Ruling (Equivalent) Span for a Line Section

It is a cube-weighted mean, so the long spans dominate it: a section of four spans where one is nearly twice the others lands well above the arithmetic average, because that long span governs how the whole section moves when the conductor heats up. Two consequences a crew acts on. First, the stringing chart is entered at the ruling span, not at the span in front of the truck. Second, once the section is sagged to one tension, the sag in any individual span scales as the SQUARE of its own length over the ruling span -- so on a 300, 450, 380, 520 ft section with a 435.96 ft ruling span sagged to 12.0 ft, the 300 ft span sags 5.68 ft and the 520 ft span sags 17.07 ft. That is a factor of three across the same conductor at the same tension on the same day, and pulling all four to 12 ft leaves the short spans badly overtensioned and the long ones in the road. The tailboard approximation, average plus two thirds of the difference between the longest and the average, is reported beside the exact value because it is what actually gets used -- and seeing the two together shows when it is good enough. On that same section it lands at 484.2 ft against 435.96, 11.1% high, which is exactly the case its own rule of thumb warns about: it is close when the spans are similar and it drifts when one span dominates. Eight span slots are provided and the unused ones are left at zero; a deadend-to-deadend section longer than that is rare, and a section is defined by its deadends rather than by how many structures it crosses. This is section geometry only. It does not compute a sag or a tension -- enter a reference sag and it distributes that -- and it does not check clearance, evaluate whether the section should be broken at a deadend, or address inclined spans, where the ruling-span assumption of equal tension throughout the section is weaker. Long, unequal sections and sections with large elevation differences depart from the ruling-span idealization enough that the utility's own stringing charts and standards govern.

ruling span RS = sqrt(sum of L^3 / sum of L) over the spans between deadends; sag in an individual span = sag at the ruling span x (L / RS)^2; field approximation = average span + (2/3)(longest - average).

The ruling (equivalent) span relation as standard overhead line practice, by name. The utility's stringing charts, sag tables, and construction standards govern.

Geometry on span lengths the crew measures; no stringing chart is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: span_1_ft, span_2_ft, span_3_ft, span_4_ft, span_5_ft, span_6_ft, span_7_ft, span_8_ft, ruling_span_sag_ft, ruling_span_ft, average_span_ft, field_approximation_ft, longest_span_sag_ft, shortest_span_sag_ft, sag_spread_ratio

• Equal tension throughout the section the ruling-span idealization, weakest on steep or very unequal spansoverhead line practice
• It computes no sag or tension a reference sag is entered and distributedtile scope
• A section is defined by its deadends not by how many structures it crossesoverhead line practice