OVERHEAD VS UNDERGROUND
Presented by Mark Contor, Engineering & Operations Manager, Northern Lights Inc., May 2015
“Why not put all overhead power lines in underground?” This is a question that surfaces
whenever there is a large wind or snow storm that damages overhead power lines and causes
prolonged outages, or when someone objects to their trees being trimmed or feel the overhead
lines are not aesthetically pleasing. A quick search on the internet will provide an astute and
objective reader numerous credible white papers and business studies that can be perused and
researched to generally provide a consistent answer. Some studies have been funded by states
that are vulnerable to hurricanes and tornados, some are subjective opinion to support their
argument, and others paint a one color fits all answer. Ultimately, it comes down to basic
economic and operational factors for each individual utility and specific line based on their own
unique and often complex characteristics. These complexities are not transparent or readily
understood by the general public and can be even more confusing when neighboring utilities
have different economic and operational characteristics. The following is a more in-depth look
at many of these unique and complex factors as well as insight to NLI’s system and service area
as well as direct bury vs. cable in conduit:
 Topography and Ground Conditions - Rocks, water table, wetlands, slope stability,
valleys, mountains, rivers, etc. These conditions tend to be much more conducive to
overhead power. Stringing wire up and over avoids the obstacles. It can be faster and
less expensive.
 Actual replacement cost of overhead vs underground for each specific line segment
depends on numerous things such as: How many taps, service drops and other
manmade obstructions are involved i.e. phone lines, gas lines, paved roads / driveways,
water/sewer lines, septic lines, private power lines, cable TV, fiber optic cables,
manicured lawns, fences, buildings, ditches, culverts etc. Converting overhead to
underground in areas congested with these obstructions significantly increases the cost
per foot of underground. Rather than simply going over the top, crews spend
considerable time locating, hand digging, directional boring, sometimes installing
conduit, pull rope, below grade warning ribbon, installing shoring or sloping the
excavation for worker safety, back filling and compacting trenches, pulling in cable and
then landscape restoration. The 2015 projected price to replace one foot of existing
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single phase underground cable with direct bury (no conduit) is $26.50 per foot. This is
because since its initial installation 30 to 40 year ago we now have to deal with the
above mentioned utilities and improvements. To rebuild a single phase overhead line
costs about $16 -$17 per foot. As our member line density continues to increase these
underground costs will also increase.
How many accounts, taps, service drops does the line serve and what type of accounts
(commercial loads)? The more service drops and taps the more costly to convert from
overhead to underground. Underground pad mount transformers are about double the
cost of overhead. Cable terminations cost a lot more and take more time to install. Pad
mount (underground) load break or sectionalizing switches can cost 10 to 20 times more
than overhead and there are not as many options in equipment or features.
Longevity of line. Underground cable has lower life expectancy than overhead wire
(sometimes 20 to 40 years less).
Remaining life of existing overhead line. Replacing an overhead line with 15 years or
more useful life is generally a low priority compared to replacing or repairing other
aging infrastructure such as: existing outage plagued underground cable, old
substations, antiquated power conditioning and sectionalizing equipment, upgrading
and replacing generation facilities, replacing old poles where underground is not an
option, tree trimming, replacing wore out equipment etc.
Outage costs – What is the outage frequency, extent of damage, cause and outage
duration. Sometimes overhead is better, sometimes underground is better.
Who is paying for conversion or installation? Private developers or the utility.
Is it a main feeder and how much load on the line? Can the line regularly be taken out
of service for repair, additions or changes? Is the line regularly switched and used to
shift load between substations. For operationally efficiency, cost and availability of
equipment overhead is preferred, and it does not get buried in snow and brush.
Generally, overhead electric service does not have to be taken out of service to cut in
new services, install taps, transformers, and power quality equipment. Unless there is
an underground junction hut with available space underground requires the line to be
de-energized to add these features. This can be a huge issue if the line has a lot of load,
a lot of members, commercial businesses, water & sewer, communications, schools etc.
Service interruption can be avoided with underground if the utility has funding and right
of way to install loop feeds to isolate the section of line being taken out of service.
However, it is much easier to establish loop feeds in cities and suburban areas with
blocked streets rather than rural areas which tend to be long radial feeds with no
parallel or looping right of way opportunities.
Would the overhead to underground conversion be direct buried or would it require
conduit and spare conduit? Due to increased heating, cable in conduit significantly
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increases the cost, both to install the conduit and because the heating reduces the
ampacity of the cable which may require larger cable to compensate for the loss of
ampacity which then also increases the size of the conduit.
 What are tree vegetation management costs for the segment of line being considered
and or NESC required clearances from other structures? In some instances the
additional cost of converting to underground may be justified because of narrow right of
way and public safety concerns.
