Why are secondary cables for underground installations generally larger than those for overhead applications?

• A. Because of reduced capacity of the cable due to temperature limitations of the cable insulation
• B. Because ACSR cables must be used and the steel does not have the capacity of an all-aluminum conductor
• C. Because underground cables only feed larger loads due to installation cost
• D. Because underground cables must be insulated

Answer: (A)

The key idea is how heat limits current-carrying capacity. Underground environments don’t shed heat as easily as overhead setups because soil and burial restrictions reduce cooling. Because of that, the same conductor would run hotter for a given current, and insulation has a maximum safe temperature. To keep the temperature within that limit, the cable is made larger so its resistance drops and its ampacity increases, allowing the same load without overheating. So the need for a larger underground secondary cable comes from temperature limitations of the insulation in a poorly ventilated, buried setting, not from the need to use a specific material or from load size alone.