1、 Determine the current carrying capacity of the cable (core electrical performance)

1. Positive correlation between current carrying capacity and cross-sectional area

• When current passes through a conductor, it generates resistance heat（$Q=I^{2}Rt$）The larger the cross-sectional area, the lower the conductor resistance（$R=\rho L/S$，$\rho$For resistivity,$L$For length,$S$The larger the cross-sectional area, the greater the current allowed to pass through.

• Example data(Copper core cable, ambient temperature 25 ℃):

  Cross sectional area (mm ²)	Single core cable current carrying capacity (A)	Corresponding power (220V, kW)
  1.5	12~15	2.6~3.3
  16	80~90	17.6~19.8
  95	260~280	57.2~61.6

2. Matching overload protection with cross-sectional area

• The rated current of the circuit breaker/fuse should match the cross-sectional area of the cable:

• Example: A 2.5 mm ² cable (current carrying capacity of 20A) needs to be equipped with a 16A or 20A circuit breaker. If a 32A circuit breaker is forcibly connected, the cable will heat up and burn out first when overloaded, rather than the circuit breaker tripping.

2、 Affects the voltage drop of cables (power supply stability)

1. The relationship between voltage drop calculation formula and cross-sectional area

• Voltage drop（$\Delta U$）Formula:$\Delta U=I\times (2L\times \rho /S)$Among them$2L$The length of the round-trip route.

• Conclusion: Cross sectional area$S$The larger the voltage drop, the smaller the voltage drop, making it suitable for long-distance power supply.

• case380V three-phase motor, 100 meters away from the distribution box, load current 50A:

• Select 10 mm ² cable:$\Delta U\approx 5.2V$(Voltage loss of 1.3%);

• Select 6mm ² cable:$\Delta U\approx 8.7V$(Voltage loss of 2.2%), which may cause difficulty in starting the motor.

3、 Mechanical strength and flexibility (physical properties) of associated cables

1. Mechanical strength: The larger the cross-sectional area, the stronger the tensile/compressive strength

• Large cross-sectional area cables (such as 35 mm ² or more) are commonly used for outdoor overhead or heavy machinery connections, as the conductors are thicker and less likely to be pulled apart by external forces.

• For example, tower crane cables on construction sites should use 16-25 mm ² rubber sheathed cables to avoid conductor breakage due to cable weight or swinging during lifting.

2. Flexibility: The larger the cross-sectional area, the poorer the bending flexibility

• For details, please refer to the previous answer: Conductors with small cross-sectional areas (such as 1.5 mm ²) are mostly twisted into thin strands, similar to "soft ropes"; A conductor with a large cross-sectional area (such as 50 mm ²) has fewer strands and a larger diameter per strand, requiring a larger radius when bending (such as R ≥ 10D).

4、 Determine the cost and installation space of cables (economy and installation adaptability)

1. Nonlinear relationship between cost and cross-sectional area

• Copper prices account for 70% to 80% of cable costs, doubling the cross-sectional area and copper usage, but the increase in insulation/sheath material costs is limited.

• Cost effectiveness recommendations：

• Short distance low power (such as household lighting): choose 1.5~2.5 mm ²;

• High power long-distance (such as factory workshops): Prioritize using cables with large cross-sectional areas to reduce voltage drops and avoid equipment failures caused by insufficient voltage.

2. Installation space limitations

• When laying through conduit, the total cross-sectional area of the cable (including insulation layer) shall not exceed 40% of the inner diameter of the conduit

• Example: A 20mm PVC pipe can accommodate up to three 4mm ² cables (each with a diameter of approximately 6mm, a total area of approximately 3.14 × 3 × 3=84.78mm ², and a cross-sectional area of approximately 314mm ², accounting for 27%); If three 16mm ² cables (each with a diameter of about 10mm) are threaded, the total area will be approximately 3.14 × 5 ² × 3=235.5mm ², accounting for 75% of the total area. Exceeding the standard will result in poor heat dissipation.

5、 Affects the heat dissipation and temperature rating of cables (safety)

1. Balance between cross-sectional area and heat dissipation efficiency

• Although cables with large cross-sectional areas have low resistance, they have a larger diameter and a relatively smaller heat dissipation area (resulting in a decrease in surface area/volume ratio), requiring insulation materials with higher temperature resistance levels

• Small cross-sectional area (≤ 6 mm ²): commonly used PVC insulation (temperature resistance of 70 ℃);

• Large cross-sectional area (≥ 16 mm ²): Cross linked polyethylene (XLPE, temperature resistant to 90 ℃) or silicone rubber (temperature resistant to 180 ℃) are often used to avoid insulation aging caused by poor heat dissipation.

2. Correction coefficient of temperature on current carrying capacity

• When the ambient temperature exceeds 25 ℃, the current carrying capacity needs to be multiplied by a correction factor (K), and the larger the cross-sectional area, the more significant the decrease in K value:

• Example: At 40 ℃, the current carrying capacity of a 16 mm ² cable decreases from 90A to 78.3A with K=0.87, while the current carrying capacity of a 2.5 mm ² cable decreases from 20A to 18.2A with K=0.91.

6、 Selection logic adapted to different application scenarios (practical application)

1. Typical case of matching cross-sectional area by scene

• Home wiring：

• Lighting: 1.5 mm ² (current carrying capacity 15A, meeting the requirements of LED lighting fixtures);

• Air conditioner/water heater: 4 mm ² (current capacity of 32A, suitable for 2-3 horsepower air conditioners).

• industrial scenario：

• Small motor (1.5kW): 3 × 2.5 mm ² cable;

• Large water pump (75kW): 3 × 50 mm ²+1 × 25 mm ² cable (three-phase five wire system).

• mobile device：

• Handheld electric drill: 3 × 1.5 mm ² rubber sheathed cable (YZ type), balancing flexibility and current carrying capacity;

• Port crane: 3 × 35 mm ²+1 × 16 mm ² heavy-duty cable (YC type), allowing for certain bending but requiring fixed installation.

Summary: A Four Dimensional Decision Model for the Cross Sectional Area of Conductors

1. Electrical requirementsWhether the current carrying capacity and voltage drop meet the requirements of equipment power and distance;

2. mechanical conditionInstallation method (fixed/mobile), bending frequency requirements for flexibility;

3. environmental factorsTemperature and heat dissipation conditions determine the insulation temperature resistance level;

4. 成本限制Under the premise of safety, avoid waste caused by "big horses pulling small cars".