With the development of my country's electrification and the implementation of rural power grid construction and renovation projects, the technical condition of the distribution network has undergone a dramatic change. Practice has proven that the development of the distribution network must have a unified plan and standards to ensure sufficient power supply capacity and meet the requirements of power supply reliability and economic operation. Below, based on years of practical experience in power distribution work, I will discuss the main contents of distribution line design. [b]1 Selection of Distribution Transformer Capacity[/b] Selecting the appropriate distribution transformer capacity has a significant impact on the safe and economical operation of the transformer and its adaptation to the development of user production. Therefore, a comprehensive consideration should be given when selecting transformer capacity. First, based on the technical characteristics of the transformer and combined with the actual local operating conditions and power consumption, the optimal economic capacity of the transformer should be calculated. Second, based on the existing load, the lighting and power loads that may increase in the next 3-5 years should be predicted, and the proposed transformer capacity should be calculated using a development coefficient. Third, the capacity of the largest power motor in the area should be selected for verification. Selecting the distribution transformer capacity with the best operating efficiency is crucial to adapting to agricultural production development, reducing investment, and achieving good social and economic benefits. [b]2 Selection of Transformer Installation Location[/b] The optimal installation location of a transformer refers to a location where the transformer can ensure low-voltage quality, reduce line loss, ensure safe operation, reduce project investment, facilitate construction, and not affect the city's appearance. Therefore, the selection of the transformer installation location should be based on the actual terrain and dense distribution of users, and should follow the following requirements: (1) Avoid areas that are prone to explosion, flammability, serious pollution, or low-lying terrain; (2) High-voltage lines should not pass over schools, squares, or housing; (3) High-voltage inlet and low-voltage outlet should be convenient; (4) Convenient for construction and operation and maintenance; (5) Consider the principle of "small capacity, dense distribution, and short radius". [b]3 Survey and Positioning of Distribution Line Path[/b] 3.1 Selection of Distribution Line Path Based on the distribution of users, the installation location of the household distribution box should be reasonably selected, generally within 30 meters of the line connection point. In this way, after the installation location of the distribution transformer and the household distribution box is determined, the route is selected on site. The route selection should meet the following requirements: (1) It should be combined with the development plan of the region, and should not occupy or occupy less farmland to facilitate mechanized farming. At the same time, it should be coordinated with agricultural mechanization, road planning, etc. to avoid relocation of the line. (2) In order to reduce power loss and voltage loss, reduce project cost, and facilitate construction, operation and maintenance, the line route should be as short as possible, with fewer corners. The low-voltage main line should also be close to the road side, but should not affect the traffic of the village road. (3) In order to ensure the safe operation of the line, the line route should avoid places that are easily washed by rainwater. It is strictly forbidden to cross the yards and warehouses where combustibles and explosives are piled up to avoid fire or explosion accidents caused by contact with or breakage of the line. In addition, the impact of power lines on weak current lines should be considered. 3.2 Pole position determination After the route is determined, the pole position is measured. First, the position of the first and last poles of the line is determined. When encountering terrain restrictions or power needs, the position of the corner poles should be determined. In this way, the entire line is divided into several straight segments based on the positions of the first, corner, and terminal poles. The length of each straight segment is then measured, the span is evenly distributed, and the positions of the straight poles are determined one by one. Determining the positions of the first, corner, and terminal poles first considers whether the location of the tension line is suitable and whether there is a location where tension pits can be dug. Furthermore, the pole positions should, as far as possible, avoid affecting local residents and consider the reasonableness and convenience of power supply lines to the distribution boxes. Simultaneously, the span distance should be considered: for 10kV distribution lines in towns (entering villages), it should not exceed 50 meters; for non-residential areas, it should not exceed 80 meters; in some areas, due to terrain