Electrical Design Part -1

⚡ Electrical Design ⚡

Part -1

🔷 Introduction

Electrical design is a critical part of any project, ensuring safe, reliable, and efficient power distribution. Proper calculation of load, transformer capacity, cable sizing, protection devices, and panel design is essential for successful project execution.

This article explains a practical example of electrical design calculation in a simple step-by-step method suitable for engineers, students, and site professionals.

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🔷 Design Data (Assumed)

• Total Connected Load = 500 kW (Detail break-up at last)
• Demand Factor = 0.7
• Power Factor (PF) = 0.9
• System Voltage = 400 V (3-phase) { 3 phase voltage in India: 400V ±10% → 360V to 440V}
• Single-phase voltage: 230V ±10% → 207V to 253V

• 1. Line-to-Phase (Neutral) Voltage

  $$V_{ph}=\frac{V_{LL}}{\sqrt{3}}$$

  • For $V_{LL}=400\text{V}$:

  $$V_{ph}=\frac{400}{\sqrt{3}}\approx 230\text{V}$$

• 2. Power in a 3‑Phase System

• Active Power (Real Power):
• $$P=\sqrt{3}\cdot V_{LL}\cdot I_L\cdot \cos \varphi$$
• Reactive Power:
• $$Q=\sqrt{3}\cdot V_{LL}\cdot I_L\cdot \sin \varphi$$
• Apparent Power:
• $$S=\sqrt{3}\cdot V_{LL}\cdot I_L$$

  Where:

  • $V_{LL}$ = Line-to-line voltage (400 V here)

  • $I_L$ = Line current

  • $\cos \varphi$ = Power factor

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🔷 Load Estimation

Maximum Demand = Connected Load × Demand Factor
               = 500 × 0.7
               = 350 KW

(Connected Load =500KW, Demand Factor =0.7)

Apparent Power (KVA)

             ✅ KVA Calculation

$$\text{kVA} = \frac{\text{kW}}{\text{Power Factor}}$$

S = P / PF
  = 350 / 0.9
  = 389 KVA

✔ Maximum Demand = 350 KW
✔ Required Capacity = 389 KVA

👉 Always consider future expansion (10–20%) in design.

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🔷 Transformer Sizing

The transformer should be selected above the calculated load.

Required Capacity = 389 KVA

✅ Transformer Selection Formula

$$\text{<span style="font-size: medium;">Transformer Rating ≥ Load KVA × Safety Factor</span>}$$

(Safety Factor = 1.1 to 1.25)

Example:

389 KVA × 1.25 ≈ 486 KVA

👉 Select Standard Rating = 500 KVA

👉 Reason:

• Standard rating
• Provides margin for future load
• Prevents overloading

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🔷 DG (Diesel Generator) Sizing

DG set is selected to supply essential loads during power failure.

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✅ DG KVA Calculation

$$\text{DG KVA}=\frac{\text{Running Load (kW)}}{PF}$$

OR

$$\text{DG KVA}=\text{Total KVA Load}\times \text{Diversity Factor}$$

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🔹 Example

Assume:

Essential Load = 300 kW
Power Factor = 0.8

$$\text{DG kVA} = \frac{300}{0.8}$$ $$\text{DG kVA} = 375 kVA$$

Add 15–20% margin:

375 × 1.2 = 450 KVA

👉 Select Standard DG = 500 KVA

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🔷 Important DG Considerations

✔ Starting current of motors
✔ Voltage dip during starting
✔ Future expansion
✔ Fuel consumption
✔ Synchronization (if multiple DGs)

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🔷 Cable Sizing

Load Current Calculation

Ib = P / (√3 × V × PF)
   = 350,000 / (1.732 × 400 × 0.9)
   ≈ 560 A

✔ Design Current ≈ 560 A

✅ Select Cable Current Rating (Iz)

As per IS:

$$I_z\ge I_{\mathrm{b}}$$

But in practical design:

$$I_z \geq 1.25 \times I_b$$ $$I_z\ge 700A$$

Now refer to IS 3961 / IS 7098 tables for 1.1 kV XLPE cable.

Option 1: Copper (Cu) XLPE Cable

Typical current capacities (approx., 40°C air, single cable):

Size (Cu XLPE)	Current Capacity
300 mm²	520–560 A
400 mm²	600–650 A
500 mm²	680–730 A

👉 For 700 A requirement:

✔ 400 mm² → Marginal

✔ 500 mm² → Safer

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✅ Apply Derating Factors 

Derating required for:

• Ambient temperature
• Grouping of cables
• Method of laying
• Underground installation

Example:

If derating factor = 0.9

Effective capacity of 400 mm²:

$$650 \times 0.9 = 585 A$$

❌ Not adequate for 700 A

So increase to 500 mm².

