Electrical Engineering Part-3

 

Electrical Engineering

Part-3

🔷 Electrical Load Calculation & Estimation 

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🔷 Introduction

Load calculation and estimation are crucial steps in designing any electrical system. Proper calculation ensures that the system is safe, efficient, and cost-effective.

For engineers involved in construction, MEP, and tendering, understanding load calculation helps in preparing BOQs, selecting equipment, and avoiding overloading.

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🔷 What is Electrical Load?

Electrical load is the amount of power consumed by electrical devices such as lights, fans, motors, air conditioners, and equipment.

👉 Unit:

Watt (W)

Kilowatt (kW)

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🔷 Types of Loads

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🔸 1. Lighting Load

Includes lights, lamps, and fixtures
Usually low power

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🔸 2. Power Load

Includes sockets, appliances
Moderate load

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🔸 3. Motor Load

Pumps, lifts, HVAC
High starting current

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🔷 Basic Load Calculation

👉 Formula:

Total Load (kW) = Sum of all connected loads

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🔸 Example:

Equipment	Quantity	Watt	Total
Lights	10	20W	200W
Fans	5	70W	350W
AC	2	1500W	3000W

👉 Total Load:

3550 W = 3.55 kW

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🔷 Demand Factor

Not all equipment operates at the same time.

👉 Formula:

Maximum Demand = Connected Load × Demand Factor

👉 Typical values:

Residential: 0.5 – 0.7

Commercial: 0.7 – 0.9

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🔷 Diversity Factor

Different loads operate at different times.

👉 Formula:

Diversity Factor = Sum of Individual Loads / Maximum Demand

✔ Helps in reducing system size

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🔷 Connected Load vs Maximum Demand

Type	Meaning
Connected Load	Total installed load
Maximum Demand	Actual load at a time

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🔷 Load per Area Method

Used for quick estimation.

👉 Example:

Residential: 30–50 W/sq.m

Office: 50–100 W/sq.m

Industrial: 100–200 W/sq.m

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🔷 Electrical Estimation

Electrical estimation is the process of calculating material, cost, and labor required for installation.

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🔷 Components of Estimation

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🔸 1. Material Cost

Cables
Conduits
Switches
DB panels

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🔸 2. Labor Cost

Installation
Testing
Commissioning

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🔸 3. Equipment Cost

Transformers
Panels
Generators

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🔷 BOQ (Bill of Quantity)

BOQ is a document listing all items with quantity and rate.

👉 Example:

Item	Quantity	Unit
Cable 4 sq.mm	200	Meter
Switch	50	Nos
MCB	20	Nos

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🔷 Cable Selection Based on Load

👉 Steps:

1. Calculate load

2. Find current

3. Select cable size

4. Check voltage drop

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🔷 Current Calculation

👉 Formula:

I = P / (√3 × V × PF)

👉 Example:
For 10 kW load:

I = 10000 / (1.732 × 415 × 0.8)
≈ 17.4 A

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🔷 Selection of MCB/MCCB

👉 Choose rating:

125% of load current

✔ Ensures safe operation

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🔷 Transformer Sizing

👉 Formula:

kVA = kW / PF

👉 Example:

10 kW / 0.8 = 12.5 kVA

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🔷 Practical Site Tips

✔ Always consider future load
✔ Provide spare capacity
✔ Balance load in 3-phase
✔ Avoid undersizing

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🔷 Common Mistakes

❌ Ignoring demand factor
❌ Wrong load assumption
❌ No future provision
❌ Incorrect cable sizing

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🔷 Conclusion

Load calculation and estimation are essential for designing safe and economical electrical systems. Accurate estimation helps in proper equipment selection, cost control, and smooth project execution.

🔷 Wiring System & Cable Sizing : Practical Guide 

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🔷 Introduction

In any electrical installation, the wiring system and proper cable selection are critical for safety, efficiency, and long-term performance. Incorrect wiring or undersized cables can lead to overheating, voltage drop, and even fire hazards.

