PG&E Commitment: Latest Updates on Wildfire Response and Recovery Effort

Posted on October 31, 2017

Facts About Undergrounding Electric Lines

Electric undergrounding is the process of building electric infrastructure, such as electric distribution lines and other electrical facilities, in trenches underground. Most often our underground lines are located in trenches with other utilities’ infrastructure, such as gas and communications infrastructure, to minimize construction costs. Transformers are installed on concrete pads or in subsurface concrete boxes.

Electric distribution lines can be placed aboveground or underground.

  • Underground distribution lines cost significantly more to build than above ground lines and typically have a longer construction timeline. If PG&E began the process of undergrounding all of its power lines today, the entire process would take decades to complete.
  • The cost of undergrounding a distribution line can vary depending on several variables, such as road width (work access), nearby sidewalks (to comply with the Americans with Disabilities Act), density of nearby residences and businesses, surrounding vegetation, the number of power lines involved, other existing structures underground and other environmental issues.

Benefits and Challenges of Undergrounding 

In some scenarios, underground infrastructure can result in better reliability and performance, such as when severe weather event occurs.

However, underground lines are not immune to weather damage and are vulnerable to equipment issues, lightning strikes and flooding or digging damage. 

When underground systems are damaged, locating fault areas and undertaking excavation processes can be time-consuming. Underground lines can take almost twice as long to repair when damage occurs. 

Undergrounding is very costly. The costs versus the benefits must be carefully considered.

  • A report prepared by the Edison Electric Institute (EEI) found that burying above-ground electric distribution systems can range up to $5 million a mile in urban areas. Also, the life expectancy of an underground line is about half an overhead line.
  • The San Francisco Chronicle reported October 21 on the high financial costs of undergrounding. Mark Toney, executive director of The Utility Reform Network, was quoted as saying, “We think it’s so expensive that it’s really not feasible.”  The entire article can be found here.
  • According to PG&E estimates, it costs approximately $3 million per mile to convert underground electric distribution lines from overhead. The cost to build new overhead lines is approximately $150 per foot or $800,000 per mile.
  • New underground construction costs vary depending on trenching and paving costs.  If paving costs are not included and trench costs are shared between multiple parties (i.e., gas, communications) the overall new underground costs will be lower than the conversion costs.

By the Numbers

PG&E currently maintains approximately 100,000 miles of overhead lines and approximately 26,000 miles of underground lines across its service area.  Most underground lines are located in more densely populated areas and are installed as part of new residential and commercial subdivision developments. 

In the areas affected by the recent wildfires, approximately 20 percent of electric lines are underground with the majority of those located in the more densely populated communities, such as Santa Rosa.  

Overhead-Underground Conversions 

Per a 1967 California Public Utilities Commission (CPUC) decision, energy companies annually allocate work credits (synonymous with frequent flier miles) to communities (cities or counties) to convert overhead electric facilities to underground electric facilities in accordance with CPUC Rule 20A.

CPUC’s Tariff Rule 20 also serves as a guide for implementation of the underground conversion program.  Projects to convert overhead lines in special public interest areas (e.g., major arterial streets, congested lines near a substation, or streets near parks and civic centers) are covered by Rule 20A – the majority of the costs for which are paid by all PG&E customers through electric rates.

Underground conversions may also occur for specific developments (Rule 20B) or small projects involving one or more property owners (Rule 20C). In these cases, most of the costs are paid for by the developer or property owner. 


Since 2012, more than $250 million has been invested in converting overhead electric lines to underground.  As mentioned above, not all costs are born by PG&E customers through rates.

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