Under what conditions must all high-voltage electric transmission and distribution circuits be installed underground?

The requirement for high-voltage electric transmission and distribution circuits to be installed underground primarily pertains to practical and regulatory considerations. Many municipalities and city planners have regulations that dictate when and where underground installation is deemed necessary to enhance safety, aesthetic value, and land use. When overhead lines may pose safety risks, create visual obstructions, or interfere with land development, local ordinances often mandate underground placement to mitigate these concerns.

While other options touch upon important considerations, such as environmental impacts or socioeconomic factors, they do not directly address the specific regulatory framework that governs the installation of high-voltage circuits. Therefore, practicality and compliance with city regulations are the driving factors in deciding to install these circuits underground.