⚡ Electrical Design & Calculations
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.
🔷 Design Data (Assumed)
- Total Connected Load = 500 kW (Ref at last of the blog)
- Demand Factor = 0.7
- Power Factor (PF) = 0.9
- System Voltage = 400 V (3-phase)
🔷 Step 1: Load Estimation
Maximum Demand Calculation
Maximum Demand = Connected Load × Demand Factor
= 500 × 0.7
= 350 kW
Apparent Power (kVA)
S = P / PF
= 350 / 0.9
= 389 kVA
✔ Maximum Demand = 350 kW
✔ Required Capacity = 389 kVA
👉 Always consider future expansion (10–20%) in design.
🔷 Step 2: Transformer Sizing
The transformer should be selected above the calculated load.
Required Capacity = 389 kVA
Standard Transformer Selection
✔ Selected Transformer = 500 kVA
👉 Reason:
- Standard rating
- Provides margin for future load
- Prevents overloading
🔷 Step 3: Cable Sizing
Load Current Calculation
Ib = P / (√3 × V × PF)
= 350,000 / (1.732 × 400 × 0.9)
≈ 560 A
✔ Design Current ≈ 560 A
🔹 Cable Selection (Copper vs Aluminium)
✔ Option 1: Copper Cable (Cu)
✔ Selected Cable:
- 3.5 Core × 400 mm² Cu XLPE Cable
👉 Typical Current Capacity:
- ~600–650 A (depending on installation)
👉 Advantages:
- Higher conductivity
- Smaller size
- Better mechanical strength
- Lower voltage drop
✔ Option 2: Aluminium Cable (Al)
✔ Selected Cable:
- 3.5 Core × 630 mm² Al XLPE Cable
👉 Typical Current Capacity:
- ~550–600 A
👉 Advantages:
- Lower cost
- Lightweight
👉 Disadvantages:
- Larger size required
- Higher voltage drop
- Proper termination required
🔹 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)
🔷 Step 4: 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
Breaking Capacity
✔ Icu Rating = 50 kA (Typical value)
👉 Must be selected based on fault level calculation.
🔷 Step 5: Busbar & Panel Design
🔹 Busbar Sizing
Busbar size is calculated using current density.
✔ Current Density Values
| Material | Current Density |
|---|---|
| Copper (Cu) | 1.2 – 1.6 A/mm² |
| Aluminium (Al) | 0.8 – 1.2 A/mm² |
👉 Aluminium requires larger size due to lower conductivity.
🔹 Busbar Area Calculation
Design Current = 700 A
For Copper:
Area = 700 / 1.25 ≈ 560 mm²
For Aluminium:
Area = 700 / 1.0 ≈ 700 mm²
🔹 Busbar Selection
✔ Option 1: Copper Busbar
✔ Selected:
- 1 × (100 × 10 mm) Cu Busbar = 1000 mm²
👉 Advantages:
- Compact size
- High conductivity
- Better heat dissipation
✔ Option 2: Aluminium Busbar
✔ Selected:
- 1 × (100 × 10 mm) Al Busbar = 1000 mm²
👉 Practical note:
- Aluminium busbars are usually oversized for safety
- Often 1.5 times copper area is preferred
🔹 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 |
🔹 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
🔹 Important Design Considerations
✔ Check temperature rise
✔ Provide proper spacing between phases
✔ Ensure proper insulation
✔ Use proper supports and clamps
✔ Maintain clearance inside panel
🔥 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)
🔥Panel Rating
✔ Panel Rating = 800 A to 1000 A
✔ Form of Separation:
🔷 Basic Single Line Diagram (SLD)
Transformer (500 kVA)
|
ACB (800 A)
|
Busbar
/ | \
Feeder Feeder Feeder
| | |
Load Motor DB
👉 SLD helps in understanding system layout.
🔷 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
🔷 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.
🔷 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
🔷 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.
🔷 Total Connected Load Calculation (500 kW)
Assume a building with the following loads:
🔹 1. Lighting Load
- Total lights = 1000 Nos
- Each light = 40 W
Lighting Load = 1000 × 40
= 40,000 W = 40 kW
🔹 2. Socket Load
- Total sockets = 200 Nos
- Each socket = 200 W
Socket Load = 200 × 200
= 40,000 W = 40 kW
🔹 3. Air Conditioning Load
- Total AC units = 50 Nos
- Each AC = 2 kW
AC Load = 50 × 2
= 100 kW
🔹 4. Motor Load (Pumps, Lifts, etc.)
- Total motors = 10 Nos
- Each motor = 10 kW
Motor Load = 10 × 10
= 100 kW
🔹 5. Miscellaneous Load
Includes:
- Office equipment
- Kitchen equipment
- Spare loads
Misc Load = 50 kW
🔹 6. Future Provision
Future Load = 50 kW
🔷 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
🔷 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
0 Comments