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How do we choose the right cross-section of cable?

The selection of cable cross-section involves determining the appropriate size of a cable based on its current carrying capacity and voltage drop. The most commonly used method for selecting the cable cross-section is the ampacity method.

Ampacity method:

Determine the ampacity of the cable by using the cable's current rating, voltage rating and the type of installation (indoor or outdoor). The ampacity of a cable is usually provided by the cable manufacturer and is based on standard testing and rating procedures.

Determine the actual current that will flow through the cable. This can be done by using the cable's load current and the cable's voltage drop. The voltage drop is the difference in voltage between the two ends of the cable and is affected by factors such as the cable's resistance and the length of the cable.

Compare the actual current flowing through the cable to the ampacity of the cable to ensure that the cable is capable of safely carrying the current without overheating.

For example, if the ampacity of a 3-phase cable is 400 A, and the load current is calculated to be 350 A, then the cable size is appropriate. However, if the load current is calculated to be 450 A, then a larger cable cross-section would be required to safely carry the current.

Voltage drop method:

Determine the voltage drop in the cable by using the cable's resistance and the length of the cable.

Compare the calculated voltage drop to the acceptable voltage drop, which is typically around 3%.

For example, if the cable length is 100m, and the load current is 400A, if the voltage drop is calculated to be 2.2%, then the cable size is appropriate. However, if the voltage drop is calculated to be 4.5%, then a larger cable cross-section would be required to keep the voltage drop within the acceptable limit.

In conclusion, choosing the right cable cross-section is a crucial step in ensuring the safe and efficient operation of any electrical system. It is important to consider factors such as the load current, ampacity, voltage drop, and ambient temperature when selecting a cable cross-section. It is always recommended to consult with a professional electrician or engineer to ensure the correct cable cross-section is chosen for your specific application.

Source : How do we choose the right cable cross-section of cable? (https://www.linkedin.com/pulse/how-do-we-choose-right-cable-cross-section-tacettin-i%25CC%2587ki%25CC%2587z)

Scenario:
- Cable Type: 3-core, 3x95 mm² copper cable (XLPE insulation, PVC sheath)
- Installation: Underground, in duct (typical for long-distance industrial applications)
- Length: 300 meters (one-way)
- Load: 3-phase, 400 A
- Voltage: 400 V
- Power factor (pf): 0.9
- System: 3-phase AC, 50 Hz

Step 1: Ampacity Check

Let's assume the ampacity of a 3x95 mm² Cu XLPE cable underground is about:
Ampacity ≈ 330 A (based on 30°C ambient temp and standard installation)

⇒ Result: Your load is 400 A, but the cable's ampacity is only 330 A. 
This means 3x95 mm² is not sufficient for current carrying capacity. 
You would need a larger size such as 3x150 mm² (typically rated around 445–470 A underground).

Step 2: Voltage Drop Calculation

Voltage drop in a 3-phase system:

ΔV = √3 × I × (R × cosφ + X × sinφ) × L

Where: 
- ΔV = voltage drop (V) 
- I = 400 A 
- R = 0.195 Ω/km = 0.000195 Ω/m 
- X = 0.080 Ω/km = 0.000080 Ω/m 
- L = 300 m 
- cosφ = 0.9 
- sinφ = √(1 - 0.9²) ≈ 0.436

ΔV = √3 × 400 × [ (0.000195 × 0.9) + (0.000080 × 0.436) ] × 300
ΔV ≈ 1.732 × 400 × (0.0001755 + 0.0000349) × 300
ΔV ≈ 1.732 × 400 × 0.0002104 × 300
ΔV ≈ 43.74 V

Percentage voltage drop:

%ΔV = (43.74 / 400) × 100 ≈ 10.94%

⇒ Result: This is above the recommended 3% limit. 
Therefore, 3x95 mm² fails the voltage drop requirement as well.

Conclusion:

The 3x95 mm² cable: 

• Fails ampacity requirement (330 A < 400 A) 
• Fails voltage drop requirement (10.94% > 3%)

Recommendation:
- Use 3x150 mm² or 3x185 mm² cable (recheck values for confirmation) 
- Or use parallel cables such as 2x(3x95 mm²) if larger cross-sections are not practical