When a large number of cables are laid in groups, the current carrying capacity of the cables is reduced due to mutual heating. Sometimes it is necessary to consider replacing large-sized cables with two or more parallel cables of smaller sizes, because large-section cables will reduce the carrying capacity per unit section due to the skin effect and proximity effect. On the other hand, the reduced ratio of the surface area to the cross-sectional area of a cable with a large cross-section makes the large cable poor in its ability to dissipate heat. If multiple cables are used in parallel, the relative position of each cable should be considered to reduce the effect of uneven distribution of cable ampacity.

For cables laid in underground pipelines, when using the load factor, the heat capacity of the average heat loss of the pipe group and its surrounding soil should be considered. The temperature of the underground part changes with the average heat loss, so a higher short-term load factor is allowed, which is the ratio of the average load to the peak load, and is usually measured on the basis of the average day and night 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 direct-buried cables, the average surface temperature can be limited between 0 and 60°C according to soil conditions to prevent 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 of the specified cable ampacity, the rated ampacity of the cable must be reduced. The normal ambient temperature of the cable installation refers to the temperature at which the cable is installed when the cable is not loaded. This temperature should be well known in order to properly determine the required cable size for a given load. For example, the ambient temperature of a cable laid separately from other cables in the air refers to the temperature before the cable is loaded. For cables in 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.

Cable lines should be used for urban high and medium voltage distribution 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; sections of key scenic tourist areas; major coastal areas that are vulnerable to salt pollution or tropical storms Important power supply sections of the city; other sections required for grid structure and operational safety.

Urban low-voltage power distribution lines should be used in urban centers with high load density of cable lines in the following situations; newly-built residential areas and high-rise building areas with large building areas; streets or areas where overhead lines are not suitable for passing through according to planning, and areas where incoming and outgoing lines are crowded; Other situations where it is more appropriate to use the Dianxian regiment-level line after technical and economic comparison. Insulated cables can be used for overhead laying when cable lines should be used but the underground conditions are not available.

For low-voltage cables indoors, the ampacity meter in the National Electrical Code is based on an ambient temperature of 30 °C. However, during the summer months in most areas, 40°C is advisable for at least some parts of the building. When determining the ampacity of the cable, the heat source that is most unfavorable to the cable nearby must be considered. Local overheating of cables may be caused by steam pipes or heat sources close to the cable, or by cables passing through boiler rooms or other high temperature locations. To avoid such problems, rerouting may be required.

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 part of the day, or when the sun is shining the hardest, some circuits are not operating at full capacity. Under such conditions, it is more reasonable to use an ambient temperature of 40°C for outdoor cables in terms of safety.

Underground In different regions of a country, the ambient temperature used for underground cables varies. In northern my country, the ambient temperature is usually 20°C, and in the central region, 25°C is commonly used; while 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 ampacity of the cable. The type of soil where the cable or cable pipe block is buried has a significant impact on the ampacity of the cable. Porous and loose soils, such as gravel and ash backfill, generally have higher temperatures and lower ampacity than sandy or clay soils.

Therefore, before calculating the cable specification, the type of soil and the thermal resistivity of the soil should be known. The moisture content of the soil also has an important influence on the carrying capacity of the cable. In dry areas, in order to compensate for the increase in thermal resistance due to lack of moisture, the rated ampacity of the cable must be reduced, or other precautionary measures must be taken. On the other hand, in often wet underground or areas affected by tides, cables can carry higher than normal currents.

When direct burial is laid in permafrost areas, it should be buried below the permafrost layer. If deep burial is not possible, it can be buried in a dry permafrost layer with good soil drainage or backfill, or other measures can be taken. Directly buried cables are strictly prohibited from being directly above or below underground pipelines. 0.25m for partitions; 0.1m for cables passing through pipes; it can be reduced in special cases. When direct burial is laid in non-frozen soil areas, the embedment depth of the cable shall not be less than 0.3m from the cable sheath to the foundation of the underground structure.

When the directly buried cables intersect with railways, highways or streets, protective tubes shall be worn, and the protection scope shall exceed 0.5m beyond the roadbed, both sides of the street and the side of the drainage ditch. For direct-buried cables leading into structures, protective tubes shall be installed at the holes through the walls, and water-blocking shall be implemented at the tube openings. For the joint configuration of direct buried cables, the net distance between the joint and adjacent cables shall not be less than 0.25m. The joint positions of parallel cables should be staggered from each other, and the clear distance should not be less than 0.5m. The placement of joints on slope terrain should be horizontal. For the cable joints of important circuits, it is advisable to lay the cables in the local section starting from about 100mm on both sides, with a spare amount reserved. When the direct buried cable is backfilled with special replacement soil, the soil quality of the backfill should be non-corrosive to the outer sheath of the cable.