Arizona Generation & Transmission Cooperatives (AZG&T) of Arizona Electric Power Cooperative, Inc. (AEPCO) is a not-for-profit transmission cooperative owned by six Class A members. These Class A members are non-profit distribution cooperatives that deliver power at retail to rural areas of Arizona and other parts of the Southwest. AEPCO transmits power to members primarily from AEPCO’s Apache Generating Station, located near Cochise, Arizona.

AEPCO and Tucson Electric Power (TEP) are planning new electrical infrastructure to serve the town of Marana, Arizona and the surrounding area in Pima County. The Project consists of two separate transmission line projects, which may be built on the same structures in select areas. The Project requires:

  • A new AEPCO 115 kilovolt (kV) transmission line between the planned Trico Electric Cooperative Inc. Adonis Substation and existing AEPCO Marana Substation. The Project will connect to the existing Saguaro Substation located approximately 7 miles northwest of Marana using an existing transmission line.
  • A new TEP 138kV transmission line connecting to a planned TEP substation in the Town of Marana near the intersection of Grier Road and Sanders Road.

The Project will increase electric reliability and serve customers’ growing needs in the surrounding Project area. Although both utilities have the necessary infrastructure to meet current energy needs in this area, electrical demand and future growth require new 115/138 kV transmission lines/substations. The new electrical facilities may also support the development of future renewable energy generation projects as well.

The new facilities are intended to further our commitment to high quality, affordable, and reliable electric service for all our customers. These new facilities are needed to serve AEPCO and TEP customers and respective service territories in the vicinity of the Town of Marana and Northeastern Pima County. This additional power capacity is needed to serve the new residential, commercial, industrial, and agricultural needs in the Marana area.

This project will bring direct and indirect benefits to individuals and the community. Providing safe and reliable electric service enables economic growth, bringing high-end jobs and revenue to the area. This project will improve the transmission infrastructure in the vicinity of the Town of Marana, which benefits economic development in the long term, including:

  • The ability to better meet the growing energy needs, and increase the reliability to residents
  • Help create new businesses and job opportunities
  • Help sustain a larger tax base to better support the communities

The planning and approval process of a project like this typically takes 12-18 months to complete. AEPCO has been working through the planning process in 2021 and 2022. A proposed route was selected in March 2022. We are now in the process of applying for a Certificate of Environmental Compatibility (CEC) with the Arizona Corporation Commission (ACC) for a transmission line route corridor. Public hearings for the CEC are scheduled for June 6-10, 2022. The Project is expected to be approved in June 2022.

Construction of the AEPCO 115kV transmission line is anticipated to start in the second half of 2022. TEP’s 138kV transmission line and substation are anticipated to start construction in 2026.

Approximately 9-12 miles of new transmission line are needed, depending on final routes selected to connect the planned Adonis Substation and the existing Marana Substation. The new Saguaro to Marana 115/138kV transmission line would begin at the planned Adonis Substation and proceed west across Interstate 10, continuing to the intersection of North Sanders Road and West Marana Road. This portion of the Project will be constructed as a double circuit transmission line. One circuit will be owned and operated by AEPCO, and one circuit will be owned and operated by TEP. The AEPCO circuit will be energized at 115kV, while the TEP circuit will be energized at 138kV. The co-location of AEPCO and TEP assets will minimize environmental impacts and provide efficiencies in the permitting processes required to build the facilities. From North Sanders Road and West Marana Road, the remainder of the line proceeding west to the existing AEPCO Marana Substation will be single-circuit 115kV, solely owned by AEPCO.

The transmission lines will utilize steel monopole (single pole) structures ranging in height from 65-95 feet tall, with spans of 500-700 feet in length. Right-of-way will be approximately 100 to 120 feet and may be co-located along existing roads or other utility rights-of-way.

Existing transmission lines are often considered opportunities to locate new transmission lines. Although the existing structures in this area are not large enough to support the new 115/138kV transmission lines for this project, we will evaluate the opportunity to consider rebuilding existing transmission lines with larger poles that would enable us to consolidate the new 115/138kV transmission lines with the existing transmission lines.

The siting process will evaluate a range of alternatives that may include routes within existing roadway right-of-way, private property, or a combination of both. Prior to identifying a proposed route, AEPCO will consider the availability of land, ownership, and cost of right-of-way in their decision process.

Right-of-way for 115/138kV facilities could be up to 100 to 120 feet; however, this requirement can be less if the right-of-way is shared with roadways, utility lines, canals, etc.

AEPCO works with landowners to secure necessary easements. There are necessary restrictions within the right-of-way to ensure the lines can be constructed, operated, and maintained safely and according to electrical codes.

Placing transmission and sub-transmission voltage transmission lines underground is significantly more expensive than placing transmission lines overhead. Actual cost differences depend on various elements, including terrain, project length, environmental concerns, labor, and material differences, etc. An overhead line that spans trenches, washes, or other difficult terrain may not be compatible for running underground and would require added time and expense for additional labor and material. Depending on the voltage and the location of the transmission line and considering the inability to be cooled by the ambient (surrounding) air as in an overhead configuration, underground transmission lines may require special technology to keep the wires cool. Underground 115/138kV transmission lines require the cable conduits to be encased in a special concrete slurry, which allows for greater heat dissipation while adding a layer of protection from accidental excavation.

Buried transmission and distribution lines may also extend power outages, as it may take additional time to locate a specific issue, access the fault, and repair it. Additionally, underground systems can be prone to flooding in certain conditions.

Typically, lower voltage 12kV (distribution) lines are buried with new developments as the additional cost to place the lines underground is passed onto homebuyers or business owners by the developer through impact fees. The bulk of the 12kV distribution lines found in the project study area are located underground. High voltage transmission lines are not typically buried underground.

The cost of building a 115/138kV transmission line overhead can vary depending on several factors. A typical transmission line, built on flat terrain, costs approximately $1.8 million per mile, but can increase due to terrain, geology, or other factors. The cost to build a 115/138kV transmission line underground can be 5-10 times (or more) the cost of building overhead.