1. Engineering Background: A Single Number Hiding a Whole Load Case

In a pole schedule, span length appears as one figure that the line designer sets and the pole supplier works to. That single number quietly determines vertical load, wind area, axial compression, moment at every angle, and often whether the pole needs one section or two. A 132kV line with 320 m spans is a different structural problem from a 132kV line with 180 m spans, even with identical conductors and identical terrain, because every load on the pole scales with the length of conductor it carries. In practice, span and tension decisions are made early in route design and are rarely revisited, yet they are the cheapest place to reduce structural cost. This article explains how the two variables interact, what the pole supplier actually needs to know, and where the usual design errors occur.

2. Technical Analysis: Vertical, Horizontal and Eccentric Loads

Vertical load. Each pole carries half of the back span plus half of the ahead span of conductor weight, plus fittings, insulators and ice where applicable. On undulating terrain, the weight span can exceed the wind span substantially, and in extreme cases a pole on a crest carries double the conductor length implied by the nominal span. Designing from span alone, without the profile, under-sizes the shaft in compression.

Horizontal load. Conductor wind load uses wind span times conductor diameter times design pressure times drag coefficient. Because conductor diameter is fixed by the electrical design, the only lever is span, and horizontal load therefore scales linearly with span. Combined with the lever arm at the arm elevation, conductor wind moment typically dominates the base moment on long-span suspension poles.

Everyday and maximum tension. Tension is set by sag limits at everyday conditions and rises at maximum wind and minimum temperature. Everyday tension in the range of 18 to 25 percent of ultimate tensile strength and maximum tension in the range of 40 to 50 percent are common practice for ACSR, but the governing values must come from the project stringing chart, not from a rule of thumb.

Line angle. At every angle pole the two tension vectors produce a resultant directed toward the outside of the angle. Up to about 15 degrees the resultant may be carried by a suspension pole with reserve capacity; between 15 and 45 degrees the pole becomes a tension structure requiring guys or a much heavier section; beyond 45 degrees, or at a dead-end, the full tension acts in one direction only and the broken-wire case usually governs.

Unbalanced and broken-wire loads. The loss-of-one-conductor case is highly eccentric: a single 20 kN tension at 12 m produces 240 kNm, which often exceeds the entire wind load case. Three levers control it: reduce ruling tension, add a guy at critical poles, or specify a heavier wall and deeper foundation.

3. Technical Parameters: Load Comparison and Worked Example

Indicative values only; confirm against the project stringing chart and wind zone.

Example, indicative: 150 mm squared ACSR at 1.2 kg/m, three phases, 250 m wind span, 40 m arm height above ground and 12 m arm height above base plate. Vertical load per phase is about 300 kg before fittings; three phases plus earthwire give roughly 1.1 tonnes, or about 11 kN of axial load on the shaft. Conductor wind load at 700 N/m squared with a drag coefficient of 1.0 on a 27 mm conductor gives about 4.7 kN per phase, or roughly 170 kNm at the base. Increasing the span to 320 m raises the same figures to about 14 kN axial and 220 kNm base moment, an increase of nearly 30 percent in moment for a 28 percent increase in span, before second-order effects. That is why span changes must be re-run through the full load case rather than scaled linearly.

4. Design and Verification Recommendations

1. Provide nominal span, ruling span, wind span and weight span separately; do not send a single span figure.
2. Send the stringing chart with everyday and maximum tension, and state the temperature and wind conditions used.
3. List every angle pole with its angle and the pole type proposed, so the supplier can check tension-pole capacity rather than assume suspension duty.
4. Confirm which broken-wire case the utility requires and at which poles, because this single case often sets the base plate and foundation size.
5. State the ice and wind zones for each section of route, not the route average.
6. Require the supplier to return a pole schedule that shows the governing load case for each pole, so the buyer can see why a particular section was selected.

5. Technical FAQ

Q1. Is it cheaper to shorten spans or to strengthen poles?
Usually shortening spans, where right-of-way and terrain allow. A 20 percent span reduction cuts conductor wind area and vertical load proportionally and frequently allows a lighter shaft and a smaller base plate, while a strengthened shaft adds weight to every pole in the section.

Q2. Can a suspension pole be used on a small angle?
Yes where the resultant is within the section capacity and the utility permits it. The check must include the broken-wire case, which is often more severe than the everyday angle load.

Q3. Why does the supplier ask for ruling span as well as nominal span?
Ruling span governs the tension calculation for the whole section. A pole designed on a 180 m ruling span but installed in a 300 m crossing span will see a much higher maximum tension than the design assumed.

Q4. How much does conductor type change pole design?
Directly through weight and diameter, and indirectly through tension limits. Moving from ACSR to a larger diameter AAAC at the same tension can change both the wind area and the sag behaviour, so the load case must be re-run.

6. Conclusion: Give the Supplier the Load Case, Not Just the Span

Span, tension and angle are a single design problem. Supplying the pole manufacturer with a complete load package, including wind and weight spans, stringing tension, angles and the broken-wire requirement, is the fastest way to obtain a pole schedule that is both safe and economical. Our Q355 and Q460 pole ranges for 10-35kV distribution and 69kV sub-transmission lines are configured pole by pole against the actual load case, so a long-span crossing pole and a standard tangent pole are never quoted as if they were interchangeable.