Other Factors Include:
System wide how much line would the utility have to convert? Many utilities similar to NLI
started building out their lines in the 1930’s when there was virtually no reliable underground
cable and equipment available. Consequently, the largest percentages of most electric systems
were built out overhead. NLI’s system was financed over many decades through member aid to
construction and low interest financing through the U.S. Department of Agriculture Rural
Electrification Administration program. NLI has almost three times more overhead than
underground. Its old underground is failing faster than its overhead. To convert large sections
of existing overhead to underground is very expensive and not always practical. Unlike new line
extension it does not generate new load or establish new territory and receives no aid to
construction (financing) from developers or new members. Financing for these projects is only
supported through electric rates and monthly service fees. Cost conscientious members and
member elected Boards are reluctant to finance or increase debt to replace existing lines with
no or very little return on investment or that don’t significantly improve power quality or
increase the cooperatives equity.
A cost that receives no consideration is the utilities huge cost to finance the Call Before You Dig
law for buried facilities. To cover 3 states and 6 counties NLI has to pay 3 different one call
services (811) to receive Call Before You Dig notifications. Then NLI pays either NLI personnel
(including vehicle and equipment) or a contractor to actually field mark the cable location. In
2014 NLI received 6,200 locate requests that had to be screened by NLI dispatchers or contract
locating services. Of these 6,200 calls NLI had to pay locating personnel to drive to the location,
hook up equipment and perform almost 3,000 locates. The total 2014 annual cost for this was
over $150,000. As underground facilities and human development expand this expense also
increases.
It takes longer and more equipment to locate underground faults than over overhead faults.
After the specific location of the underground fault has been located the crew must notify
dispatch who calls 811 Call Before You Dig. Then the crew waits about 2 to 4 hours on overtime
before a locator(s) arrives to locate all the other buried utilities in the vicinity before they can
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start digging to repair the faulted underground cable. This significantly increases the outage
cost and the outage duration. Phone locates in Idaho generally require about 2 hours while
Sanders County phone locates may take as long as 4 hours for the locator to drive from the east
side of Missoula. This waiting time is often misunderstood by members without power who
question crew productivity.
 Cost of wire Underground vs. Overhead (not including labor, equipment or
terminations) - In 2015 underground 1/0, 260 mil wire was $2.20 per foot. Overhead
1/0 ACSR was $0.235 per foot. Underground cable includes a neutral wire so to fairly
compare the cost of single phase wire you double the cost of the overhead wire which
then makes overhead $0.47 cents per foot. However, for a 3 phase line there is still only
one neutral so the cost per foot cost for 3 phase underground cable is $6.60 and
overhead is $0.94.
 Underground transformers and junction huts routinely get damaged by snow plows.
This is a problem at Schweitzer as well as all areas when there is a large snow event.
 Underground is much more vulnerable to lightning.
 Underground is out of site and damaged by excavators who don’t follow Call Before You
Dig laws. This is similar to loggers and members falling trees into overhead lines.
 Because underground is out of site new property owners and contractors tend to be
unaware or ignore our easement rights and encroach on our right of way when building,
excavating, installing new roads and landscaping. They remove ground reducing
required depth or place large amounts of fill or obstructions over the line. Years later
when the encroachment is discovered it is hard to determine when it was done and who
was responsible. This is similar to members who occasionally build under the overhead
lines however these encroachments are easier to discover.
 Grounding - Long stretches of underground (especially in conduit) make it more difficult
to install or add system grounds. Good grounding is essential for electric systems to
operate properly.
 Underground can be more confusing to map and for line crews to determine feed
directions.
 Underground is difficult and expensive to install in narrow road right of way corridors or
where there is a lot of steep cut and fill slopes, other utilities, drainage ditches or
fencing.
 Underground is very susceptible to washouts on steep slopes or sandy loams.
 Repairing underground in the winter when it’s buried in snow or under frost is slow and
difficult.
Outage Expense Overhead vs. Underground NLI
Based on the total footage of overhead wire compared to the total footage of underground
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wire over a 10 year period revealed there is no significant difference in cost. The average cost
per ft. of overhead outages was $.0456 per ft. and underground was $.0458 per ft.
Cost of Installing New Underground VS Cost of Replacing Old Underground
New Services constructed with underground cable can usually be installed fairly quickly and is
much less labor intensive than replacing old existing cable. The new member understands the
ground is going to be disturbed and is responsible for any cosmetic clean up or landscaping
rehabilitation. If there are road bores, poor plowing or digging conditions or other obstructions
the member is required to pay to provide the ditch and conduit and pay for the directional
bores and vacuum trucks. All these costs are the utilities responsibility when it comes time for
cable replacement. And as previously stated after the cable is installed and additional utilities
and improvements are made these costs significantly increase.