For site engineers, understanding wiring methods and cable sizing is essential for execution, inspection, and billing work.

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🔷 What is Electrical Wiring?

Electrical wiring is the system of conductors used to carry electrical power from the source to various loads such as lighting, sockets, motors, and equipment.

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🔷 Types of Wiring Systems

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🔸 1. Conduit Wiring

Most commonly used system in modern buildings.

👉 Types:

Surface conduit

Concealed conduit

👉 Features:
✔ Safe and durable
✔ Neat appearance
✔ Suitable for residential and commercial buildings

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🔸 2. Casing and Capping Wiring

Used in old installations
Wooden casing used

👉 Not recommended for modern projects

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🔸 3. Batten Wiring

Cables fixed on wooden batten
Used in temporary installations

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🔸 4. Cable Tray System

Used in industrial and commercial projects.

👉 Types:

Ladder type

Perforated type

👉 Used for:

Power cables

Control cables

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🔷 Types of Cables

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🔸 1. Single Core Cable

Used for internal wiring

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🔸 2. Multi-Core Cable

Used for heavy loads

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🔸 3. Armoured Cable

Has protective layer
Used underground

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🔸 4. Flexible Cable

Used for appliances

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🔷 Cable Size & Units

Cable size is expressed in square millimeters (sq.mm).

👉 Example:

1.5 sq.mm, 2.5 sq.mm, 4 sq.mm, 10 sq.mm

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🔷 Factors Affecting Cable Sizing

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🔸 1. Load Current

Higher load requires bigger cable.

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🔸 2. Length of Cable

Longer length increases voltage drop.

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🔸 3. Voltage Drop

Voltage should not drop beyond permissible limit.

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🔸 4. Installation Method

In conduit
Underground
On cable tray

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🔸 5. Ambient Temperature

Higher temperature reduces current capacity.

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🔷 Cable Sizing Formula

👉 Basic formula:

I = P / (V × PF)

Where:

I = Current, P = Power, V = Voltage, PF = Power factor

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🔷 Simple Cable Selection Chart

Load	Cable Size
Lighting	1.5 sq.mm
Socket	2.5 sq.mm
AC	4 sq.mm
Motor	6 sq.mm & above

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🔷 Voltage Drop Calculation

Voltage drop must be within limit:

👉 Acceptable limit:

Lighting: 3%

Power: 5%

👉 Formula:

VD = (2 × L × I × R) / 1000

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🔷 Earthing in Wiring

Proper earthing is essential for safety.

👉 Always provide:
✔ Separate earth wire
✔ Proper earthing pit
✔ Continuity of earthing

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🔷 Color Coding of Wires

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🔸 Single Phase

Phase → Red
Neutral → Black
Earth → Green

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🔸 Three Phase

R → Red
Y → Yellow
B → Blue
Neutral → Black

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🔷 Distribution of Load

✔ Separate circuits for lighting and power
✔ Balance load in three-phase system
✔ Avoid overloading

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🔷 Practical Site Tips

✔ Use proper conduit size
✔ Avoid sharp bends
✔ Use proper lugs and ferrules
✔ Label all cables
✔ Follow drawing and SLD

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🔷 Common Mistakes

❌ Undersized cable
❌ No earthing
❌ Mixing lighting and power circuits
❌ Loose connections

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🔷 Safety Guidelines

✔ Use ISI marked cables
✔ Provide MCB protection
✔ Avoid joints in cable
✔ Ensure insulation integrity

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🔷 Conclusion

A well-designed wiring system and correct cable sizing ensure safety, efficiency, and reliability of electrical installations. For engineers, these are critical aspects of execution and maintenance in real-world projects.

🔷 Electrical Protection (MCB, MCCB, ELCB, Earthing)

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🔷 Introduction

In any electrical system, safety is the top priority. Faults such as short circuits, overloads, and leakage currents can damage equipment and pose serious risks to human life.

