor, the switch may, if desired, be mounted at a height of 1.3 m above the floor level. Socket outlets accessible to children should be shuttered or interlocked type. No socket outlet of rating higher than 15 A is to be installed. Appliances requiring a current of more than 15 A shall be connected through a double pole switch of appropriate rating. Socket outlets are not to be located centrally behind the appliances connected to them.
10) The power (or heating) circuit must be drawn separately in AEH (All Electric Home)installations from the meter board itself.
11) When the total load exceeds 5 kVA, the installation must have an ELCB (Earth Leakage Circuit Breaker).
Department of EEE, SJBIT Page 8 12) All the metal parts like metal sheaths / conduits of wiring and metal casings of all consumer appliances (starter body, iron clad switches etc.) must be earthed properly to avoid danger due to electric shock.
13) The current rating of the conductor used should be corresponding to the connected load to ensure safety to the consumers. Also, all light conductors are to be insulated or otherwise safeguarded to avoid danger.
15) All incandescent lamps should have a minimum clearance of 2.5 m from the floor level and ceiling fan should have a minimum clearance of 2.75 m from the floor.
16) The height at which conduit runs on a wall (horizontal run) must be a minimum of 3 m from the floor.
17) A switch board which contains switches, sockets, regulators etc. is to be installed such that its bottom lies 1.25 m above the floor.
18) The distance between the ceiling and the horizontal run may vary from 0.25 m to 0.5 m and depends on the type of building.
19) No fuse or switch is provided on an earthed conductor.
20) Each circuit is to be protected from drawing excessive current (due to overload or insulation failure) by a fuse or automatic circuit breaker.
21) Every installation is to be properly protected near the point of entry of supply cables by a main switch and fuse unit. The main switch shall have two poles in case of single phase supply three poles in case of three phase supply.
22) Depending on the size of the kitchen, one or two 3-pin, 15 A socket outlets are to be provided to plug-in hot plates and other appliances. Dining rooms, living rooms and bed rooms, if required, are to be provided with at least one 3-pin, 15 A socket outlets in each.
23) Every circuit or apparatus is to be provided with a separate means of isolation such as a switch.
24) No additional load is to be connected to an existing installation without ensuring that the installation
Department of EEE, SJBIT Page 9 25) Lamp holders used in bath rooms are to be constructed or shrouded in insulating materials and fitted with protective shield and the earth continuity conductor shall not be less than 7/0.915 mm size.
26) The switch board and socket outlets should not be fixed at locations where there are chances of water entering even in traces.
27) Looping of neutral and phase wires may be done on any one of the brass connectors embedded in insulating material such as in junction box terminals or switch terminals or holder terminals, etc.
28) After completion of work, the installations are to be tested for insulation resistance, polarity of single pole switches, earth resistance and earth continuity before energization.
2.List out the general rules and guidelines for residential installation. (Jun -2015) 1. Every installation is to be properly protected near the point of entry of supply cables by 2-linked main switch and a fuse unit.
2. Conductor used is to be of such a size that it carry load current safely. 3. Every sub-circuit is to be connected to a distribution fuse board.
4. A switch board is to be installed so that its bottom lies 1.25mts above the floor. 5. All plugs & socket outlets are of 3-pin type
6. All incandescent lamps are to be hung at ht of 2.5mt above the floor 7. No fuse or switch is to be provided in earthed conductor
8. In any building , light, fan power wiring are to be kept separately.
9. Unless otherwise specified, the clearance between the bottom most point of the ceiling fan and the floor shall be not less than 2.4 m. the minimum clearance between the ceiling and the plane of the blade shall be not less than 30 cm.
10. Each 15 A socket outlet provided in building for the use of domestic appliances such as AC, water cooler etc.
11. Each socket outlet shall be controlled by a switch which shall preferably be located immediately adjacent thereto or combined therewith.
12. Ordinary socket outlet may be fixed at any convenient place at a height above 20 cm from the floor level. In a situation where the socket outlet is accessible to children, socket outlet which automatically gets screened by the withdrawal of plug is preferable.
Department of EEE, SJBIT Page 10 3.Estimating the quantity of materials required for writing a newly constructed building where plan is shown in fig Assume the details of the load All dimensions are in meters
(Jun -2015)
Total Load = 4.5kW = 4.5 x1000W = 4500W 20% additional load = 4500 x (20/100) = 900W Total Load = 4500W + 900W = 5400W
Total Current = I = P/V = 5400W /220V =24.5A
Now select the size of cable for load current of 24.5A (from Table 1) which is 7/0.036 (28 Amperes) it means we can use 7/0.036 cable according table 1.
Now check the selected (7/0.036) cable with temperature factor in Table 3, so the temperature factor is 0.94 (in table 3) at 40°C (104°F) and current carrying capacity of (7/0.036) is 28A, therefore, current carrying capacity of this cable at 40°C (104°F) would be
Current rating for 40°C (104°F) = 28 x 0.94 = 26.32 Amp.
Since the calculated value (26.32 Amp) at 40°C (104°F) is less than that of current carrying capacity of (7/0.036) cable which is 28A, therefore this size of cable (7/0.036) is also suitable with respect to temperature.
