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When a large number of cables are laid in groups, the current-carrying capacity of the cables is reduced due to the mutual heating effect. Large-gauge cables sometimes need to be replaced with two or more smaller-gauge cables in parallel, because large-section cables will reduce the ampacity per unit cross-section due to skin effect and proximity effect. On the other hand, the reduction in the ratio of surface area to cross-sectional area of ​​large cross-section cables results in poor heat dissipation capabilities of large cables. If multiple cables are used in parallel, the relative position of each cable should be considered to reduce the uneven distribution effect of the current carrying capacity of the cables.

For cables laid in underground pipes, when using the load factor, the heat capacity of the average heat loss of the pipe group and the surrounding soil should be considered. The temperature of the subsurface varies with the average heat loss, so a higher short-term load factor is allowed. It is the ratio of the average load to the peak load and is usually measured on the basis of the day and night average load. The peak load generally refers to the average value of the maximum load during 0.5 to 1 hour that occurs within 24 hours. For directly buried cables, the average surface temperature can be limited between 0 and 60°C according to soil conditions to prevent the loss of soil moisture and thermal breakdown of cables.

When the cable is close to other loaded cables or heat sources, or when the ambient temperature exceeds the ambient temperature that specifies the cable's current-carrying capacity, the rated current-carrying capacity of the cable must be reduced. The normal ambient temperature of a cable installation is the temperature at which the cable is installed when the cable is not under load. This temperature should be well known in order to properly size the cable required for a given load. For example, the ambient temperature of a cable laid apart from other cables in the air refers to the temperature before the cable is loaded. For cables in the air, also assume that there is enough space around the cable to dissipate the heat generated by the cable and not raise the temperature of the entire room. If the above correct conditions are specified, then the following environmental conditions can be used to calculate the ampacity of the cable.

Urban high and medium voltage distribution lines should use cable lines in the following situations. Prosperous urban areas, important areas, main roads, and areas with special requirements for urban planning and city appearance; areas with severe corrosion that are technically difficult to solve; areas in key scenic tourist areas; major coastal areas susceptible to salt pollution or tropical storms Important power supply sections of the city; other sections required for grid structure and operation safety.

Urban low-voltage distribution lines should be used in the urban center areas with high cable line load density in the following situations; new residential quarters and high-rise buildings with large building areas; streets or areas that are not suitable for passing overhead lines according to planning and crowded areas of incoming and outgoing lines; After technical and economic comparison, it is more appropriate to adopt the Dianxian regiment-level line. When the cable line should be used but the underground is not available, the insulated cable can be used for overhead laying.

For low-voltage cables indoors, the ampacity meter on the National Electrical Code is based on an ambient temperature of 30°C. However, during the summer months in most areas, at least 40°C for some parts of the building is appropriate. The most detrimental nearby heat source for the cable must be considered when determining the cable ampacity. Local overheating of the cable can be caused by steam pipes or heat sources close to the cable, or it can be caused by the cable passing through a boiler room or other high temperature locations. To avoid this type of problem, wiring may need to be changed.

Outdoors, for cables installed in the shade, the maximum ambient temperature is generally 40°C, and for cables installed in the sun, the maximum ambient temperature is generally 50°C. When using these ambient temperatures, it is assumed that the maximum load occurs exactly at the specified ambient temperature. During the hottest hours of the day, or when the sun is at its worst, some circuits are not running at full capacity. Under such conditions, it is reasonable to use an ambient temperature of 40°C for the safety of outdoor cables.

Underground In different parts of a country, the ambient temperature used for underground cables varies. In northern China, the ambient temperature is often 20°C, and for the central region, 25°C is commonly used; and for the southernmost and southwest ends, the ambient temperature may be 30°C. The geographic boundaries of these ambient temperatures are impossible to draw precisely. The thermal properties of the medium surrounding the cable are important parameters when determining the current carrying capacity of the cable. The type of soil in which the cable or cable duct block is buried has a significant impact on the current carrying capacity of the cable. Porous loose soils, such as gravel and ash backfill, generally have higher temperatures and lower current carrying capacities than sand or clay.

Therefore, the type of soil and soil thermal resistivity should be known before calculating cable specifications. The moisture content of the soil also has an important influence on the current carrying capacity of the cable. In dry areas, to compensate for the increased thermal resistance due to lack of moisture, the rated ampacity of the cable must be reduced or other precautions must be taken. On the other hand, in frequently wet underground or areas affected by moisture, the cables can carry more current than normal.

When direct burial is laid in the permafrost area, it should be buried below the permafrost layer. When deep burial is not possible, it can be buried in the dry permafrost layer or backfill with good soil drainage, or other measures can be taken. Directly buried cables are strictly prohibited from directly above or below the underground pipeline. It can be separated by 0.25m with a partition; it can be 0.1m when the cable is passed through the pipe; it can be reduced in special circumstances. When directly buried in non-frozen areas, the cable burial depth shall not be less than 0.3m from the cable sheath to the foundation of underground structures.

When directly buried cables intersect with railways, highways or streets, they should wear protective pipes, and the protection scope exceeds the subgrade, both sides of the street pavement and the side of the drainage ditch by more than 0.5m. For directly buried cable entry structures, protection pipes shall be provided at the penetration wall holes, and the pipe openings shall be blocked by water blocking. For the joint configuration of directly buried cables, the clear distance between joints and adjacent cables shall not be less than 0.25m. The joint positions of the parallel cables should be staggered from each other, and the clear distance should not be less than 0.5m. The joint placement at the slope terrain should be horizontal. For the cable joints of important circuits, the cables should be laid in the local section starting from about 100mm on both sides of the cable joints according to the method of leaving a spare amount. When using special soil replacement and backfill for directly buried cables, the soil quality of the backfill should be non-corrosive to the outer sheath of the cable.