SRI RAMAKRISHNA ENGINEERING COLLEGE

[Educational Service: SNR Sons Charitable Trust]

[Autonomous Institution, Reaccredited by NAAC with ‘A+’ Grade]
[Approved by AICTE and Permanently Affiliated to Anna University, Chennai]
[ISO 9001:2015 Certified and all eligible programmes Accredited by NBA]
VATTAMALAIPALAYAM, N.G.G.O. COLONY POST, COIMBATORE – 641 022.

Department of Electrical and Electronics Engineering

Internal Test – II– Answer Key
Date 29.04.2025 Department EEE
Semester VI Class/ Section III year A& B
Duration 2:00 Hours Maximum marks 50
Course Code &Title: 20EE2E35- ELECTRICAL ESTIMATION AND COSTING
Course Outcomes Addressed:
CO3: Estimate electrical installation estimation and costing for commercial and small industries.
CO4: Estimation cost of overhead transmission & distribution system equipment and
components.

QUESTIONS

PART – A (Answer All Questions) (10*1 =10 Marks)

1. c) Three-phase supply

2. b) Near the main entrance or utility room

3. c) Load demand and safety compliance

4. b) Dead-end clamp

5. c) Maintain mechanical stability of poles

6. d) Water supply provision

7. c) Current-carrying capacity and mechanical strength

8. Higher than residential

9. Power factor correction in commercial buildings is used to

10. Two

PART – B (Answer All Questions) (5*2 =10 Marks)

11. Define a commercial electrical installation and differentiate it from a residential installation.
(Reason: 2 Marks)
A commercial electrical installation refers to the electrical wiring, systems, and equipment
installed in buildings used for business purposes, such as offices, shops, malls, and restaurants.

Difference:

Feature Residential Installation Commercial Installation

Load Type Mostly lighting, fans, home Lighting, HVAC, elevators, office
 Feature Residential Installation Commercial Installation
appliances equipment
Load Demand Low to moderate High and continuous
Wiring Standards Basic (single-phase mostly) Advanced (3-phase preferred)
Control and Safety Extensive (MCBs, RCCBs, surge
Minimal
Gear protectors)
12. List two key design considerations for the electrical installation system in a commercial building.
How does load calculation affect the selection of service connection size and nature of supply?
(Reason: 2 Marks)

Design considerations include:

1. Estimating total connected and demand load (lighting, HVAC, equipment).

2. Ensuring future expansion and safety compliance (IS codes, energy efficiency).

Impact of load calculation:

Accurate load calculation determines the correct size of service cable, main switchgear rating,
and whether a single-phase or three-phase supply is needed for efficient and safe operation.
13. What factors influence the selection of cable and bus bar size in an electrical installation, and
how does it affect the positioning of switchboards and distribution boards? (Reason: 2 Marks)

Cable and bus bar size selection depends on current-carrying capacity, voltage drop limits,
short-circuit rating, and future load expansion. Improper sizing can lead to overheating or
energy loss.

Positioning of switchboards and DBs must ensure easy access, adequate ventilation, and
compliance with safety clearances, improving maintainability and operational safety.
14. Explain the importance of determining the correct conductor size in overhead transmission lines
and list two key cost components involved in their erection. (Reason: 2 Marks)

The correct conductor size ensures adequate current-carrying capacity, minimal voltage
drop, and mechanical strength to withstand environmental stress.

Two key cost components are:

1. Erection of supports (poles/towers)

2. Fixing of cross arms and insulators

15. Explain the proper jointing and earthing essential in overhead distribution lines, and what is one
key test performed before commissioning the line? (Reason: -2 Mark)

Proper jointing ensures electrical continuity and mechanical strength, while earthing provides
fault protection and lightning safety.

