To determine the appropriate wire size for use in the distribution box

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To determine the appropriate wire size for use in the distribution box, it is necessary to consider multiple factors comprehensively. The following is a detailed analysis:

### Load Current - **Calculating the Total Load Current**: First, tally up the rated power of all electrical equipment connected to the distribution box. Calculate the total load current according to the formula \(I = \frac{P}{U\cos\varphi}\) (where \(I\) represents current, \(P\) represents power, \(U\) represents voltage, and \(\cos\varphi\) represents the power factor). Different types of equipment have different power factors. For example, the power factor of an incandescent lamp is \(\cos\varphi = 1\), while that of an electric motor is usually between 0.75 and 0.9.

- **Considering the Coefficient of Simultaneous Use**: Not all electrical equipment will operate simultaneously. Therefore, it is necessary to multiply by a coefficient of simultaneous use based on the actual usage situation. Generally, the value of this coefficient ranges from 0.6 to 0.8.

### Line Length - **Short Distance**: If the line length is short, usually within dozens of meters, the resistance of the wire is relatively small and has a relatively minor impact on the current. The wire size can be selected according to the load current by referring to the conventional ampacity table.

- **Long Distance**: When the line length exceeds 100 meters, the resistance of the wire will cause a relatively large voltage drop, resulting in too low a voltage at the end equipment and making it unable to operate normally. At this time, it is necessary to appropriately increase the cross-sectional area of the wire to reduce the resistance and lower the voltage drop.

### Installation Method - **Exposed Installation**: The wire is directly exposed to the air, and the heat dissipation conditions are relatively good. For wires with the same cross-sectional area, the ampacity is relatively large, and the wire size can be selected according to the normal ampacity table.

- **Concealed Installation**: The wire is installed in pipes buried in walls, under floors, or in ceilings, etc. The heat dissipation conditions are relatively poor, and the ampacity will be reduced to some extent. Generally, it is necessary to correct the ampacity according to the conditions of concealed installation, usually by multiplying by a coefficient of about 0.8.

### Safety Factor - **Ambient Temperature**: If the distribution box is installed in a high-temperature environment, such as near heating equipment or outdoors under direct sunlight, the heat dissipation of the wire will be affected, and the ampacity will be reduced. At this time, the ampacity should be corrected according to the ambient temperature. Generally, for every 10 °C increase in ambient temperature, the ampacity should be reduced by about 10%.

- **Mechanical Strength**: For some wires that need to be moved frequently or may be pulled by external forces, such as the wires connected to movable equipment, the mechanical strength of the wire needs to be considered, and a larger wire size should be selected.

### Voltage Level - **220V**: This is the voltage level commonly used in households and general small commercial premises. For general lighting, socket circuits, etc., wires with a cross-sectional area ranging from 1.5 square millimeters to 4 square millimeters can be selected according to the calculated load current; for circuits of high-power equipment such as air conditioners and water heaters, wires with a cross-sectional area ranging from 4 square millimeters to 10 square millimeters are required.

- **380V**: This is the voltage level used in industrial and some large commercial premises. For the power cords of general electrical machinery, electric welding machines and other equipment, the wire size needs to be selected according to the rated current of the equipment, ranging from 4 square millimeters to 25 square millimeters or even larger.

### Short-Circuit Current - **Calculating the Short-Circuit Current**: Calculate the possible magnitude of the short-circuit current based on parameters such as the power supply capacity of the distribution box, transformer impedance, and line impedance.

- **Checking Thermal Stability**: The selected wire should be able to withstand the heat generated by the short-circuit current without being damaged. Generally, it is determined by checking the thermal stability coefficient of the wire.

### Special Requirements

- **Fire Protection Requirements**: In the fire protection system, for the power cords of equipment such as fire pumps and fire fans, fire-resistant or flame-retardant wires need to be used, and their specifications should meet the requirements for long-term power supply in the event of a fire. Generally, the wire size selected is larger than that for ordinary occasions. 、

- **Electromagnetic Compatibility Requirements**: In some places with high requirements for electromagnetic compatibility, such as hospitals and communication equipment rooms, shielded wires may be required to reduce electromagnetic interference. When selecting the size of shielded wires, in addition to considering the current-carrying capacity, factors such as the shielding effect also need to be considered.