Electric Power Generation, Transmission, and Distribution

whereRS¼Ruling span for the line section containingnsuspension spans
S 1 ¼Span length of first suspension span
S 2 ¼Span length of second suspension span
Sn¼Span length ofnth suspension span
Alternatively, a generally satisfactory method for estimating the ruling span is to take the sum
of the average suspension span length plus two-thirds of the difference between the maximum span
and the average span. However, some judgment must be exercised in using this method because a
large difference between the average and maximum span may cause a substantial error in the ruling
span value.
As discussed, suspension spans are supported by suspension insulators that are free to move
in the direction of the conductor axis. This freedom of movement allows the tension in each suspension
span to be assumed to be the same and equal to that calculated for the ruling span. This assumption
is valid for the suspension spans and ruling span under the same conditions of temperature and load,
for both initial and final sags. For level spans, sag in each suspension span is given by the parabolic
sag equation:

Di¼

w(Si^2 )

8 HRS

(14:27)

whereDi ¼sag in theith span
Si ¼span length of theith span
HRS¼tension from ruling span sag-tension calculations
The sag in level suspension spans may also be calculated using the ratio:
where DRS¼sag in ruling span
Suspension spans vary in length, though typically not over a large range. Conductor temperature
during sagging varies over a range considerably smaller than that used for line design purposes.
If the sag in any suspension span exceeds approximately 5% of the span length, a correction factor
should be added to the sags obtained from the above equation or the sag should be calculated using
catenary Eq. (14.29). This correction factor may be calculated as follows:

Correction¼D^2

w

6 H

(14:28)

whereD¼sag obtained from parabolic equation
w¼weight of conductor, lb=ft
H¼horizontal tension, lb
The catenary equation for calculating the sag in a suspension or stringing span is:

Sag¼

H

w

cosh

Sw

2 H

 1



(14:29)

whereS¼span length, ft
H¼horizontal tension, lb
w¼resultant weight, lb=ft

14.4.4 Stringing Sag Tables

Conductors are typically installed in line section lengths consisting of multiple spans. The conductor is
pulled from the conductor reel at a point near one strain structure progressing through travelers
attached to each suspension structure to a point near the next strain structure. After stringing, the