Direct Buried Cable (DBC) VS Cable In Conduit (CIC)
Direct Bury Cable
 DBC is cheaper and faster to install.
 Causes less ground disturbance at time of initial installation because it is generally
buried with a plow tooth where CIC requires trenching, backfilling and compacting.
 Faster and cheaper to fix faulted cables or cut into for adding taps and services.
 DBC causes a lot more ground disturbance to replace which is problematic in yards and
under driving surfaces. For this reason NLI usually requires lots in subdivisions to have
cable in conduit.
 DBC does not offer the same amount of cable protection as CIC.
 With Direct bury the ground helps dissipate heating caused by current, whereas conduit
traps the heat which reduces the ampacity of the wire and reduces the life.
 Direct bury cable can be replaced while leaving the existing cable in service. Cable in
conduit requires the old cable to be de-energized so it can be pulled out and new cable
installed.
Cable In Conduit
 CIC is a lot more expensive to install.
 Pinpointing the location of faults in CIC is sometimes harder than DBC.
 CIC offers greater cable protection.
 CIC does not dissipate heat caused by current as efficiently as DBC and therefor the
cable ampacity is less and or larger cable needs to be installed to compensate.
 When CIC faults and the cable needs to be repaired there are 2 options;
1) Cut the conduit and add a cable splice which is much faster, cheaper and
minimizes outage time. However, once a splice is added the cable cannot be
pulled out and new cable installed at a later time.
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2) Pull the faulted cable out and pull all new cable in which takes a lot of time,
and more money. It also increases the outage restoration time because it
requires all new cable and terminations and requires more equipment. The
advantage is the cable can usually be pulled out and replaced at a later date.
 Conduit (CIC) occasionally gets damaged or gets water infiltration. Over time this
infiltration plugs the conduit and eliminates the ability to pull the old cable out and new
cable in. Therefore, replacement sometimes requires trenching a new ditch and
installing new conduit. Sometimes spare conduits are added to eliminate this problem.
However, experience has shown after 15 plus year’s unused conduit is prone to getting
broke and infiltration which renders it useless and ends up being a waste of money.
 To replace cable in conduit the crew has to de-energize the line to pull out the old cable
and pull new cable in. The member is out of service during this time. This is not the
case with direct bury cable.
Overhead Issues
 Overhead is more susceptible to tree damage from wind and snow.
 Overhead is more susceptible to bird and squirrel caused outages.
 Overhead may be a concern with raptor related issues in certain areas.
 Aesthetically, the public prefers underground rather than overhead.
 Overhead requires more money to side clear right of way. Underground requires
clearing but generally requires only minimal low level side trimming.
 Overhead presents additional concerns of tree related fires and public exposure to
downed wire.
NLI’s basic philosophy of replacing overhead with underground is determined by economics,
operational efficiency, terrain and opportunity. That being said we have replaced several
significant portions of outage prone overhead line with underground when conditions and
timing make it economically feasible. These conditions typically include several of the
following:
 Areas with a lot tree related outages.
 Overhead lines that have come to the end of their useful life or need to be
relocated.
 Areas that have relatively favorable terrain and plowing conditions - Few large
rocks, stable ground (no swamps, creeks, steep sandy slopes prone to washout
etc.)
 Low member density. This reduces a lot of additional expense for hand digging,
directional pipe bores, vacuum trucks to avoid damaging other buried facilities,
and it minimizes the cost to convert, transformers, service drops and taps.
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 Lateral taps or lightly loaded feeder lines that do not require expensive
sectionalizing and power conditioning equipment. Or don’t require frequent
switching and load transfer between substations.
 Lines that are not in or adjacent to narrow road corridors with a lot of cut and fill
slopes and don’t crisscross roads / highways, creeks, gas lines, phone lines,
yards.
We have converted several long sections of overhead to underground. Some were converted
with FEMA funding during the ice storm of 1996/1997. Some were funded by NLI on long
stretches of relatively flat terrain going through U.S. Forest Service land to replace old overhead
lines. Some were done in conjunction with large developments, highway projects or where the
developer contributed to the conversion.
When terrain allows most new services are designed to be underground and we may require
developers and new service applicants to install underground lines in areas that we know will
be susceptible to tree related outages.
As our aging overhead lines near the end of their useful life and underground cable technology
and equipment improves NLI will look at opportunities to convert more overhead to
underground where and when conditions are favorable. However, in NLI’s service terrain and
for reasons mentioned above converting overhead to underground may not always be the most
economical and operationally the best solution.
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