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Unlike smaller distribution power lines that deliver power to homes, high-voltage power lines are extremely expensive to build underground. Underground construction of transmission lines often costs 5 to 10 times more than overhead construction. On a recent project, the cost estimate for a section of 230-kV line a little more than a mile was $16 million for underground.
Cost is the main reason why most U.S. utilities reserve underground construction for places where overhead rights of way are highly restricted, such as airports and major cities, or where other parties are willing to pay the extra cost. It’s also why underground construction is used mostly on short spans of longer overhead lines.
Insulated cables, underground surveying and excavation, splicing vaults and concrete-encased conduits to protect lines from dig-ins contribute to higher costs. The lifespan of underground lines is 30 to 35 years, about half that of overhead lines.
Underground lines are less susceptible to storm-related outages than overhead lines. When outages do occur on underground, however, it takes an average of a week or more to locate and repair the problem, compared to a few hours for overhead lines. That’s an important reliability concern since tens of thousands of customers can be affected by a transmission line outage. Cleared rights of way are required for both types of construction.
Even with the disadvantages, we evaluate underground construction on most projects, and we will consider it when third parties agree to pay the difference in cost between overhead and underground construction.