limitations, the maximum should not exceed 100 meters. For 0.4kV distribution lines, it is generally 30-40 meters, and should not exceed 50 meters at most. In special circumstances, such as when encountering crossings or the need for service lines, the span distance can be reduced. To obtain a larger vertical distance for crossings, when a new line passes over an object being crossed, the pole should be as close as possible to the object being crossed, while ensuring the required pole-falling distance. If the new line is under an object that is crossed, the crossing point should be placed in the middle of the span of the new line as much as possible. [b]4 Selection of conductor type and cross section[/b] 4.1 Determine the meteorological conditions for line design. For the mechanical calculation of overhead lines operating in the atmosphere, the three meteorological factors of temperature, wind speed and icing thickness are the most important. Therefore, it should be comprehensively considered based on the relevant local meteorological data and the operating experience of existing lines in the area. According to the meteorological data of Meizhou City, Guangdong Province: (1) the maximum wind speed is not greater than 25 m/s and the temperature is -5℃; (2) the icing thickness is not greater than 5 mm and the temperature is -5℃; (3) the highest temperature is not higher than +40℃; (4) the lowest temperature is not lower than -40℃. Based on the above data, it can be determined that: LGJ type steel core aluminum stranded wire is used for 10kV distribution lines, and overhead insulated wire, namely aluminum core plastic wire (BLV) type, is used for 0.4kV distribution lines. 4.2 Selection of conductor cross section. Generally, it is determined according to the allowable voltage loss, while meeting the requirements of heating conditions and mechanical strength. A margin for future development should be allowed based on load conditions. However, to ensure the quality and safety of line operation, the cross-sectional area of ​​10kV lines and 0.4kV main lines should not be less than 35mm², and the cross-sectional area of ​​the neutral wire in a three-phase four-wire system should not be less than 50% of the phase wire cross-section. The cross-sectional area of ​​other 0.4kV branch lines and service lines should be determined according to the actual capacity of the electrical equipment. [b]5 Selection of Conductor Sag and Arrangement[/b] 5.1 Determination of Conductor Sag. The sag of a conductor is related to the conductor cross-section, meteorological conditions, and the span of the pole. Based on meteorological conditions and operational experience, a conductor sag table for different spans and temperatures can be calculated by referring to relevant tables. 5.2 Selection of Conductor Arrangement. To reduce pole height and prevent wire breakage or conductor bridging accidents due to sag changes, 10kV distribution line conductors are arranged in a triangular pattern, while 0.4kV distribution line conductors are generally arranged horizontally. In special cases, a vertical arrangement can be used, but the neutral wire should be installed below the phase wires. When conductors are arranged horizontally, if there are buildings near the line, the neutral line should be close to the building to increase the distance between the phase lines and the building, reducing the chance of electric shock. At the same time, pay attention to the distance between conductors. According to operational experience, when overhead insulated conductors are arranged horizontally, the distance between lines should not be less than 0.4m; the clearance between the conductor's bow wire (also known as a jumper wire), down conductor, and grounding electrode/conductor should not be less than 150mm to prevent short circuits caused by lead wire swaying. The clearance between conductors and guy wires/pole should never be less than 50mm. [b]6 Selection of Pole Type[/b] To extend the service life of poles and reduce line maintenance costs, distribution lines should preferably use standardized products that conform to the current national standard "Ring-shaped Prestressed Concrete Pole," i.e., prestressed reinforced concrete poles. The pole height can be selected based on local conditions, generally requiring: the pole height for 10kV distribution lines should not be less than 10 meters, and the pole height for 0.4kV distribution lines should not be less than 7 meters. Secondly, determine the pole burial depth. The foundation of the power pole should be calculated and determined based on local operating experience, material sources, soil conditions, and load conditions, but its burial depth should not be less than 1/6 of the pole height. [b]7 Selection of Distribution Line Hardware and Insulators[/b] 7.1 Crossarm Selection. To ensure sufficient bending strength of the crossarms, 10kV distribution lines use angle steel crossarms with specifications not less than ∠63mm×63mm×6mm; 0.4kV distribution lines use angle steel crossarms with specifications not less than ∠50mm×50mm×5mm. 7.2 Insulator Selection. The performance of distribution line insulators should comply with relevant national standards. For 10kV distribution lines, straight poles and small-angle poles with a bend of less than 5 degrees use pin insulators or porcelain crossarms. Tension poles should preferably use insulator strings composed of two X-4.5 type suspension insulators. For 0.4kV distribution lines, straight poles generally use pin insulators, porcelain crossarms, and low-voltage suspension clamps. For vertical wiring, angle iron street clamps can be used. Tension poles use butterfly insulators, spool insulators, or tension clamps. [b]8 Selection of Guy Wires[/b] 8.1 Selection of Guy Wire Material. Guy wires should preferably be made of galvanized steel stranded wire, with a minimum cross-section not less than GJ-25mm2, and a strength design safety factor greater than 2.0. 