Answers to Frequently Asked Questions about Underground Transmission Lines

Tucson Electric Power does not install transmission lines underground. While lower-voltage distribution lines are sometimes installed underground, TEP’s higher-voltage transmission and sub-transmission lines are installed overhead. The following questions and answers provide more details about this practice.

What is the difference between transmission lines and distribution lines?

TEP’s transmission and sub-transmission lines carry energy at voltages above 34.5 kilovolts (kV) between energy resources, substations and some industrial, institutional or large commercial customers that pay for higher-voltage service. Distribution lines deliver energy at lower voltages, usually from substations to customers. Residential customers are typically served at 110 and 220 volts.

Why doesn’t TEP install transmission lines underground?

It costs significantly more to build a transmission line underground than to build the same line overhead. Underground lines also have higher maintenance costs. Because our costs are passed along to customers, TEP avoids unnecessary expenditures to help keep our rates as affordable as possible.

Why would it cost more to build and maintain underground transmission lines?

The higher construction cost typically reflects civil engineering expenses, right-of-way acquisition, additional labor and materials such as conduit, insulated wire and pull-boxes that are not required for overhead projects. Transmission lines conduct energy flows at higher amperages than distribution cables, generating far more excess heat that must be managed to avoid overloads. This requires the use of higher-cost conductors and other insulating infrastructure. Maintenance also costs more and takes more time, and requires specialized training for utility workers as well as police, firefighters and others who might respond to an incident involving a high-voltage underground transmission line.

Could nearby property owners subsidize the additional cost of underground installation?

Arizona state law (ARS 48-620) provides for the creation of an underground utility improvement district that can allow nearby property owners to pay the additional cost of installing facilities below ground. While such districts have been used to fund the underground installation of distribution lines, the extremely high cost of installing higher-voltage lines below ground makes this option less realistic for transmission line projects.

For answers to questions about underground distribution lines, click here.