Now find the voltage drop for 100feet for this (7/0.036) cable from Table 4 which is 7V, But in our case, the length of cable is 35 feet. Therefore, the voltage drop for 35feet cable would be Actual Voltage drop for 35feet = (7 x 35/100) x (24.5/28) = 2.1V
And Allowable voltage drop = (2.5 x 220)/100 = 5.5V
KITCHEN ROOM HALL BATH VERANDAH 2 1.(Ju n -201 5)(Ju n -201 5)5 3 4 4 3 3.(J un -201 5)(J un -201 5)5
Department of EEE, SJBIT Page 11
Here The Actual Voltage Drop (2.1V) is less than that of maximum allowable voltage drop of 5.5V. Therefore, the appropriate and most suitable cable size is (7/0.036) for that given load for Electrical Wiring Installation
4. What are the general rules to be followed for internal wiring. (Jan-2015) 1. Every installation is to be properly protected near the point of entry of supply cables by 2-linked main switch and a fuse unit.
2. Conductor used is to be of such a size that it carry load current safely. 3. Every sub-circuit is to be connected to a distribution fuse board.
4. A switch board is to be installed so that its bottom lies 1.25mts above the floor. 5. All plugs & socket outlets are of 3-pin type
6. All incandescent lamps are to be hung at ht of 2.5mt above the floor 7. No fuse or switch is to be provided in earthed conductor
8. In any building , light, fan power wiring are to be kept separately.
9. Unless otherwise specified, the clearance between the bottom most point of the ceiling fan and the floor shall be not less than 2.4 m. the minimum clearance between the ceiling and the plane of the blade shall be not less than 30 cm.
10. Each 15 A socket outlet provided in building for the use of domestic appliances such as AC, water cooler etc.
11. Each socket outlet shall be controlled by a switch which shall preferably be located immediately adjacent thereto or combined therewith.
12. Ordinary socket outlet may be fixed at any convenient place at a height above 20 cm from the floor level. In a situation where the socket outlet is accessible to children, socket outlet which automatically gets screened by the withdrawal of plug is preferable.
5. The fig shows the plan of a low income group government quarter. Draw the single line dia for lighting and heating circuits on the sketch. Calculate total load , length and size of the wire by taking safety factor equals to two. (June-2014)
Department of EEE, SJBIT Page 12
Total load of Sub-Circuit 1 = (2 x 1000) + (4 x 80) + (2×120) = 2000W + 320W + 240W = 2560W
Current for Sub-Circuit 1 = I = P/V = 2560/230 = 11.1A Total load of Sub-Circuit 2
= (6 x 80) + (5 x 100) + (4 x 800) = 480W + 500W + 3200W= 4180W
Current for Sub-Circuit 2 = I = P/V = 4180/230 = 18.1A
Therefore, Cable suggested for sub circuit 1 = 3/.029” (13Amp) or 1/1.38mm (13Amp) Cable suggested for Sub-Circuit 2 = 7/.029” (21Amp) or 7/0.85mm (24Amp)
Total Current drawn by both Sub-Circuits = 11.1A + 18.1A = 29.27
So cable suggested for Main-Circuit = 7/.044” (34Amp) 0r 7/1.04mm (31Amp)
Unit – 3
1. List out the design consideration of electrical installation in commercial bldgs.(Jan-2016) Design consideration of Electrical Installation in Commercial building:
i. Deciding the number of Sub-circuits: The total load in a commercial building is calculated taking into consideration the general lighting load,the motor load and other power loads. The total requirements are then tabulated and the number and size of sub-circuits are determined. The load on each light-fan sub-circuits shall be restricted to 800 watts or 10 outlets and the load on each power sub-circuit should be restricted to 3000 watts or 2 outlets.
ii. Deciding the size of rating of switch boards and distribution boards: Sub circuits are fed from sub-distribution boards, which is turn are fed from main distribution boards and to which supply comes from the main switch board. The sub-distribution boards, main- distribution boards, sub-switch boards, and main switch boards are designed stage by stage considering the load at different levels. The distribution fuse boards shall be located as near
Department of EEE, SJBIT Page 13 as possible to the centre of the load they are intended to control. They shall be marked "Lighting" or "Power" as the case may be, and also marked with voltage and number of phases of the supply. (Refer IS: 732 - 1983)
iii. Deciding the size of Cables: The size of cables or conductors feeding the different stages of supply connection can be found out by calculating the actual current value at each stage. For electricity distribution from a substation or main switch board to a number of sub-switchboards, PVC insulated armored and PVC sheathed cable installed in under ground trenches should be made use of.
iv. Deciding the size of conduits: The size of conduit is determined from the size of the cables and the number of cables to be drawn though it. The conduit size is stated in term of its outer diameter.
v. Bus bar and bus bar chamber: Bus bar camber consists of bus bars which are strips of copper or aluminium. The incoming lines are connected to these distribution bus bars through the main switch fuse and the load circuits are supplied from the bus bar through the switch fuse units. In a bus-bar chamber these are fixed four of which three are for the three phases and the fourth for the neutral. The size of busbar chamber depend on (a) size and number of strips used, (b) number and rating of switches to be mounted on it.
vi. Mounting arrangement of switchboards and distribution boards: Switchboards and distribution boards can be mounted on to the wall or on the floor. Any type of mounting frame can be made with suitable angle iron. The size of angle iron depends upon the weight and size of the switchboard or distribution board to be mounted on the frame.
2.Explain the determination of load calculation selection of size of service connection and
nature of supply. (Jun -2015)
Conduit wiring :
• Rigid non-metallic conduits are used for surface, recessed and concealed conduit wiring. Conductors of ac supply and dc supply shall be bunched in separate conduits. The numbers of insulated cables that may be drawn into the conduit are given in table. Maximum permissible number of 1.1 kV grade single core cables that may be drawn into
Department of EEE, SJBIT Page 14 rigid non metallic conduits Conduit shall be fixed by saddles secured to suitable wood plugs or other plugs with screws at an interval of not more than 60 cm. whenever necessary, bends or diversions may be achieved by bending the conduits or by employing normal bends, inspection bends, inspection boxes, elbows or similar fittings.
