Download Unit – 3

Unit – 3
1. List out the design consideration of electrical installation in commercial building.
(Jan-2016)
Solution: - 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
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 underground
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)
Solution: - 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 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)
Solution: 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:
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 draw a current of 17.4A. So 30A DP isolator can serve as incomers to these DB's.
The average load & DB's 2 and 4 draws a current of 10.3A. So 15A DP isolator can serve as
incomers. Since 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 will 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 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. 30A TPN/DP switch = No. 4 SWG aluminium wire.
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)
Solution: -: 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)
Solution: 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.
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 4.1mV (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.