How to Calculate the Correct Cable Size | Cable Selection Calculation | Step by Step
Introduction
When installing a three-phase motor, selecting the correct cable size is crucial to ensure efficient power delivery and minimize voltage drop. In this guide, we will go through a step-by-step process to determine the appropriate cable size for a 15-horsepower motor located 750 meters from the power supply. The motor has a full load current of 20 amps, an applied voltage of 415 volts, and 80% efficiency.
Considerations for Cable Selection
When selecting a cable, three key factors must be considered:
- Voltage Drop – The voltage lost across the cable length should not exceed acceptable limits.
- Cable Resistance – A higher resistance leads to a higher voltage drop; selecting a larger cable size helps reduce resistance.
- Current Carrying Capacity – The cable must safely handle the full load current of the motor.
Understanding Voltage Drop Calculation
For a three-phase circuit, the voltage drop is calculated using the formula:
Voltage Drop = (√3) × I × R × L
Where:
- √3 = 1.732
- I = Current (Amps)
- R = Resistance of cable (Ohms per km)
- L = Length of cable (km)
Step 1: 3-Core 16 mm² Cable Analysis
- Resistance: 2.5 Ohms/km
- Voltage Drop Calculation: Voltage Drop = 1.732 × 20 × 2.5 × 0.75 = 65V
- Percentage Voltage Drop: (65 / 415) × 100 = 15%
- Conclusion: This voltage drop is too high and not suitable for installation.
Step 2: 3-Core 25 mm² Cable Analysis
- Resistance: 1.5 Ohms/km
- Voltage Drop Calculation: Voltage Drop = 1.732 × 20 × 1.5 × 0.75 = 39V
- Percentage Voltage Drop: (39 / 415) × 100 = 10%
- Conclusion: This is an improvement but still on the higher side.
Step 3: 3-Core 35 mm² Cable Analysis
- Resistance: 0.9 Ohms/km
- Voltage Drop Calculation: Voltage Drop = 1.732 × 20 × 0.9 × 0.75 = 23V
- Percentage Voltage Drop: (23 / 415) × 100 = 5%
- Conclusion: This voltage drop is within acceptable limits.
Final Comparison of Voltage Drops
| Cable Size | Resistance (Ohms/km) | Voltage Drop (V) | Percentage Voltage Drop |
|---|---|---|---|
| 16 mm² | 2.5 | 65 | 15% |
| 25 mm² | 1.5 | 39 | 10% |
| 35 mm² | 0.9 | 23 | 5% |
Conclusion
The recommended cable size for this installation is 35 mm², as it results in the least voltage drop (5%). This ensures that the motor operates efficiently over long distances, reducing power loss and improving motor performance and longevity. By following proper cable selection guidelines, we ensure compliance with voltage drop limits and enhance overall system efficiency.
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