3. Fig Shows the plan of ground floor of school building .School building consists at ground floor ,1st floor and 2nd floor having same plan that of ground floor. Draw single line diagram for ground floor and calculate materials required for three floors. (Jun -2015)
i. Number of Sub-circuits:
Taking 8 points per circuit or 560 W per circuit
No. of Sub-circuits in ground floor main building = 45 / 8 = 6
No. of Sub-circuits in first floor main building = 44 / 8 = 2 No's 10 way single phase ICDB's are required for wiring the circuits in the two floors.
No. of Sub-circuits in ground floor Auxillary building = 30 / 8 = 4 No. of Sub-circuits in first floor Auxillary building = 27 / 8 = 4
2 No's 10 way single phase ICDB's are required for wiring the circuits in the two floors.
Then 1 no. 4 way single phase ICDB is required for wiring the 4 no's 15A power socket outlets. ii. Deciding the cable size:
Class Room 1 Class Room 2 Class Room 3 Staff room d d d d Passage (all dimensions are in meters)
Wc 1 Wc 2 Wc 3 Wc 4 7.( Ju n -20 15 )(J un -20 15 )5 1.(J un -201 5)(J un -201 5)5 2 7.( Ju n -20 15 )(J un -20 15 )5 10 1.( Ju n -20 15 )(J un -20 15 )5 7.( Ju n -20 15 )(J un -20 15 )5 7.( Ju n -20 15 )(J un -20 15 )5
Department of EEE, SJBIT Page 15 i. Total wattage of the scheme = 16900 watts = 16.9 kw, 3-phase 4 wire system of supply is
choosen.
Load current = 16900 23.5 3 415 L IA = = ×
3 1/2 core 25 sq mm PVC insulated aluminium conductor cable or four core 7/2.24 mm aluminium conductor cable of current carrying capacity 42A can be used.
(Higher size of cable is choosen taking into account the future expansion of the building)
ii. The distribution boards are located at different locations as shown in fig. 3.3 and 3.4. Cable has to be run from the main switch board to the different distribution boards in each area. a) Average load on DB.1 and DB.3 = 3960 watts.
Load current = 3960 16.5 240 A =
Single core 1/2.24 mm aluminium conductor cable of current carrying capacity 20A can be used for connecting MSB and DB's 1 and 3.
b) Average load on DB.2 and DB.4 = 2640 watts. Load current = 2640 11.0 240 A =
Single core 1/1.80 mm aluminium conductor cable of current carrying capacity 15A can be used for connecting MSB and DB's 2 and 4.
c) Average load on DB.5 = 4000 watts. Load current = 4000 16.67 240 A =
Single core 1/2.80 mm aluminium conductor cable of current carrying capacity 27A can be used for connecting MSB and DB.5.
iii. a) Wiring of light, Fan and 5A Socket points from the distribution boards can be done by 1/1.40 mm single core aluminium conductor cable.
b) Wiring of 15A power socket points from the DB.5 can be done by 1/1.80 mm single core aluminium conductor cable.
iii. Deciding the switchboards and distribution boards:
The rating of switch boards & distribution boards are decided by knowing the load current, each level. 10 way, 6 way or 4 way. Single phase 15A per way ICDB's can control the different sub-circuits. The average load on DB's 1 & 3 draws a current of 15.5A, & the load on DB.5 draws a current of 17.4A. So 30A DP isolator can serve as incomers to these DB's. The avrage load & DB's 2 and 4 draws a current of 10.3A. So 15A DP isolator can serve as incomers. Since
Department of EEE, SJBIT Page 16 the total load current of the scheme is 23A, the main switch incoming is choosen as 60A TPN switch, also considering the future expansion of the building. 3 No's 30A DPIC switches and 2 no's 15A DPIC switches are provided for controlling the five DB's located at different places. 1 no 30A TPN switch is kept as spare for future expansion. The schematic or line diagram shown in fig. 3.5 w ill help in understanding this arrangement more clearly.
iv. Bus bar and Bus bar chamber. a. 25.4 mm x 3.18 mm AL strips are c
for neutral bar considering the maximum current flowing through the bus bar is 60A. Length of 25.4 mm x 3.18 mm Al. Strips for phase bars 1 x 3 = 3m
Length of 12.7 mm x 3.18 mm Al. Strips for phase bars 1 = 1m
4 no's 30A/15A/DPIC switches are to be fixed on to the top of the bus bar chamber and 1 no, 60A TPN, 1 no 30A TPN and 1 no. DPIC switches to the bottom of bus bar chamber.
.3 m. The busbar chamber is made up of 16 SWG MS sheets.
Total area of 16 SWG MS sheets required for the bus bar 2) Allowance for wastage and cutting 20% = 0.28 sq. m (m2) Total = 1.66 sq. m (m2)
b. To keep the bus bars in fact inside the bus bar chamber, bakelite supports of thickness 0.3 mm
Total area of 2 no's bakelite sheets for bus bar supports.
Allowance for wastage = 0.02 sq. mm & cutting 20%
Total = 0.14 sq. mm
c. For inter connecting the switches of different ratings to the bus bars, different sizes of Al. strips/ wires can be used.
Department of EEE, SJBIT Page 17 15A DP switch = No. 8 SWG aluminium wire.