One key test before commissioning is the insulation resistance test to check for leakage or
faults in the line.
PART-C (3*10=30 Marks)

16 Compulsory Question:
Create a load calculation and service connection proposal for a mixed-use building (residential +
commercial) and justify your choices based on load profiles and usage patterns.
(Reason: Explanation- 10Marks)

Assumptions for the Scenario

 Building Type: G+4 (Ground + 4 floors)

 Use:
o Ground Floor: Commercial (shops/offices)

o First to Fourth Floors: Residential (4 flats per floor = 16 flats total)

 Location: Urban area with standard utility supply (per Indian norms)

Step 1: Load Estimation

A. Residential Load (Per Flat)

Load Type Quantity Wattage Total Load (W)

Lights & Fans 10 points 60 W 600 W
Refrigerator 1 300 W 300 W
TV + Set-top box 1 150 W 150 W
Washing Machine 1 750 W 750 W
Kitchen Loads 1 1000 W 1000 W
Miscellaneous - 200 W 200 W
Total per flat 3000 W

Total for 16 flats = 16 × 3 kW = 48 kW

Apply diversity factor (0.6) → 48 × 0.6 = 28.8 kW

B. Commercial Load (Ground Floor - 5 Shops)

Load Type Quantity Wattage Total Load (W)

Lights & Fans 10/shop 60 W 600 W/shop
AC 1/shop 2000 W 2000 W/shop
Computers/Devices 2/shop 300 W 600 W/shop
Signage, etc. - 500 W 500 W/shop
Total/shop 3700 W

Total for 5 shops = 5 × 3.7 kW = 18.5 kW

Apply diversity factor (0.8) → 18.5 × 0.8 = 14.8 kW

Step 2: Total Estimated Load

 Residential: 28.8 kW
 Commercial: 14.8 kW
 Total Load: 43.6 kW
Add 10% future provision: 43.6 × 1.1 = 47.96 kW → ~48 kW

Step 3: Service Connection Proposal

1. Type of Supply

 Three-phase, 4-wire, 415V, 50Hz from utility provider (TANGEDCO or equivalent)

 Reason: Load exceeds single-phase capacity (above 5-10 kW), balanced three-phase
distribution is necessary

2. Sanctioned Load

 Apply for 60 kW (with provision for EV charging or solar integration)

3. Main Switchgear & Cable Sizing

 Main Circuit Breaker: MCCB 125A, 4P, 25kA breaking capacity

 Cable from Service to Panel: 3.5-core 70 sq.mm XLPE aluminium armored cable

4. Sub-Metering

 Separate meters for:

o Each residential flat (16 meters)

o Each commercial unit (5 meters)

o Common services (lift, lighting, pump, etc.)

Justification Based on Lad Profile & Usage

Criteria Justification
Residential loads are intermittent and predictable (lighting, appliances),
Load Nature while commercial loads involve HVAC, signage, computers — higher
power density per area
Residential diversity is higher due to staggered use; commercial spaces
Diversity Factor
often operate simultaneously (lower diversity)
Three-phase is essential for balanced load sharing and handling peak
Type of Supply
demands
Cable and
Sized for peak load + 25% margin, following IS 732 and IErules
Switchgear
Metering and
Segregated billing promotes accountability and energy management
Billing
Future
Allows EV charging, solar tie-in, or shop expansion without overloading
Provisioning

Any Two Questions

17. Design an electrical distribution layout for a medium-sized commercial building (connected
load ~100 kW) by deciding the appropriate size of cables, busbars, and busbar chambers. Justify
the mounting arrangement and positioning of switchboards, distribution boards, and main
switches based on safety, maintenance, and future expansion considerations. (Reason:
Explanation- 10 Marks)
 Key Impacts

1. Overloading of Electrical Infrastructure

 Undersized cables, transformers, and circuit breakers may get overloaded, leading to
overheating, insulation damage, or fire hazards.
 Premature aging or failure of equipment.

2. Frequent Tripping and Power Disruptions

 Load demand exceeding breaker or panel capacity causes nuisance tripping, affecting
business continuity.
 Sensitive devices (servers, POS systems) may malfunction due to inconsistent power.

3. Inadequate Provision for Future Expansion

 If loads are underestimated, there’s no room for scalability, requiring costly and
disruptive infrastructure upgrades.

4. Non-Compliance with Standards

 Violates IS 732 (Code of practice for electrical wiring installations), National Building
Code, or local electricity supply rules.
 Could lead to penalties or rejection of electrical safety audits.

5. Increased Operational Costs

 Emergency upgrades post-installation are costlier.

 Frequent repairs and replacements inflate maintenance budgets.

6. Safety Hazards

 Risk of fire, electric shock, or equipment damage increases in overstressed systems.

