Ruling Span in Transmission Line Design: Definition, Formula, Examples, and Engineering Standards
Ruling Span in Transmission Line Design: Definition, Formula, Examples, and Engineering Standards
An authoritative guide for electrical, transmission, and power systems engineers.
What Is Ruling Span?
Ruling Span is the single "equivalent span" used to compute sag and tension in a tension section with multiple unequal spans. It represents the mechanical behavior of the entire section and ensures uniform tension and safe clearances.
Engineering definition: The weighted average span length that produces the same total tension as the actual individual spans.
Why Ruling Span Matters
- Determines sag and tension
- Affects ground clearance
- Critical for safety and reliability
- Required by ASCE, NESC, IEC, NGCP TLDS
- Used in PLS-CADD for accurate mechanical modeling
Ruling Span Formula
LR = (L1³ + L2³ + ... + Ln³) / (L1² + L2² + ... + Ln²)
Example Calculation
Given spans: 220 m, 180 m, 260 m
- Numerator: 220³ + 180³ + 260³ = 34,056,000
- Denominator: 220² + 180² + 260² = 148,400
Ruling span = 229.4 m
Ruling Span vs. Maximum Span
| Term | Meaning | Purpose |
|---|---|---|
| Ruling Span | Weighted average mechanical span | Sag & tension |
| Maximum Span | Longest actual span | Tower loading |
Ruling Span in PLS-CADD
PLS-CADD automatically computes ruling spans and applies them to:
- Sag/tension tables
- Stringing tensions
- Clearance checks
- Insulator swing and blowout
Terrain Effects on Ruling Span
Uneven terrain changes span lengths. Ruling span smooths these variations and provides a stable basis for sag design.
Rules of Thumb
- Use ruling span for sag/tension only
- Use actual span for tower loads
- Recalculate ruling span after re-spotting
- Very long spans heavily influence ruling span
Standards on Ruling Span
- ASCE 74 – Defines equivalent span
- IEC 60826 – Mechanical reliability design
- NESC – Safety and clearance requirements
- NGCP TLDS – Uses ruling span for sag tables
Common Mistakes
- Using simple average instead of correct formula
- Mixing spans from two tension sections
- Using ruling span for tower loading (incorrect)
- Ignoring extreme span lengths
Summary
Ruling Span is the backbone of sag and tension design in transmission engineering. It ensures uniform conductor behavior, maintains safe clearances, complies with IEC/ASCE/NESC/NGCP standards, and produces accurate results in tools like PLS-CADD.
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