(1 set for 30A TPN & 3 sets for 30A DP) = 3 m
Length of n
4. List and explain the design considerations of electrical installation in commercial
buildings (Jan-2015)
Ans : Same as Unit-3 (1a)
5. fig shows the plan of ground floor of school building. School building consists of gnd floor, 1st floor, 2nd floor having same plan that of gnd floor. Draw single line dia for gnd floor & calculate material required for three floors. (June-2014) Load = 5.8kW = 5800W
Voltage = 230V
Current = I = P/V = 5800 / 230 = 25.2A
20% additional load current = (20/100) x 5.2A = 5A Total Load Current = 25.2A + 5A = 30.2A
Now select the size of cable for load current of 30.2A (from Table 1) which is 7/1.04 (31 Amperes) it means we can use 7/0.036 cable according table 1
Now check the selected (7/1.04) cable with temperature factor in Table 3, so the temperature factor is 0.97 (in table 3) at 35°C (95°F) and current carrying capacity of (7/1.04) is 31A, therefore, current carrying capacity of this cable at 40°C (104°F) would be
Current rating for 35°C (95°F) = 31 x 0.97 = 30 Amp.
Since the calculated value (30 Amp) at 35°C (95°F) is less than that of current carrying capacity of (7/1.04) cable which is 31A, therefore this size of cable (7/1.04) is also suitable with respect to temperature.
Department of EEE, SJBIT Page 18
Now find the voltage drop for per ampere meter for this (7/1.04) cable from (Table 5) which is 7mV, But in our case, the length of cable is 35 meter. Therefore, the voltage drop for 35 meter cable would be:
Actual Voltage drop for 35meter = = mV x I x L
(7/1000) x 30×35 = 7.6V
And Allowable voltage drop = (2.5 x 230)/100 = 5.75V
Here the actual Voltage drop (7.35V) is greater than that of maximum allowable voltage drop of 5.75V. Therefore, this is not suitable size of cable for that given load. So we will select the next size of selected cable (7/1.04) which is 7/1.35 and find the voltage drop again. According to Table (5) the current rating of 7/1.35 is 40Amperes and the volte drop in per ampere meter is 4.1 mV (See table (5)). Therefore, the actual voltage drop for 35 meter cable would be
Actual Voltage drop for 35meter = = mV x I x L
(4.1/1000) x 40×35 = 7.35V = 5.74V
This drop is less than that of maximum allowable voltage drop. So this is the most appropriate and suitable cable size.
Unit - 4
1.List out the points to be checked while carrying out inspection of wiring install(Jan-2016) Internal wiring should be inspected once a year and the following points should be checked
while carrying out inspection of tho wiring installation.
1. Service Connections: In case of overhead line, check and ensure that: (i) The lines are terminated at a sufficient distance from the building. (ii) The danger notice exists to warn the staff.
(iii) The fuse wire of correct rating is provided on the phase line.
Department of EEE, SJBIT Page 19 (v) The lead-in pipe is properly earthed and bonded and pipe ends are provided with insulating bushes to protect the wires from mechanical damage.
ln case of underground cable check up and ensure that (i) the cables are properly sealed and there is no leakage of cable oil (ii) there are earth connections to the cable armouring. 2. Main Switchboard: In case of main switchboard check and ensure that
(i) The voltage available is correct. (i.e. within permissible limits of declared voltage.) (ii) The main switch is provided close to the point of commencement of supply. (iii) The fuse of correct size is provided on the live pole.
(iv) The main switch is easily identified and is easily accessible so that in case of emergency the entire supply to the building can be switched off at once.
(v) There is a clear working space all round the board (as mentioned in IE rule 51 i.e. 0.914 m). (vi) The phase and neutral wires are clearly marked for identification.
(vii) Caution notice in Hindi or other local language is placed. 3. Miscellaneous: The points to be checked are :
(i) No branch circuit feeds more than 10 points or 800 watt load.
(ix) The leakage current is not more than 1/5,000 of maximum supply current.
(iii) The insulation resistance between conductor and earth and between conductors is more than permissible value as per IE rule.
(iv) The single pole switches are provided on the live conductor.
(v) The electrical resistance from the point of connection with the earth electrode to any point on ECC in the complete installation is not more than one ohm.
(vi) The metallic frames of all power equipment are earthed by two independent earth conductors.
(vii) The metallic covering of iron clad switch, distribution board. submain distribution boards. GI pipe, conduit pipes enclosing VIR or PVC cables are properly earthed.
2. Mention the different types of tests conducted on wiring installations. Explain in detail
Department of EEE, SJBIT Page 20 Testing of Polarity of Single Pole Switches. It in necessary that single pole switches are placed in + ve side or live side so that by making switch off the lamp can be made quite dead. The reason of it is that if the switch in provided on neutral wire, then lamp holder or the fan as well as part of wiring will remain alive, even when the single pole switch is in open position which may easily lead to accidents. For example, a person who is replacing lamp even after opening the single pole switch is liable to get shock if he comes in contact with the line terminal of the lamp holder. As regard the function of a single pole switch, it is equally effective whether it is connected in live or neutral wire but from the safety point of view it is necessary that all single pole switches are provided on phase or outer wire, never on neutral wire.
To ensure that all the switches are placed in phase or live conductors and not in neutral conductor, this test is performed.
A convenient and quicker method of performing this test is by means of a small neon tube tester. While performing polarity test by means of a small neon tube tester, its one terminal is held in the hand and the other against the feed terminal of the switch; if the switch is correctly connected the neon lamp will glow. Pocket neon testing tubes for the purpose are available An alternative method is by means of a test lamp. In this method all the lamps are removed, main switch is put in ‘on’ position, main fuse is inserted, one end of test lamp is connected to earth and the other end is tapped by lead to each contact of each switch in turn. If the test lamp lights on one of the two contacts, it indicates that switch is on the live wire as shown in Fig. 5.2 (a) and if test lamp, does not give light on either contact of the switch, it indicates that the switch is on neutral wire, so must be connected correctly.