8.2 Determination of Installation Location. Guy wires are pole reinforcement devices used to balance conductor tension or wind pressure. Therefore, selecting the right installation location for the guy wire is particularly important. The guy wire is generally installed 0.1 to 0.3m below the crossarm at the fixed position on the pole. For horizontal guy wires crossing roads, the vertical distance to the road surface should not be less than 6 meters, and the inclination angle of the guy wire post is generally 10°–20°. The angle between the guy wire and the pole is generally 45°, but when limited by terrain, it should be appropriately reduced to no less than 30° to maintain effective balanced tension. 8.3 Requirements for the guy wire handles and guy wire reel. The guy wire handle should be made of round steel with a diameter of not less than 16mm, and the handle should protrude 0.3–0.5m above the ground to allow for adjustment of the guy wire length using UT clamps. The guy wire reel should be made of reinforced concrete, and its dimensions should not be less than 50mm × 250mm × 500mm. 8.4 Determination of guy wire length. Due to the influence of actual terrain and external factors, the guy wire length should be determined by actual calculation. **9 Lightning Protection Measures** Lightning protection devices for distribution transformers and switchgear should use metal oxide zinc surge arresters (YWS) to prevent lightning surge waves from entering the transformer and equipment along high-voltage lines. Low-voltage zinc oxide surge arresters should be installed on the neutral line of the low-voltage side to prevent high voltage from entering the low-voltage outgoing lines. The grounding terminals of high-voltage and low-voltage surge arresters, iron parts, the neutral point on the low-voltage side, and the metal casing of the transformer should be connected to the same grounding device respectively. The grounding resistance of the grounding device for distribution transformers below 100kVA should not exceed 10Ω, and the grounding resistance of the grounding device for distribution transformers of 100kVA and above should not exceed 4Ω. **10 Low-Voltage Energy Metering Devices** 10.1 Selection of Distribution Boxes (also known as Meter Boxes). Currently, there are two types of distribution boxes: one is made of sheet metal, and the other is a plastic meter box. Both types of meter boxes have their own advantages and disadvantages. The former, made of sheet metal, is less prone to discoloration and has a clearer meter mirror; however, the manufacturing process is not precise enough, and the power lines inside the junction box are not easily sealed. The latter has neat wiring terminals and a beautiful appearance; however, the meter mirror is prone to fading and becomes blurry, which makes meter reading difficult. After comparing the advantages and disadvantages, different models of meter boxes can be selected to install single-phase, three-phase, three-phase or three single-phase energy meters with current transformers. 10.2 Selection of metering device. (1) Selection of model and capacity of lighting user meter: According to the characteristics of rural residential electricity consumption and its development law, the actual electricity load is calculated to correctly select the capacity of the energy metering device and select its model accordingly. Generally, the DD862 type wide load, long life and low power consumption energy meter is selected. (2) Selection of metering devices for power users: ① Selection of metering method: For rural power users, three single-phase energy meters with current transformers can be selected for phase metering, or three-phase four-wire direct-connection energy meters can be used. When the power load reaches 22kW, the load current I = 39.3A can be calculated using the formula I = P/Ucosф (U = 380V, cosф = 0.85). If the current exceeds 40A, the junction box is prone to overheating and burning when using a direct-connection meter. Therefore, when the load exceeds 22kW, it is advisable to use a current transformer-type meter. ② Selection of capacity: When using a direct-connection meter, first calculate I according to the formula I = P/Ucosф, and then determine the capacity of the meter according to the size of I. Generally, I should be between 50%Ib and 350%Ib (Ib is the rated current of the energy meter). If a current transformer-type meter is used, a 1.5 (6)A energy meter should be used.