Tension and Sag
The 2007 National Electrical Safety Code (NESC) addresses Tension and Sag in Sections
235. 251. 252. 253, 261, 263, and 277
Table of Contents:
A. Definitions of Tension and Sag
B. Engineering Design Description
C. Methods of Tensioning
D. Reduced Tension Construction
E. Guy Tension
F. Line of Site
G. Sag Charts
H. Loading Zone
I. Grades of Construction
A1. Definition of Tension: Tension is defined in the NESC as “The longitudinal force
exerted on a cable during installation”. In other words tension can be explained as force
pulling the cables or wires at either end by what they are attached to or the weight of the cable itself. Tension is also applied to insulators
There are two main types of tension
; Initial is defined in the NESC as “ The longitudinal tension in the conductor prior
to the application of any external load” In other words the characteristics of the
cable before time, temperature, weight etc. of the conductor stretch or shrink the
; Final is defined in the NESC as “The longitudinal tension in a conductor after it
has been subjected for an appreciable period of the loading prescribed for the
loading district in which it is situated, or equivalent loading, and the loading
removed. In other words what the characteristics of the cable are expected to be
after time, temperature, weight and other factors have effected it.
A2. Definition of Sag: Sag is defined in the NESC as “The distance measured vertically
from a conductor to the straight line joining its two points of support.” In other words sag is usually the lowest point on a cable or wire between two points. This low point is measured as though there were a straight line between the attachment points on either end and the distance between this low point and the straight line is the sag.
B. Engineering Design: The appropriate sag and tension is determined by
; Span lengths
; Strand size
; Storm loading area
; Vertical clearances above grade ; Vertical clearances from other utilities ; Pole lengths
; Class of pole
C. Methods of Tensioning:
Types of Tensioning
Dynamometer are the preferred
method for testing tensioning
A light rope tossed over the conductor near one end of a span and give it a good hard jerk down. At the same instant press the button on the watch to start it. You then feel for return pulses in the rope as the shock wave you created runs up and down the conductor. At the instant you feel the third or fifth return you stop the watch. Read the number on the appropriate scale on the face of the watch and you have your sag in feet. The scales are direct reading and no math is needed.
; Third wave return
The smaller cables represent the
difference in sag after
environmental changes- notice how
the sag between conductors differs.
NESC clearances may not be met
during different temperatures, ice,
wind, etc. or over time.
This method is not recommended
; Matching sag
D. Reduced Tension Construction (Slack Span): Reduced tension is when traditional
guying is not practical. This should be avoided if possible. Reduced tension usually requires guying in the same direction as the reduced tension span. Guying can be avoided with stronger pole
Guying in same direction Using larger class pole
Double Slack Span to provide corner poles and street clearance
E. Guy Tension: Per NESC 261C2 (in layman’s terms) the guy should be considered a part of the structure and designed and installed with the proper tension to support the tension of the attachments it supports. The guy can create an imbalance in tension if installed to tight. The note for this section also clarifies that guys must not be loose.
F. Line of Site: Per NESC 235C3 (in layman’s terms) primary power cables cannot sag below the attachment points of the highest communication cable in spans over 150 feet.
Secondary, Fiber etc is OK Primary below line of site- Violation
G. Sag Charts: Sag Charts are used by most aerial utilities in one form or another to determine how much tension to use to pull the wire up to the appropriate sag. They come in many forms based on a variety of formulas. Some are commercially provided like those mentioned in Resources, some are created in house. They all consist of some basic information:
; Wire Diameter
; Span Length
; Wire Weight
; Supporting Cable Characteristics
; Rated Breaking Strength
H. Loading Zone: Varying environmental conditions create hazards that effect aerial cables differently. When looking at the United States Map the loading zone appears that all of Oregon has a “Medium” loading zone. Special Wind Regions change the standard
“Medium” Loading Zone to “Extreme” loading. There are 34 types of loading zones Heavy, Medium, Light and Extreme.
Oregon appears to be only Medium
Notice the Special Wind Regions
Normally Oregon can expect 85 mile winds- however in “Extreme” areas those winds
may increase to 120 miles or more. The Loading Zone or district determine some of the overload factors used for engineering.
H. Loading Zone (cont.)
; Heavy Loading Districts are generally in the central and northeast U.S. states.
There is an assumption of lower temperatures and greater ice buildup on cables
and conductors. This may require adjustments made such as open-wire
conductors having breaking strength reduced by 50%. Where there are copper or
steel cables or conductors span lengths should be kept to a minimum
; Medium Loading District covers much of the northwestern states including
Oregon. Where the standard for ice in the Heavy Loading District is ? inch, in
the Medium zone it is assumed ? inch. This may require adjustments are made.
One example is an open-wire conductors having breaking strength reduced by
33%. Where a limiting span length in the Heavy zone is 150 ft it may be increased
to 175 in a Medium or Light zone.
; Light Loading District covers most of the southern USA. It has the lightest, most
flexible loading zone. Where the Medium Loading District assumes ? of ice. The
Light Loading District assumes no ice.
; Extreme Loading District covers the entire coast, Columbia Gorge and some
other areas of Oregon. This zone does not have ice but has higher than usual
winds. The current code assumes wind above 60 feet must withstand extreme
wind. The 60 feet exception will likely be removed in the NESC 2007 code.
H. Grades of Construction: There are 3 types of grades of construction. Above
ground utility construction must meet one of the 3 depending on environmental concerns.
; Grade N construction per NESC 263 is the most reduced type. Per NESC 014A2
grade N may be used for emergency construction. This construction must be
upgraded to Grade C or above as soon as possible. Construction must always meet
the minimum of grade N. A planned Grade N construction may be required when
installing and removing facilities overlap. Planned Grade N requires approval of
the OPUC. Grade N does not usually apply to communication facilities where no
supply facilities exist. (NESC 263G)
; Grade C construction is the most common type of construction. Grade C
provides the standard of minimum requirements for items such as strength of
poles, structures, hardware, cross-arms, guys, anchors, foundations and sizes and
sag for supply conductors. Grade C also provides the overload factors needed to
meet the minimum standard.
; Grade B is the highest or most stringent type of construction. Grade B provides
the standard of minimum requirements at greater values then Grade C. This type
of construction is the minimum for installations crossing over railroads,
communication lines or limited access highways. It is also used when the high
voltage of the supply conductor will not be de-energized during breaker
operations. Grade B may be used in Extreme Wind loading areas. Grade B has
more stringent strength and overload factors than either grade N or C.
Engineering for Grade B may require doubling cross-arms, brackets, ties and pins.
; Alcoa Sag 10 (Supply Cables)
; CommScope (Communication Cables)
; NESC 2007