Polarity Test
Testing of Earth Continuity Path. For safety all the metal pieces or coverings such as conduits, metal covers of switches etc. must be solidly connected to earth otherwise on the damage of insulation, the leakage current will start giving severe shock to the person touching it.
Department of EEE, SJBIT Page 21 In case of conduit wiring there is a possibility of the conduit joints to become loose or to be separated resulting in high resistance in the earth path. For earth continuity test, main switch should be opened, main fuse withdrawn, all other switches in on position and lamps in their respective holders. One end of the earth continuity tester is connected to an independent earth and the other end is connected to the wiring say to a switch or conduit. The pointer will indicate the earth resistance, which should not exceed the value of one ohm. Higher than this value shows that conduit or switch has not been properly earthed.
3. Estimate the material required for single phase overhead service line of a house located 20 meters away from the pole. With following loads: Lighting load = 800 watts, Heating = 2
kW. Take factor of safety 2. (Jan-2016)
4. Write short note on service lines. (Jun -2015)
This describes various types of utility electrical services and supply voltages. The nominal system supply voltages listed below can vary by ±10% or more. Watt Node meter models are available in seven different versions that cover the full range of electrical services types and voltages. Meters and current transformers are designed for use on either 50 or 60 Hz systems.
Classification of Different Services:
Alternating current electric power distribution systems can be classified by the following properties:
Frequency: 50 Hz or 60 Hz
Number of phases: single or three phase
Number of wires: 2, 3, or 4 (not counting the safety ground) Neutral present:
Wye connected systems have a neutral
Delta connected systems typically do not have a neutral Voltage levels:
Department of EEE, SJBIT Page 22 Medium Voltage: 601 volts to about 34,500 volts
High Voltage: 46,000 volts and up
Line-to-line voltages are typically 1.732 times the phase-to-neutral voltages:
In symmetrical three-phase electrical system, the phase-to-neutral voltages should be equal if the load is balanced.
5. Write the reason for excess recording of ene rgy consumption by energy meter(Jan -2015) Meters can be manipulated to make them under-register, effectively allowing power use without paying for it. This theft or fraud can be dangerous as well as dishonest.
Power companies often install remote-reporting meters specifically to enable remote detection of tampering, and specifically to discover energy theft. The change to smart power meters is useful to stop energy theft.
When tampering is detected, the normal tactic, legal in most areas of the USA, is to switch the subscriber to a "tampering" tariff charged at the meter's maximum designed current. At US$0.095/kWh, a standard residential 50 A meter causes a legally collectible charge of about US$5,000.00 per month. Meter readers are trained to spot signs of tampering, and with crude mechanical meters, the maximum rate may be charged each billing period until the tamper is removed, or the service is disconnected.
Department of EEE, SJBIT Page 23 A common method of tampering on mechanical disk meters is to attach magnets to the outside of the meter. Strong magnets saturate the magnetic fields in the meter so that the motor portion of a mechanical meter does not operate. Lower power magnets can add to the drag resistance of the internal disk resistance magnets. Magnets can also saturate current transformers or power-supply transformers in electronic meters, though countermeasures are common.
Rectified DC loads cause mechanical (but not electronic) meters to under-register. DC current does not cause the coils to make eddy currents in the disk, so this causes reduced rotation and a lower bill.
Some combinations of capacitive and inductive load can interact with the coils and mass of a rotor and cause reduced or reverse motion.
All of these effects can be detected by the electric company, and many modern meters can detect or compensate for them.
The owner of the meter normally secures the meter against tampering. Revenue meters' mechanisms and connections are sealed. Meters may also measure VAR-hours (the reflected load), neutral and DC currents (elevated by most electrical tampering), ambient magnetic fields, etc. Even simple mechanical meters can have mechanical flags that are dropped by magnetic tampering or large DC currents.
Newer computerised meters usually have counter-measures against tampering. AMR (Automated Meter Reading) meters often have sensors that can report opening of the meter cover, magnetic anomalies, extra clock setting, glued buttons, inverted installation, reversed or switched phases etc.
Some tampers bypass the meter, wholly or in part. Safe tampers of this type normally increase the neutral current at the meter. Most split-phase residential meters in the United States are unable to detect neutral currents. However, modern tamper-resistant meters can detect and bill it at standard rates.[34]
Disconnecting a meter's neutral connector is unsafe because shorts can then pass through people or equipment rather than a metallic ground to the generator or earth.
A phantom loop connection via an earth ground is often much higher resistance than the metallic neutral connector. Even if an earth ground is safe, metering at the substation can alert the
Department of EEE, SJBIT Page 24 operator to tampering. Substations, inter-ties, and transformers normally have a high-accuracy meter for the area served. Power companies normally investigate discrepancies between the total billed and the total generated, in order to find and fix power distribution problems. These investigations are an effective method to discover tampering.
Power thefts in the U.S. are often connected with indoor marijuana grow operations. Narcotics detectives associate abnormally high power usage with the lighting such operations require.[35] Indoor marijuana growers aware of this are particularly motivated to steal electricity
simply to conceal their usage of it.
6. Find the materials required for 1- overhead service lines of a house located 10 meters away from pole, with following loads :Lightning =300 watts, Heating= 2500 watts.Assume
safety factor=2 (Jan -2015)
power supply and another for domestic supply.
