Sag Calculations for Transmission Lines

A set of sag formulas is presented, in the form of convergent series which give accurately the results of the hyperbolic catenary formulas. The series are useful for calculating almost any practical transmission line span, long or short. Trial and error methods are not used. First, the sag or deflection is calculated for a given maximum tension in the cable. Then, changes in temperature corresponding to changes in deflection and loading are computed. This procedure is followed whether the supports are at equal heights or at unequal heights. A formula is presented for the deflection from the line joining the supports, when the elevations are unequal. This is useful in sighting from tower to tower, as shown in Fig. 2. A few examples are worked out and some derivations of formulas are given. In the last appendix instructions are given with regard to allowing for the change in weight per ft. with change in length of cable.