18. Design an overhead transmission line for a rural feeder covering 10 km. Evaluate and
determine the appropriate size of the conductor, considering current-carrying capacity and
voltage drop. Estimate the cost components involved in the erection of supports, setting of
stays, and fixing of cross arms, insulators, and conductors. Justify your design and cost
choices based on safety, economy, and efficiency. (Reason: Explanation- 10Marks)

1. Comprehensive Load Survey and Diversity Application

 Conduct a detailed load survey of equipment, usage schedules, and peak demands.
 Use appropriate diversity factors based on commercial activity type (retail, office,
hospitality).

2. Load Forecasting for Future Growth

 Include 10–25% buffer for future expansion, additional equipment, or unforeseen

needs.
 Use modular design practices to support add-ons without overhauls.
3. Simulation and Demand Estimation Tools

 Use software-based load estimation tools (like ETAP, Dialux, or Revit MEP) for
accuracy.
 Simulate load variations based on operational hours and seasons.

4. Standard Adherence and Peer Review

 Follow national/international standards (IS, IEC, NEC).

 Conduct peer review or third-party verification of electrical design documents.

5. Segregation of Loads

 Classify loads into essential, non-essential, and critical to prioritize power

distribution and backup sizing.
 Design separate circuits for high-power or sensitive equipment.

6. Installation of Smart Monitoring Systems

 Enable real-time load monitoring via smart energy meters, which help identify early
signs of overload and energy misuse.

19. Develop a comprehensive plan for the repair, jointing, earthing, and commissioning of a
newly installed 11 kV overhead distribution line. Analyze the role of dead-end clamps and
jumper connections in mechanical and electrical integrity. Evaluate the safety and operational
effectiveness of your plan based on standard testing procedures. (Reason:Explanation-
10Marks)

1. Load and Design Parameters

 Total connected load: 100 kW

 Supply voltage: 415 V, 3-phase, 4-wire system
 Estimated full load current:

I=P3⋅V⋅cos⁡ϕ=100,0003⋅415⋅0.9≈Assume power factor = 0.9

 Allow 25% future expansion → Design current = 155 × 1.25 ≈ 195 A

2. Cable Sizing

 From main LT panel to DBs:

o Use 3.5-core XLPE aluminum armored cable

o For 200 A: 70 sq.mm cable (considering permissible current rating and

derating factors)
o Confirm using IS 3961 for conductor ampacity and installation conditions

3. Busbar Sizing

 Material: Copper (preferred for compact size and high conductivity)

  Current Rating: ~200 A

Busbar Type Suggested Size Justification

Phase bars 25 mm × 6 mm Cu Current density ~1.5 A/mm²
Neutral bar 25 mm × 5 mm Cu Lower current, but sized for unbalanced loads
Earth bar 25 mm × 3 mm Cu Follows IS 3043 grounding standards
 Use air-insulated chamber with PVC sleeving

 Ensure clearances per IS 8623 and maintain temperature rise < 40°C

4. Busbar Chamber Design

 Rated for 200 A with appropriate IP protection (IP42/IP54)

 Provide natural ventilation louvers or forced cooling
 Isolate using shrouds and interlocks for maintenance safety

5. Mounting Arrangement & Positioning

Component Mounting Arrangement Positioning Guidelines

Wall-mounted or floor- Near the service entrance; accessible but
Main Switchboard
standing secured from public
Distribution Boards Recessed or surface- Centrally on each floor for load balance and
(DBs) mounted cable economy
Floor-mounted, inside
Busbar Chamber Near the transformer/utility intake
main panel
Cables Cable trays or conduits Segregate power and communication cables

6. Justification

 Safety:
o Sufficient clearance (min. 1 m front), shrouded busbars, IP-rated enclosures

o DBs away from wet areas, switchgear mounted at 1.2–1.5 m height

 Maintenance:
o Isolated zones for switchgear and cable entry

o Labels, mimic diagrams, and accessible MCCBs simplify troubleshooting

 Future Expansion:
o Extra space in panel for additional breakers

o Oversized busbars and cable trays for added loads

 Compliance:
o IS 732 (Electrical Wiring), IS 8623 (LT Panels), NBC India, IE Rules
Course Instructor Programme Assessment Committee HoD–EEE
Dr.P.Sebastain Vindro Jude Dr. Dr.R.Krishnakumar Dr.S.Allirani