On paper, an overhead line project is a set of straight lines and pole symbols. In the field, the conductor hangs as a curve between two poles, and that curve changes constantly with the weather. On a hot summer afternoon the conductor lengthens and sags; on a cold, icy winter morning it shortens, tightens and transfers far greater forces to the poles. Whether a project is sound depends on both of these extremes having been calculated in advance.
In practice, this calculation often runs separately from the drawing. The route is read from a CAD file, spans and elevations are typed into a spreadsheet, sag values are worked out in another calculation file, and clearance is checked by hand on the profile. Every transfer is a chance for error, and tracing which value rests on which assumption gets harder.
What Is Sag and Why Does It Change?
Sag is the maximum distance by which the conductor between two poles hangs below the straight line joining its support points. It is determined by the conductor's own weight, any additional loads on it (ice, wind) and the tension in the conductor. Higher tension means less sag; lower tension means more.
The difficulty is that tension is not constant. When temperature changes, the conductor changes length through thermal expansion; when extra load is applied, it stretches elastically. These two effects balance each other, producing a different tension–sag pair for every weather condition. So there is no single sag value; a project needs a sag table, calculated for a range of temperature and load conditions.
The Change-of-State Equation and the Catenary Solution
At the heart of the sag table is the change-of-state equation. Starting from the conductor's tension in a known reference condition, it calculates the tension in another temperature and load condition. It accounts for the conductor's modulus of elasticity, thermal expansion coefficient and span length.
Once tension is known, the conductor's shape follows. A cable hanging freely under its own weight between two points forms a catenary curve. A parabolic approximation may be enough for short spans, but on long spans and terrain with large elevation differences the catenary solution is more accurate.
The Tension Section: The Right Unit of Calculation
A line should be calculated tension section by tension section, not span by span. On suspension poles the conductor hangs from insulator strings, and tension differences between neighbouring spans are balanced by the swing of those strings. Dead-end poles, by contrast, fix the conductor and separate one tension section from the next.
Spans within the same tension section therefore work at a common tension, determined by the span structure of that section. A sag calculation that does not separate tension sections correctly makes some spans look tighter or slacker than they really are.
Another key output of the tension section calculation is the uplift check. A pole at the bottom of a deep valley can lose its load in cold weather as the conductors on either side pull upwards. If this is not flagged on the profile, it shows up in the field as insulators loaded in the wrong direction.
Ground Clearance: The Distance Between Conductor and Ground
Ground clearance is the minimum distance from the conductor, in its least favourable state (maximum sag), to the ground or any object beneath it. It is the most critical check for public safety. The maximum sag temperature is a project assumption and must be stated explicitly.
Clearance is not checked only at mid-span. On sloping terrain, the point where the conductor comes closest to the ground may not be in the middle of the span; a ridge, an embankment or a rise in the side profile may be the real critical point. That is why reading the terrain profile, side profiles and obstacles correctly from the field survey matters as much as the check itself.
Crossings: Roads, Railways, Power Lines and Pipelines
When the route crosses other infrastructure, the check gains another dimension. At road, railway, power line and pipeline crossings, both vertical and horizontal distances are checked against rules that differ by crossing type. At power line crossings, the crossed line's own conductor also sags with temperature; the closest approach between the two lines should be calculated using both lines' condition-corrected positions.
Recording which rule each crossing was checked against also eases approval: a reviewer wants to see how a "compliant" result was reached.
Making Assumptions Visible
In line calculations, the assumptions behind a result matter as much as the result itself: the reference condition, maximum sag temperature, conductor type, ice and wind conditions, acceptance limits. Left in hidden spreadsheet cells, they make errors hard to find when the project is revised or reviewed. Showing each result next to its assumptions makes the calculation reviewable.
Conclusion: Run Sag and Clearance Checks from the Drawing with GeoEner Proje Analiz
GeoEner Project Analysis reads the line project from the preliminary project or as-built and site handover drawing version in GeoEner. The main line and branches are defined by clicking poles on the map, and a field survey file brings terrain, side profiles and obstacles into the calculation. In the Route Workspace, the profile is drawn at sheet scale, with the conductor curve, clearance violations and uplift points marked directly on it.
The sag table is calculated per tension section using the change-of-state equation and catenary solution: dead-end poles split the sections, uplift and conductor breakage are checked, and the maximum sag temperature is configurable. The minimum conductor-to-ground distance in each span is compared with the acceptance limit, and vertical and horizontal distances at road, railway, power line and pipeline crossings are checked by type, with the crossed line's conductor height corrected for temperature. Every result is shown with the assumptions it rests on, and sag and clearance sheets come out as PDF.
If you would like to produce your line project calculations straight from the drawing, with no manual transfer, get in touch and let's plan a demo on one of your own projects.
