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

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.

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.