Assuming efficiency of motor 85% and power factor 0.8, we have full- load currents of motors as ( ) ( ) ( ) 1 2 34 0.75 1, 000 0.75 , 415 , 3 1.54 3 415 0.85 0.8 3.7 1, 000 3.7 , 415 , 3 7.57 3 415 0.85 0.8 5.5 1, 000 5.5 , 415 , 3 11.25 3 415 0.85 0.8 I I For kW V phase motor A I For kW
V phase motor A I For kW V phase motor A × =
Total full- load current for these motors = 1.54 + 1.54 + 7.57 + 11.25 = 21.9 A
Hence cable from meter board to main board shall have a current carrying capacity 1.6 times of full- load current of the mot
Hence 3-core, 1100 V grade PVC insulated, 16 mm2 aluminium conductor having current carrying capacity of 38 A shall be used from meter board to main board.
The current rating of main switch in the starting current of one motor of highest rating plus full
1. Length of 31 mm exible conduit from meter board to main switch board and from main switch board to distribution board, including wastage = 1.0 m
2. Length of 25 mm exible conduit from HG conduit to motor terminals fan machine no. 4, including wastage = 1.0 m
Department of EEE, SJBIT Page 25 3. Length of 19 mm exible conduit required for connecting motor switches to motor starters for all the four machines and HG conduit to motor terminals for machines no. 1,
7. Explain points to be checked while carrying out inspection of wiring installat(Jan-2015) Conductors configuration spacing and clearances, Span lengths
1) No conductor of an overhead line, including service lines, erected across a street shall at any part thereof be at a height less than-
(a) for low and medium voltage lines 5.8 metres (b) for high voltage lines 6.1 metres
(2) No conductor of an overhead line, including service, lines, erected along any street shall at any part thereof be at a height less than-
(a) for low and medium voltage lines 5.5 metres (b) for high voltage lines 5.8 metres
(3) No conductor of an overhead line including service lines, erected else- where than along or across any street shall be at a height less than-
(a) for low, medium and high voltage lines up to and including 11,000 volts, if bare ;4.6 metres (b) for low, medium and high voltage lines up to and including 11,000 volts, if insulated 4.0 (c) for high voltage lines above 11,000 volts 5.2 metres
(4) For extra-high voltage lines the clearance above ground shall not be less than 5.2 metres plus 0.3 metre for every 33,000 volts or part thereof by which the voltage of the line exceeds 33,000 volts: PROVIDED that the minimum clearance along or across any street shall not be less than 6.1 metres.
8. Write the reason for excess recording of ene rgy consumption by energy mtr (June-2014) Meters can be manipulated to make them under-register, effectively allowing power use without paying for it. This theft or fraud can be dangerous as well as dishonest.
Department of EEE, SJBIT Page 26 Power companies often install remote-reporting meters specifically to enable remote detection of tampering, and specifically to discover energy theft. The change to smart power meters is useful to stop energy theft.
When tampering is detected, the normal tactic, legal in most areas of the USA, is to switch the subscriber to a "tampering" tariff charged at the meter's maximum designed current. At US$0.095/kWh, a standard residential 50 A meter causes a legally collectible charge of about US$5,000.00 per month. Meter readers are trained to spot signs of tampering, and with crude mechanical meters, the maximum rate may be charged each billing period until the tamper is removed, or the service is disconnected.
A common method of tampering on mechanical disk meters is to attach magnets to the outside of the meter. Strong magnets saturate the magnetic fields in the meter so that the motor portion of a mechanical meter does not operate. Lower power magnets can add to the drag resistance of the internal disk resistance magnets. Magnets can also saturate current transformers or power-supply transformers in electronic meters, though countermeasures are common.
Rectified DC loads cause mechanical (but not electronic) meters to under-register. DC current does not cause the coils to make eddy currents in the disk, so this causes reduced rotation and a lower bill.
Some combinations of capacitive and inductive load can interact with the coils and mass of a rotor and cause reduced or reverse motion.
All of these effects can be detected by the electric company, and many modern meters can detect or compensate for them.
The owner of the meter normally secures the meter against tampering. Revenue meters' mechanisms and connections are sealed. Meters may also measure VAR-hours (the reflected load), neutral and DC currents (elevated by most electrical tampering), ambient magnetic fields, etc. Even simple mechanical meters can have mechanical flags that are dropped by magnetic tampering or large DC currents.
Newer computerised meters usually have counter-measures against tampering. AMR (Automated Meter Reading) meters often have sensors that can report opening of the meter cover, magnetic
Department of EEE, SJBIT Page 27 anomalies, extra clock setting, glued buttons, inverted installation, reversed or switched phases etc.
Some tampers bypass the meter, wholly or in part. Safe tampers of this type normally increase the neutral current at the meter. Most split-phase residential meters in the United States are unable to detect neutral currents. However, modern tamper-resistant meters can detect and bill it at standard rates.[34]
Disconnecting a meter's neutral connector is unsafe because shorts can then pass through people or equipment rather than a metallic ground to the generator or earth.
A phantom loop connection via an earth ground is often much higher resistance than the metallic neutral connector. Even if an earth ground is safe, metering at the substation can alert the operator to tampering. Substations, inter-ties, and transformers normally have a high-accuracy meter for the area served. Power companies normally investigate discrepancies between the total billed and the total generated, in order to find and fix power distribution problems. These investigations are an effective method to discover tampering.