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✅ Voltage Drop Check (IS 732 Requirement)

Voltage drop should be within:

• 3% for power circuits
• 5% total system

Voltage Drop:

$$VD=\sqrt{3}\times I\times (R\cos \varphi +X\sin \varphi )\times L$$

If VD exceeds limit → Increase cable size.

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✅ Short Circuit Withstand Check

As per IS 7098:

$$S=\frac{I_{sc}\sqrt{t}}{\mathrm{k}}$$

Where:

• Isc = Short circuit current
• t = Fault clearing time
• k = Material constant

Cable must withstand system fault level.

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🔷 Final Decision 

For 560 A load:

✔ After applying 25% margin → 700 A required
✔ After derating → 400 mm² insufficient
✔ 500 mm² Cu XLPE preferred

OR

✔ Two runs of 300 mm² in parallel

Option 2: Aluminium (Al) XLPE Cable

Aluminium has lower conductivity (~61% of Copper). So larger size is required.

Typical Current Capacity:

Size	Current Capacity
400 mm²	450–500 A
500 mm²	520–560 A
630 mm²	580–620 A
800 mm²	650–700 A

👉 For 700 A requirement:

✔ 630 mm² → Marginal
✔ 800 mm² → Recommended

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✅ Apply Derating Factors (Mandatory as per IS 732)

Derating due to:

• Ambient temperature
• Grouping
• Buried installation
• Soil thermal resistivity

Example: 

If derating factor = 0.9

For 630 mm² Al:

$$600 \times 0.9 = 540 A$$

❌ Not sufficient

For 800 mm² Al:

$$700 \times 0.9 = 630 A$$

✔ May still require parallel cables depending on condition.

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✅ Voltage Drop Check

Voltage Drop:

$$VD=\sqrt{3}\times I\times (R\cos \varphi +X\sin \varphi )\times L$$

Since Aluminium has higher resistance:

✔ Voltage drop will be higher
✔ Cable length becomes critical

If VD > 3% → Increase cable size

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✅Short Circuit Withstand Check (IS 7098)

$$S=\frac{I_{sc}\sqrt{t}}{\mathrm{k}}$$

k value:

• Copper ≈ 143
• Aluminium ≈ 94

👉 Aluminium requires larger cross-section for same fault level.

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🔷 Final Comparison (Practical Selection)

Parameter	Copper	Aluminium
Conductivity	High	Lower
Size Required	Smaller	Larger
Cost	Higher	Lower
Voltage Drop	Lower	Higher
Short Circuit Strength	Better	Lower
Termination	Easy	Needs Bimetallic Lugs

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🔥 Final Engineering Decision (For 560 A Load)

Material	Recommended Size
Copper	500 mm²
Aluminium	800 mm²

OR

✔ Two runs of smaller cables in parallel.

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🔹 Important Checks for Cable Selection

✔ Current carrying capacity
✔ Voltage drop (must be within limits)
✔ Short-circuit withstand capacity
✔ Installation method (tray, duct, buried)
✔ Derating factors (temperature, grouping)

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🔷 Protection Device Selection

Breaker Rating

Breaker Current ≥ 1.25 × Ib
               ≥ 1.25 × 560
               ≈ 700 A

✔ Selected Breaker = 800 A ACB

👉 Provides:

• Safe operation
• Protection against overload & short circuit

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Breaking Capacity

✔ Icu Rating = 50 kA (Typical value)

👉 Must be selected based on fault level calculation.

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🔷 Busbar Design 

🔹 Busbar Sizing

Busbar is the main current-carrying conductor inside LT panels.

It must be designed based on:

✔ Rated Current
✔ Short Circuit Level
✔ Temperature Rise
✔ Material (Copper / Aluminium)

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🔹 Step 1: Determine Design Current

From earlier calculation:

Design Current (after margin) ≈ 700 A

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🔹 Step 2: Select Current Density

Typical values used in Indian practice:

Material	Current Density
Copper (Cu)	1.2 – 1.6 A/mm²
Aluminium (Al)	0.8 – 1.2 A/mm²

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🔹 Step 3: Calculate Required Busbar Area

✔ For Copper

$$Area = \frac{I}{Current\ Density}$$ $$Area = \frac{700}{1.25}$$ $$Area \approx 560\ mm²$$

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✔ For Aluminium

$$Area = \frac{700}{1.0}$$ $$Area = 700\ mm²$$

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🔹 Step 4: Select Standard Busbar Size

✔ Copper Option

Selected:
1 × (100 × 10 mm)

Area = 1000 mm²

✔ Provides safety margin
✔ Lower temperature rise

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✔ Aluminium Option

Selected:
1 × (100 × 10 mm)

Area = 1000 mm²

👉 Practical rule:
Aluminium busbar area ≈ 1.5 times copper for better reliability.

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✅ Short Circuit Withstand Check

Busbar must withstand system fault level.