Electrical protection devices are used to detect these faults and disconnect the supply automatically, ensuring safe and reliable operation.

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🔷 What is Electrical Protection?

Electrical protection is the system that protects electrical circuits, equipment, and users from faults by interrupting the flow of current when abnormal conditions occur.

👉 Main objectives:

Protect human life

Prevent equipment damage

Ensure system stability

Avoid fire hazards

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🔷 Types of Electrical Faults

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🔸 1. Overload Fault

Occurs when current exceeds the rated capacity of the system.

👉 Causes:

Excess load

Undersized cables

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🔸 2. Short Circuit

Occurs when phase and neutral (or phase-phase) come into direct contact.

👉 Result:

Very high current

Instant damage

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🔸 3. Earth Fault

Occurs when current flows through the ground due to insulation failure.

👉 Dangerous for human safety

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🔷 Protection Devices

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🔸 1. MCB (Miniature Circuit Breaker)

MCB is a small protective device used in residential and light commercial installations.

👉 Functions:

Protect against overload

Protect against short circuit

👉 Features:

✔ Automatic operation

✔ Easy to reset

✔ Compact size

👉 Common ratings: 6A, 10A, 16A, 32A

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🔸 2. MCCB (Moulded Case Circuit Breaker)

MCCB is used in industrial and large electrical systems.

👉 Functions:

Overload protection

Short circuit protection

Adjustable settings

👉 Features:

✔ Higher current capacity

✔ Adjustable trip settings

✔ Suitable for heavy loads

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🔸 3. ELCB / RCCB (Earth Leakage Protection)

These devices protect against leakage current.

👉 Function:

Detects leakage current

Trips the circuit immediately

👉 Important for:

✔ Human safety

✔ Preventing electric shock

👉 Typical sensitivity: 30 mA (for human protection)

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🔷 Difference Between MCB, MCCB, RCCB

Feature	MCB	MCCB	RCCB
Use	Domestic	Industrial	Safety
Protection	Overload/ Short Circuit	Same/ Adjustable	Earth Leakage
Capacity	Low	High	Medium
Trip Setting	Fixed	Adjustable	Fixed

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🔷 Fuse vs Circuit Breaker

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🔸 Fuse

Melts when current exceeds limit

Needs replacement

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🔸 Circuit Breaker

Trips automatically

Can be reset

👉 Circuit breakers are preferred in modern systems

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🔷 Earthing System

Earthing is one of the most important safety measures in electrical installations.

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🔸 What is Earthing?

Earthing is the process of connecting electrical equipment to the ground to safely discharge fault current.

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🔸 Purpose of Earthing

✔ Protect human life

✔ Prevent electric shock

✔ Stabilize voltage

✔ Protect equipment

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🔸 Types of Earthing

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1. Plate Earthing

Uses metal plate buried in ground

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2. Pipe Earthing

Most common method

Uses GI pipe

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3. Strip Earthing

Used in substations

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🔷 Distribution Board (DB)

A Distribution Board is used to distribute electrical power to different circuits.

👉 Contains:

MCB

RCCB

Busbars

👉 Types:

Single phase DB

Three phase DB

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🔷 Practical Installation Tips

As a site engineer, always follow:

✔ Proper selection of MCB/MCCB rating

✔ Separate circuits for lighting and power

✔ Proper earthing resistance (<1 ohm preferred)

✔ Use RCCB for safety

✔ Label all circuits

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🔷 Common Mistakes

❌ Using wrong MCB rating

❌ No earthing connection

❌ Overloading circuits

❌ Ignoring leakage protection

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🔷 Maintenance of Protection System

✔ Check tripping operation regularly

✔ Inspect connections

✔ Test RCCB monthly

✔ Maintain earthing system

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🔷 Conclusion

Electrical protection systems are essential for ensuring safety and reliability. Devices like MCB, MCCB, and RCCB protect against faults, while proper earthing prevents electric shocks.

A well-designed protection system reduces risks and ensures smooth operation of electrical installations.