Digging Into Underground Power Lines
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Members often ask why Northern Lights does not install all power lines underground. The answer is not simple. It comes down to balancing the ability to provide quality electric service and offering it at an affordable price.
To understand the pros and cons of underground and overhead power lines, let’s look at how each is built.
Take, for example, a mile-long route for a new distribution power line extension over rocky, rugged terrain with road and creek crossings.
Before construction starts, the area is marked through 811 underground locates. If needed, the path is cleared of trees. This is done for both overhead and underground power lines. Underground line rights-of-way do not need regular tree trimming after installation.
A mile-long overhead power line requires 13 to 15 wood poles. The pole ground disturbance is relatively minimal. Rocky areas can often be avoided by moving pole locations. Special equipment is used to dig holes in rocky areas.
Existing underground utilities are easy to avoid. We pass over road and creek crossings by installing poles that span across them. Uninsulated wires are strung between the poles.
If this same mile of line were installed underground, we would dig a 3-foot-deep continuous trench with a cable plow and lay insulated cable in the trench. Junction boxes are installed to connect sections of cable and services.
Using underground locate markers, crews carefully avoid other underground utilities. Crews must hand-dig or use a vacuum excavator to avoid damaging other underground utilities.
Trenching through rock and steep areas is hard work. Large boulders are dug up with a backhoe or mini excavator before workers plow in the cable. To get under roads or creeks, a contract crew bores underneath.
Installing this section of line underground takes more time and money than overhead line. Typically, underground lines cost four to eight times more to install than overhead lines. However, there are good reasons to bury power lines in some areas, even with the added costs.
Throughout the life of a line, the cost of potential outages and damage must be weighed. Areas susceptible to ice, heavy snow loading or extreme winds may be better served by underground lines. These lines are less affected by vegetation. Wildfire prevention is another consideration.
On the other hand, damage to overhead lines is typically much faster to find and repair compared to underground cable.
NLI has approximately 1,360 miles of underground power lines and 1,556 miles of overhead power lines. The mix of underground and overhead construction used by NLI provides you with the highest possible quality of service at the lowest possible cost. Cost, appearance, reliability, maintenance, and future upgrades drive which is the better approach.
Northern Lights Engineering & Operations Manager Kristin Mettke is an electrical engineer and has worked in the electric utility industry for most of her career.
