Answers to Frequently Asked Questions about Underground Distribution Lines

While most of Tucson Electric Power’s electric distribution lines are installed on poles above ground, some are built underground to comply with engineering and/or safety requirements or at customers’ request. Customers who request underground installation of facilities that would otherwise be built overhead must pay the differential cost for below ground construction. The following questions and answers provide more details about this practice.

What is the difference between distribution lines and transmission lines?

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

Why are most of TEP’s distribution lines developed above ground?

Generally speaking, overhead distribution lines are more cost-effective than underground lines. It costs more to build and maintain underground lines, though below ground installations are sometimes unavoidable due to existing conditions. TEP does not install transmission or sub-transmission lines underground.

When will TEP pay to place distribution facilities underground?

Engineering and safety concerns sometimes justify the additional cost of installing distribution facilities underground. For example, the lower voltage "feeder" lines that emerge from TEP's distribution substations are typically installed underground until these "getaways" reach a point where they can be safely brought above ground due to congestion in the vicinity of the substation. In most cases, though, engineering concerns can be satisfied more cost-effectively through an overhead installation.

Can a customer or developer request that distribution lines be placed underground?

Yes, TEP will install distribution lines underground at the request of a customer, developer or other party if those parties agrees to pay the additional cost. TEP has methods outlined within its Rules and Regulations for the undergrounding of distribution lines. For more details, visit our new construction application page.

Why is it TEP’s policy that the additional cost of underground facilities be borne by those who request such installations?

This practice avoids passing along unnecessary costs to customers through our rates. It also ensures that all customers are not asked to subsidize a discretionary expenditure that primarily benefits residents of one small area of our service territory.

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