Why Overhead? Answers to Common Questions About Our Transmission Lines

Almost all high-voltage electric transmission lines are proposed as overhead for three general reasons: cost, repair time, and environmental considerations. SCE’s position on placing transmission lines on overhead structures is rooted in our responsibility to safely provide consumers with the most affordable, reliable power possible.

The California Public Utilities Commission conducts a formal review process on utility-proposed solutions and evaluates public comments in determining the best course of action.

The most affordable industry standard is overhead power lines. That said, it is SCE’s responsibility to consider many factors, including cost and environmental impacts, when proposing new electric transmission lines. Transmission lines are rarely constructed underground, largely due to the additional time involved for repairs and higher installation and repair costs. In fact, transmission lines at the 500 kV level have not been placed underground anywhere in the United States.

There are advantages and disadvantages to building lines overhead and underground. While communities often provide funding to underground lower voltage, local electric distribution lines in newer neighborhoods, it is not the preferred method for transmission lines that carry electric power over long distances. In addition to being cost prohibitive, a key difference between underground and overhead lines is that it typically takes more time to locate, diagnose a problem and repair an underground transmission line. On average, an overhead line could be restored within hours or days, whereas restoration of an underground line may potentially take weeks or months.

Yes, there are significant extra costs associated with undergrounding transmission lines. Compared to an equivalent overhead line, undergrounding can cost 2.5 to 10 times more than the cost of an equivalent overhead transmission line. For example, placing the transmission line underground for 3.5 miles in Chino Hills would cost approximately $400 million to $700 million more than the approved overhead project.

The design, installation and maintenance costs are all higher for underground lines. Installation costs for underground transmission lines can be 2.5-10 times those of an overhead line. Site-specific factors influencing cost include:

  • Routing: Right-of-way, easement and permitting costs and whether the line will be placed in the road right-of-way
  • Terrain and obstacles: Other underground utilities, streams and railroad crossings, embankments, bridges, major roads, traffic and soil conditions
  • Permitting: Traffic and lane restrictions, noise, time of day and other construction restrictions
  • Design: Significantly more construction for trenches, cost of insulated cable vs. bare wire, additional substation facilities, and Mitigating soil and thermal characteristics

The placement of transmission lines underground requires specific engineering construction measures to ensure the safe and reliable operation of the line. Because a single transmission line circuit requires three wires, each much be installed in an individual conduit. The three conduits are encapsulated in thermal concrete and surrounded by special thermal backfill materials. These facilities require significant trenching of at least five feet in depth and width. Because the repair of underground lines can be costly, environmentally disruptive and time-intensive if a problem occurs, underground construction design includes the installation of a spare conduit that can be used to replace a damaged cable or pipe to help ensure continued reliability without reopening the entire trench. The underground design also must accommodate a dedicated fiber optic cable for operation of line protection and control devices, which protect the system during faults and other anomalies.

All electric lines produce heat and therefore have a limit on the amount of power that they can carry. Underground lines cannot dissipate heat as well as overhead lines. Factors such as the type of surrounding soil, adjacent underground utilities and the depth of installation all affect the wire’s ability to dissipate heat.

New underground lines can have higher thermal ratings than outdated overhead lines they are replacing; however, SCE has far less flexibility to make improvements as needed on underground lines. When lines are above ground, SCE can replace the wire or make other improvements to its capacity without significant disruption. This means that SCE can respond to unforeseen circumstances, such as a change in the electric demand forecast or a change in power flow on the network, much more easily on overhead lines.

Transmission lines are rarely constructed underground, largely due to the additional time involved for repairs and higher installation and repair costs. Underground placement has occurred for smaller lines for low voltages that power homes and businesses and in rare incidences for very short portions of 230 kV lines. However, 500 kV transmission lines have not been placed underground anywhere in the United States. The CPUC reviews utility-proposed solutions as well as public comments in determining when it is appropriate to put transmission lines underground. In such a case, specific construction measures are necessary for safe and reliable operation of the line.

The costs associated with new and existing networked transmission lines are passed along to retail and wholesale electric customers across California in their monthly electric bills.

Homeowners and residents who live in communities served by SCE and other utilities share the desire for an aesthetically pleasing community. During the routing and siting process for new transmission lines, SCE will look first at possible routes that follow existing utility corridors, roadways, and railroads. We then look at other linear features such as recreation trails, property lines and fence lines. We also look for opportunities to place smaller distribution lines on the same poles whenever safe to reduce the clutter of multiple lines in an area.

Once the route is selected, we work with community and landowners on the placement of individual poles and, to the extent possible, work around lines of sight that may affect the appearance of an area. Further, we ask communities to help us by sharing their concerns, asking questions, and providing input on the siting our facilities. We know that our facilities often have impacts, so our goal is to site our lines in the least objectionable and intrusive way possible while ensuring reliable electricity for all our customers. Finally, before a transmission line is approved by the CPUC, the CPUC’s Energy Division conducts a full environmental impact review that evaluates any impact on aesthetics.

Installation costs for medium voltage (4kV to 115kV) underground transmission lines can range from 2.5-10 times the cost of an equivalent overhead line High Voltage (220kV to 500kV) can range from 30-50 times the cost of overhead. As a regulated utility, SCE is required to explore low-cost options when proposing new transmission lines since costs associated with new and existing transmission lines are passed on to retail electric customers in their monthly electric bills. The rate impacts of underground construction extend beyond the project area. A community may wish to use funds for the underground placement of smaller, lower voltage lines that power the local area. Yet, if the CPUC orders SCE to place transmission lines underground based on the request of local community, citizens outside of the project area would also be responsible for the cost. This is one of many reasons why major utility projects are subject to state, rather than local, approval. In submitting proposals to the CPUC, we are mindful of the impact of our projects on electricity bills for all consumers we serve.