Study Area

UNS Electric is developing a plan to run a 138-kilovolt transmission line from Rio Rico to a new substation in the Patagonia Mountains to provide South32’s mine development project with sufficient electricity.

The plan calls for the 20-to-30-mile line to be strung across steel monopoles measuring up to 110 feet tall, with even taller structures possible at major road or line crossings.

The project would include a new switchyard connecting the line to UNS Electric’s existing 138 kV transmission system east of the Interstate 19 frontage road, about one mile north of Ruby Road in Rio Rico.

From there, the line would run to another new switchyard at South32’s privately owned Hermosa Project site in the Harshaw area of the Patagonia Mountains, according to a project description posted to the website of UniSource Energy Service, parent company of UNS Electric.

“The Hermosa Project currently is served by UNSE through an existing 13.2 kV distribution line that is not adequate to support the Hermosa Project’s anticipated energy needs,” says the project description, which also touts the line’s potential to “improve service reliability for all electric customers in the San Rafael Valley, Washington Camp and Lochiel areas.”

The power line construction would require approximately 100-foot-wide rights-of-way to cross private and government-owned land in Santa Cruz County, and UNSE plans to apply for a special use authorization from the U.S. Forest Service for a segment of the line that is expected to be built on Coronado National Forest land.

It will also have to secure a Certificate of Environmental Compatibility from the Arizona Corporation Commission.

UniSource said it is seeking public input about the project and invited the public to attend an upcoming virtual open house to ask questions and submit comments (see sidebar).

Comments can also be submitted through the project webpage at, via email to, by voicemail left at 1-833-783-0396, or by letters sent to UniSource Energy Services, Attention: Rio Rico-Harshaw, P.O. Box 711, Tucson, AZ 85702.

UniSource said the project’s development timeline will depend in part on the length of the permitting and CEC processes, but said the power line is expected to be in service in 2024.

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