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Tillamook to Oceanside Transmission Line Project
Tillamook PUD is committed to providing reliable, safe and cost-effective energy to our
customers and to strengthening the overall reliability, security and operating flexibility of our
electric power system. To accomplish these important objectives, Tillamook PUD has proposed
a new overhead 115 kV transmission line to connect the existing BPA substation in Tillamook to
a new substation to be built near Oceanside. We have collaborated with the City of Tillamook
and other landowners to site a transmission route that balances our obligation to provide
adequate, reliable power in a cost-effective and safe manner with our commitment to being a good neighbor. While the
project is currently stalled, our goal remains to continue our discussions to identify acceptable solutions to mitigate the
issues and concerns surrounding the Project. To that end, we will be addressing some of the more frequently-asked
questions we have heard, the first of which is the following:
Why Not Underground?
It’s important to understand the difference between distribution and transmission lines. One of the main differences
between the two line types is the voltage each is designed to carry.
1. Transmission lines carry high-voltage electricity (115,000 volts for the proposed line) from the generation
source (such as the Bonneville Dam) to substations.
2. At the substation, electricity is reduced from transmission voltage to distribution voltage and supplied to multiple
distribution lines.
3. Distribution lines carry the reduced voltage (25,000 volts on the PUD system) for delivery to communities and
neighborhoods where it is again lowered for use in homes and businesses.
Undergrounding distribution lines that feed communities and businesses is more common and less complex than
undergrounding high-voltage transmission lines. Overhead transmission lines are considered the industry standard.
When deciding whether or not to underground transmission lines, electric utilities must weigh the costs and benefits to its
customers. Underground transmission lines may improve aesthetics to some degree and reduce the frequency of
outages. Overhead transmission lines are less expensive to construct and maintain, quicker to repair, and less disruptive
to the environment. When Tillamook PUD chose to design the preferred transmission route overhead, we considered a
number of factors including the following:
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P ow er Outages
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Construction I m pacts
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Depending on the types of overhead structures used, an overhead transmission line typically requires a
foundation that may be 2 feet or more in diameter. Such foundations can vary from 600 to more than
800 feet apart.
At a minimum, an underground transmission line requires a continuous trench at least 5 feet in width at
the bottom and a depth of 5 feet.
Concrete manholes or large splice vaults are needed at recurring intervals. During repairs, a whole
segment between these vaults may need to be excavated again.
Visual I m pacts
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Any damage with underground transmission is difficult to pinpoint and repair. Required repairs may take
several weeks to months to complete.
Conversely, damage to overhead transmission lines is easier to locate and typically takes several hours to
days to repair.
Costs
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Underground lines have less visual impact than overhead lines. However, steel termination poles must
be used at each end and are visible.
An underground transmission line has significantly higher costs versus an overhead line. In general,
underground transmission lines can cost 8 – 12 times more than overhead transmission, and in many
cases, even greater.
An underground line must be routed to avoid other underground installations, such as water and sewer.
Due to different physical, environmental, and construction needs, underground transmission lines are
more complicated to construct and repair, and require specialized personnel to perform construction and
maintenance work.
Overhead transmission line costs are typically covered by all customers, whereas with undergrounding,
the local customers who benefit typically pay the difference in costs between the two.
At the City of Tillamook’s request, Tillamook PUD evaluated undergrounding a route segment along Second Street from
the Tillamook County Courthouse to the Tillamook County General Hospital and calculated it to be nearly 18 times the
cost of the equivalent length of an overhead route along the South bank of the Hoquarton Slough.
 Conservative construction costs to underground this segment were estimated at $5,069,000.
 Cost estimates of an overhead transmission line along an equivalent length were $289,000.
 The underground alternative for this section would require two large, self-supporting dead-end structures
measuring approximately 5 feet in diameter at the base at each end of the underground cable locations.
 It would also require eight large (8 feet deep by 8 feet wide by 17 feet long) concrete vaults to be buried under
the streets. High water tables and ground water issues would be problematic in these facilities.
For these reasons, among others, burying the transmission line was not a feasible alternative.
Underground cable and smaller overhead conductor
Workers in a typically-sized underground vault
Photos courtesy of Georgia Transmission Company