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.