Short circuit force depends on:

$$F \propto I^2$$

So higher fault current → stronger support required.

✔ Check panel fault level (e.g., 50 kA for 1 sec)
✔ Provide proper busbar supports
✔ Maintain phase spacing

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✅ Busbar Spacing (Important)

Spacing depends on system voltage (415 V LT typical):

✔ Phase-to-phase clearance
✔ Phase-to-earth clearance

Proper spacing prevents flashover.

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🔹 Comparison: Copper vs Aluminium

Parameter	Copper	Aluminium
Conductivity	High	Lower
Size Required	Smaller	Larger
Cost	Higher	Lower
Weight	Heavy	Light
Maintenance	Low	Needs proper joints

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🔹 Practical Site Selection

✔ Use Copper (Cu) for:

• Critical systems
• HT/LT panels
• High reliability projects

✔ Use Aluminium (Al) for:

• Cost-sensitive projects
• Large distribution systems

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🔹 Important Design Considerations

✔ Check temperature rise
✔ Provide proper spacing between phases
✔ Ensure proper insulation
✔ Use proper supports and clamps
✔ Maintain clearance inside panel

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🔥 Professional Tip (Important for Your Work)

Since you are working in tender + execution, always:

✔ Mention both Cu & Al options in BOQ
✔ Compare cost vs performance
✔ Check client specification
✔ Verify termination (Al requires special lugs)

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🔥Panel Design

Panel rating must be:

$$PanelRating\ge BreakerRating$$

Earlier selected breaker = 800 A

✔ Panel Rating = 800 A to 1000 A🔹 Form of Separation

Common types:

• Form 1 – No separation
• Form 2 – Busbar separated
• Form 3 – Busbar + functional unit separation
• Form 4 – Full separation (highest safety)

✔ For commercial & industrial projects → Prefer Form 3 / Form 4

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🔹 Panel Design Considerations

✔ Proper ventilation
✔ Cable alley space
✔ Top / Bottom cable entry
✔ Earthing busbar provided
✔ Proper gland plate thickness
✔ IP rating (IP42 / IP54 depending on location)

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🔷 Basic Single Line Diagram (SLD)

Transformer (500 kVA)
        |
      ACB (800 A)
        |
     Busbar
    /   |   \
 Feeder Feeder Feeder
   |       |      |
 Load    Motor   DB

👉 SLD helps in understanding system layout.

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🔷 Important Design Considerations

✔ Use maximum demand, not connected load
✔ Apply diversity factor
✔ Include future load margin
✔ Consider derating factors
✔ Check voltage drop
✔ Verify short-circuit rating
✔ Ensure proper earthing

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🔷 Common Mistakes in Electrical Design

❌ Using connected load instead of demand load
❌ Undersized cables
❌ Oversized breakers without calculation
❌ Ignoring voltage drop
❌ No fault level calculation

👉 These mistakes can lead to failure and safety risks.

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🔷 Practical Tips for Engineers

For engineers working in site execution and design, always:

✔ Verify load calculation before design
✔ Cross-check cable size with standards
✔ Ensure proper protection coordination
✔ Maintain documentation for approval
✔ Follow safety standards strictly

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🔷 Conclusion

Electrical design requires proper calculation, planning, and verification. A well-designed system ensures safety, efficiency, and long-term reliability of electrical installations.

This step-by-step approach helps engineers perform accurate design and avoid common errors in practical projects.

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🔷 Total Connected Load Calculation (500 kW)

Assume a building with the following loads:

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🔹 1. Lighting Load

• Total lights = 1000 Nos
• Each light = 40 W

Lighting Load = 1000 × 40
              = 40,000 W = 40 kW

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🔹 2. Socket Load

• Total sockets = 200 Nos
• Each socket = 200 W

Socket Load = 200 × 200
            = 40,000 W = 40 kW

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🔹 3. Air Conditioning Load

• Total AC units = 50 Nos
• Each AC = 2 kW

AC Load = 50 × 2
        = 100 kW

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🔹 4. Motor Load (Pumps, Lifts, etc.)

• Total motors = 10 Nos
• Each motor = 10 kW

Motor Load = 10 × 10
           = 100 kW

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🔹 5. Miscellaneous Load

Includes:

• Office equipment
• Kitchen equipment
• Spare loads

Misc Load = 50 kW

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🔹 6. Future Provision

Future Load = 50 kW

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🔷 Total Connected Load Calculation

Total Connected Load
= Lighting + Socket + AC + Motor + Misc + Future
= 40 + 40 + 100 + 100 + 50 + 50
= 380 kW

👉 Round off for design:

✔ Total Connected Load ≈ 400 kW

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🔷 Example to Reach 500 kW

Add additional loads:

• Extra HVAC = 50 kW
• Additional equipment = 50 kW

Final Connected Load = 400 + 100 = 500 kW

✔ Total Connected Load = 500 kW