The cable manufacturer can supply detailed data, which include thermal expansion and creep. These factors
increase the arc length after initial stringing, resulting in an increased sag.
The calculation for sag and tension is based on the equation for parabolas. This equation closely approximates a catenary curve for small deflections, as given by:
TH = ((s x s ) x w ) / 8d (in pounds)
where: TH= horizontal tension in conductor or messenger, in pounds
s = length of span between supports, in feet
w = weight of cable assembly, includes supporting conductor/messenger, saddles, lashings, etc., in pounds per foot
d = sag, in feet
The total messenger tension, at its support, consists of a horizontal and a vertical component. The vertical component has been neglected.
The tension shall not exceed:
a) 50% of rated breaking strength of the messenger under the assumed ice and
wind loading
b) 25% of rated breaking strength for final unloaded tension at 60°F (15°C)
Ice and wind loading on aboveground cables and conductors are determined by location. The NESC divides the United States into three loading districts—Light, Medium, and Heavy.
The weight of the ice, force of the wind, and resultant weight of the cable can be calculated by the following equations.
i = 1.24 x t x (D+t)
p = (P(D+2t)) / 12
WL = Sqrt (((W+I)x(W+I)) + (hxh) + K)
Where:-
i = weight of ice, in pounds per foot
t = thickness of ice, in inches
D = outside diameter of cable, in inches
h = force of wind, in pounds per foot
P = horizontal wind pressure, in pounds per square foot
WL= resultant weight of loaded cable, in pounds per foot
W = weight of cable only (i.e. without ice), in pounds per foot
K = constant from NESC Table 251-1
Values of t, P, and K are presented in Table 11. This information is extracted from Tables 250-1 and 251-1 of the NESC.
TABLE - 11
| LOADING DISTRICT VARIABLES | |||
| Variables | Loading District* | ||
| Heavy | Medium | Light | |
| Dial thickness of ice (in.) | 0.5 | 0.25 | 0 |
| **P, horizontal wind pressure (Pa) | 4 | 4 | 9 |
| K, a constant (lb/ft.) | 0.3 | 0.2 | 0.05 |
| Temperature (°F) | 0 | 15 | 30 |
Additional information can be found in ICEA Publication P-79-561 “Guide for Selecting Aerial Cable Messengers and Lashing Wires.”
Typical breaking strengths of messengers are presented in the following table:
