How is the ampacity of an underground cable determined, and what factors must be considered beyond the conductor size?

The ampacity of an underground cable is the maximum current it can carry continuously without exceeding its design temperature limit. While conductor size is a primary factor, several other factors significantly influence ampacity. Soil thermal resistivity is a critical consideration, as it affects the cable's ability to dissipate heat. Higher soil thermal resistivity means the cable will run hotter and have a lower ampacity. Burial depth affects the cable's temperature. Shallower burial depths generally result in higher ampacity due to better heat dissipation, but this must be balanced against other factors like mechanical protection. Ambient earth temperature influences the cable's operating temperature. Higher ambient temperatures reduce the cable's ampacity. Cable spacing affects the mutual heating between adjacent cables. Closer spacing increases the heating and reduces the ampacity. The type of cable insulation and jacket materials influences ampacity due to their different thermal properties. The presence of other heat sources, such as nearby steam pipes or other cables, must also be considered. The ampacity is determined through calculations based on heat transfer models, considering all of these factors. Standards like the ICEA (Insulated Cable Engineers Association) provide guidelines and formulas for ampacity calculations. Software tools are often used to perform these calculations, taking into account the specific installation conditions. Derating factors are applied to the base ampacity to account for adverse conditions. Accurate knowledge of these factors is essential for selecting the appropriate cable size and ensuring safe and reliable operation.