Power thefts in the U.S. are often connected with indoor marijuana grow operations. Narcotics detectives associate abnormally high power usage with the lighting such operations require.[35] Indoor marijuana growers aware of this are particularly motivated to steal electricity
simply to conceal their usage of it.
9. find the material required for 1pase overhead service line of a house located 10 mts away from pole, with foll loads : Lighting = 300 Watts; Heating= 2500 Watts Assume safety
factor=2. (June-2014)
power supply and another for domestic supply.
Assuming efficiency of motor 85% and power factor 0.8, we have full- load currents of motors as 1, 000 0.75 , 415 , 3 1.54 3 415 0.85 0.8 3.7 1, 000 3.7 , 415 , 3 7.57 3 415 0.85 0.8 5.5 1, 000 5.5 , 415 , 3 11.25 3 415 0.85 0.8 I I For kW V phase motor A I For kW V phase motor A I For
kW V phase motor A × = − = = × × Total full-load current for these motors = 1.54 + 1.54 +
Department of EEE, SJBIT Page 28 Hence cable from meter board to main board shall have a current carrying capacity 1.6 times of
full-Hence 3-core, 1100 V grade PVC insulated, 16 mm2 aluminium conductor having current carrying capacity of 38 A shall be used from meter board to main board.
The current rating of main switch in the starting current of one motor of highest rating plus full load curr
1. Length of 31 mm exible conduit from meter board to main switch board and from main switch board to distribution board, including wastage = 1.0 m
2. Length of 25 mm exible conduit from HG conduit to motor terminals fan machine no. 4, including wastage = 1.0 m
3. Length of 19 mm exible conduit required for connecting motor switches to motor starters for all the four machines and HG conduit to motor terminals for machines no. 1,
Unit-5
1. List out important consideration regarding motor installations. (Jan-2016) IMPORTANT CONSIDERATION REGARDING MOTOR INSTALLATION WIRING
These are as detailed below.
1. All equipment used in power wiring shall be of iron clad construction and wiring shall be of the armoured cable or conduit type (IE Rule 51).
2. Woodwork shall not be used for mounting of switchgear. 3. Looping of conductors and use of the joints shall not be done,
4. The length of exible conduit used for connections between the terminal boxes of motors andstarters, switches and motors shall not exceed 1.25 metres.
5. Every motor, regardless of its size shall be provided with a switch fuse placed near it. [IE Rule 50 clause (d)]
6. In addition to switch fuse all motors shall be provided with suitable means for starting and stopping (starters) placed at convenient places. The starters are used to limit the starting current to a desirable value. Direct-on-line starters, star-delta starters, auto transformer-starters (or rotor
Department of EEE, SJBIT Page 29 resistance starters in case of slip-ring induction motor)are used for ac motors of rating up to 0.75 kW, above 0.75 kW and below 11 kW and above 11 kW respectively.
7. The conduit should preferably be laid in covered trenches to facilitate operator movement (safe).
8. Laying of cables must be in separate conduits for separate motors.
2.The Fig. shows the plan of workshop. One 15 HP, 3 phase, 415 y induction motor is installed. Show the key diagram and estimate quantity of material required. (Jan-2016)
Assumptions made:
1. The motor and starter are to be procured through separate contract.
2. Motor disconnect switches and main switches are to be supplied by wiring contractor. 3. All the conduits are to be run exposed on walls.
4. The main switch, motor switch and starter shall be mounted at a height of 1.5 metres from ground level.
5. Two earth wires will be run side by side for earthing the motor, starter and switches. 6. The motor shall be installed on suitable foundation, 0.2 m above the floor level. 7. Motor efficiency 85% and power factor 0.8 (lagging).
Full load current =
Starting current = 1.5 times full-load current = 1.5 X 15.06 = 22.6A. Hence three-core PVC 1100V grade, 6 mm2 aluminium conductor cable of current carrying capacity 24 A may be used. The main switch and motor switch to be used will be 32 A. 415V TPIC switches. As from meter board to main board and main board to motor control board only one 3-core cable is to be run so a HG conduit of size 25mm will be run from meter board to main board and from main board to motor control board. From, motor starter to motor two 3-core cables carried so HG conduit of size 31 mm will be used. Flexible conduit of size 25 mm will be used for connecting motor switch and motor starter and of size 31 mm will be used for connecting heavy gauge conduit to motor.
Department of EEE, SJBIT Page 30 From meter board to main board = 0.3 m
from main board to motor switch (mounted on control board) = 22.5 metres Total = 22.8 metres
Wastage 10% = 2.3 metres
Total = 25.1 metres = 25 metres (say) Length of 31 mm HG Conduit
From motor starter to ground = 1.5 metres Below ground level = 0.2 metres
Along ground up to foundation = 1.0 metres
Up to top of motor foundation = 0.2 + 0.2 = 0.4 metre Total = 1.5 + 0.2 + 1.0 + 0.4 = 3.1 metres
Wastage 10% = 0.31 metre
Total = 3.1 + 0.31 = 3.41 = 3.5 metres (say)
Length of 25 mm exible conduit required for connecting motor switch to motor starter= 0.25m Length of 31 mm exible conduit required for connecting heavy gauge conduit to motor = 1 metre Length of 3 core, 1100 V grade, 1/2.80 mm (6 mm2) aluminium conductor PVC cable
(i) 1 Length from meter board to main board = 0.3 m (ii) 1 Length from main board to motor switch = 22.5 m (iii) l Length from motor switch to motor starter = 0.25 m
(iv) 2 Lengths from motor starter to motor terminal box = 2 (3.2 + l) = 8.2 m Total Length= 31.25 m
Wastage and for connections, 10 96 = 3.13 m Total = 34.38 = 36 metres (say)
Length of Earth Wire According to IE rules, the motor frame, motor switch, motor starter,main switch are to be earthed by means of two separate and distinct connections. Hence two separate earth electrodes will be provided for earthing purpose.
