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Other Code
How Hot is That Wire? |
The conductor temperature is a critical part of the design, construction, and checking clearances of aerial electric supply lines. |
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The conductor temperature is a critical part of the design,
construction, and checking clearances of aerial electric
supply lines.
Line Design
In my article on "Overhead Line Design From Scratch
– Part 1" in the March/April 1998 issue of IAEI News, I
discussed conductor choice based upon steady-state ampacity,
transient ampacity, short-time ampacity, voltage drop,
sag/tension characteristics, cost and losses. The ampacity of
the conductor is a function of the conductor’s physical
properties, conductor maximum operating temperature and
ambient weather conditions, including air temperature, wind
velocity and solar intensity. For insulated conductors, the
maximum operating temperature is the maximum temperature of
the insulation material. For bare conductors, the maximum
operating temperature is chosen by the designer of the aerial
electric supply line.
How high can we go? Economics limit the
conductor temperature well below the temperatures that can
actually damage the conductor. As the conductor temperature
increases, the sag increases. The sag increase requires us to
use taller structures or more structures to maintain good
clearances. Both increase the cost of the line. As the
conductor temperature increases, the resistance of the wire
increases. The resulting increase causes greater voltage drop
and higher losses. If we set the maximum operating temperature
too low, we end up using a much larger and more expensive
conductor than necessary. 212° F. is commonly chosen. Once we
choose a maximum operating temperature and design the line
based upon that temperature, we must make sure the operating
personnel do not allow the design ampacity to be exceeded.
An industrial plant manager once asked me
the ampacity of the 12kV lines that feed the various buildings
in the industrial complex. The lines had been designed and
constructed by a consultant about twenty years before his
request. There was no record of the ampacity/maximum operating
temperature used to design the line. We measured the ground
clearances and conductor sags for several of the longest
spans. We also measured the conductor temperatures. We
calculated the conductor tension and plugged it into a
sag/tension computer program to determine sags at various high
temperatures. Since we measured conductor heights above ground
at the points of attachment on the structures, we calculated
the maximum conductor sags, which would comply with the NESC®
requirements. The plant manager was not happy to find out the
line was constructed for a maximum operating temperature of
130° F. and that they were already exceeding the ampacity of
the line. The line was in violation of the NESC®
requirements. The line was probably field designed.
Line Construction
Once we decide what maximum operating temperature we will
use to design the line, we then have to determine the
stringing sag/tension we will use to construct the line. As
discussed in my March/April 1998 article, we get the
sag/tension stringing details from the sag/tension computer
program. The temperatures listed by the program are conductor
temperatures, not air temperature. On a hot sunny day, it is
not uncommon for a conductor not carrying any current to be 50
degrees hotter than the air temperature. The line crew
installing and tensioning the conductors must be given the
sag/tension details from the computer program and must have an
accurate way of measuring the conductor temperature at the
time of tensioning. I give our construction personnel a graph
of stringing sag in inches versus conductor temperature so
they can pick off the appropriate sag for their conductor
temperature.
Conductor Temperature
Measurement
Hand-held electronic thermometers with surface probes work
very well for measuring the temperature of "dead"
conductors like during the stringing process. When we are
checking the clearance of an existing energized line as
discussed in my July/August and September/October 1999
articles, measuring the conductor temperature is much more
difficult. The electronics of most thermocouple type
electronic thermometers don’t like high voltage. The devices
turn off when the energized wire is contacted. I understand
that some thermistor-type electronic thermometers have the
same problem. The device I use is a thermistor type and is no
longer available. I’m working with two manufacturers to find
a device that will work. When I find a production device that
works, I will let you know. Most infrared thermometers don’t
work because the target diameter is too large. For clearance
measurement, we want to be able to measure the conductor
temperature from the ground. The device I use is mounted on a
"V" shaped guide on the end of a 50-foot fiberglass
extension pole (the same pole I use to measure vertical
clearances). The probe is mounted at the bottom of the
"V" so that the probe doesn’t miss the conductor.
An electronic thermometer with a "peak" function is
necessary so that you can recall the conductor temperature
after you lower the device to the ground.
The Alternative
As discussed in my March/April 1998 article; IEEE®
Standard 738-1993 is one source that can be utilized to
determine the ampacity of bare conductors. The calculation
method presented in the standard and the associated computer
program (included with the standard) can also be used to
predict the conductor temperature given the conductor’s
physical properties, current flow in the conductor and weather
conditions. The results are only as accurate as the input
data. The input data consists of thirteen variables, which are
not easy to measure.
1. Air temperature
2. Wind speed
3. Angle between the wind direction and the
conductor
4. Altitude of the conductor above sea
level
5. Direction of the conductor relative to a
compass
6. Latitude of the conductor
7. Sun time
8. Visibility
9. Conductor diameter
10. Conductor resistance at 25° and 75°
C.
11. Conductor emissivity
12. Conductor absorptivity
13. Current flow in the conductor
Each of the variables is explained in
detail in the standard. To complicate matters, each variable
is considered by the program to be constant. The field
conditions are not constant. Variations in air temperature,
wind speed, wind direction, visibility and current flow all
affect the results. For that reason, the program can only be
used to roughly predict the conductor temperature.
Conclusion
There is no substitute for actual measurement of the
conductor temperature.
If you have enjoyed this article or have
general questions about the NESC®, please call me at
302-454-4910 or e-mail me at dave.young@conectiv.com.
National Electrical Safety Code®, NESC®
and IEEE® are registered trademarks of the Institute of
Electrical and Electronics Engineers.
Dave Young is a senior engineer with
Conectiv Power Delivery of Wilmington, Delaware where he has
been working with and teaching the NESC® for over 28 years.
He is a member of the NESC® Interpretations Subcommittee and
an alternate on the NESC® Overhead Lines Clearances
Subcommittee.
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