From the table for size for earth wire, for 10 HP motor 8 SWG GI wire will be required as earth wire.
Department of EEE, SJBIT Page 31 = 2 (25 + 3.5 + 0.25 + l) = 59.5 metres = 60 metres (say) or 6 kg.
SL N O
Description of Material With Full Specifications Quantity Required Rem a rks Qu ant ity Unit 1 2 3 4 5 6 7 8 9
32A, 415V, TPIC rewirable type switch fuse unit IC boards complete with locking arrangement etc.
Heavy gauge (HG) 16 SWG conduit i) 31 mm
ii) 25 mm Flexible conduit i) 31 mm
ii) 25 mm
3 core, 1100 V, grade 6 mm2 aluminium conductor PVC cable Conduit bends i) 31mm ii) 26 mm Conduit Saddles i) 31 mm ii) 25 mm Lock nuts i) 31 mm 2 1 1 3.5 25 1 0.2 5 35 2 6 4 25 6 2 nos no do m do do do do nos do do do do do
Department of EEE, SJBIT Page 32 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 ii) 25 mm
Flexible pipe coupling complete with locknuts i) 31 mm
ii) 25 mm
Wooden bushings i) 31 mm
ii) 25 mm
Teak wood gutties 8 SWG GI wire Shock treatment chart Iron screws 32 mm Caution plates
GI thimbles with nuts and bolts Earth wire clips
GI Plate 600 mm x 600 mm x 60 mm GI pipe 19 mm diameter
GI pipe 12 mm diameter 8 SWG GI wire
CI Cover 30 cm x 30 cm
GI bolts nuts, check nuts with washers Funnel with wire mesh
Charcoal Salt
Soldering material
Civil Engineering works (Digging, Finishing, Foundation of motor, covering up, building up of CI frame, cement etc.)
Department of EEE, SJBIT Page 33 3. Explain determination of input power, current to motors & rating of cables.(Jun -2015) Step 1: Data Gathering
The first step is to collate the relevant information that is required to perform the sizing calculation. Typically, you will need to obtain the following data:
Load Details
The characteristics of the load that the cable will supply, which includes: Load type: motor or feeder
Three phase, single phase or DC System / source voltage
Full load current (A) - or calculate this if the load is defined in terms of power (kW) Full load power factor (pu)
Locked rotor or load starting current (A) Starting power factor (pu)
Distance / length of cable run from source to load - this length should be as close as possible to the actual route of the cable and include enough contingency for vertical drops / rises and termination of the cable tails
Cable Construction
The basic characteristics of the cable's physical construction, which includes: Conductor material - normally copper or aluminium
Conductor shape - e.g. circular or shaped Conductor type - e.g. stranded or solid
Department of EEE, SJBIT Page 34 Insulation type - e.g. PVC, XLPE, EPR
Number of cores - single core or multicore (e.g. 2C, 3C or 4C)
Installation Conditions
How the cable will be installed, which includes: Above ground or underground
Installation / arrangement - e.g. for underground cables, is it directly buried or buried in conduit? for above ground cables, is it installed on cable tray / ladder, against a wall, in air, etc.
Ambient or soil temperature of the installation site
Cable bunching, i.e. the number of cables that are bunched together Cable spacing, i.e. whether cables are installed touching or spaced Soil thermal resistivity (for underground cables)
Depth of laying (for underground cables)
For single core three-phase cables, are the cables installed in trefoil or laid flat? Step 2: Cable Selection Based on Current Rating
Current flowing through a cable generates heat through the resistive losses in the conductors, dielectric losses through the insulation and resistive losses from current flowing through any cable screens / shields and armouring.
The component parts that make up the cable (e.g. conductors, insulation, bedding, sheath, armour, etc) must be capable of withstanding the temperature rise and heat emanating from the cable. The current carrying capacity of a cable is the maximum current that can flow continuously through a cable without damaging the cable's insulation and other components (e.g. bedding, sheath, etc). It is sometimes also referred to as the continuous current rating or ampacity of a cable.
Cables with larger conductor cross-sectional areas (i.e. more copper or aluminium) have lower resistive losses and are able to dissipate the heat better than smaller cables. Therefore a 16 mm2 cable will have a higher current carrying capacity than a 4mm2 cable.
Department of EEE, SJBIT Page 35
Base Current Ratings
Table 1. Example of base current rating table (Excerpt from IEC 60364-5-52) International standards and manufacturers of cables will quote base current ratings of different types of cables in tables such as the one shown on the right. Each of these tables pertain to a specific type of cable construction (e.g. copper conductor, PVC insulated, 0.6/1kV voltage grade, etc) and a base set of installation conditions (e.g. ambient temperature, installation method, etc). It is important to note that the current ratings are only valid for the quoted types of cables and base installation conditions.
In the absence of any guidance, the following reference based current ratings may be used
4. A 10 HP (metric), 415 v, 3 ,50Hz squirrel cage IM is to be installed in a flour mill, the plan of which is shown in fi Shows the wiring dia of the layout and estimate the quantity of
materials required and its cost. (Jun -2015)
We depend on electricity to light our homes, turn on our television sets, and even cook our meals. When the power goes out because of a storm, a short circuit, or another problem in the electrical circuit, understanding what the basic components of an electrical system is a must.