Aerial cable sag & clearance calculator
For lashed aerial fiber and other spans: the mid-span sag at a given tension, and whether the low point still clears the road or driveway under it.
Sag grows with the square of the span and shrinks as you pull tighter, so a long span sags fast. Tightening cuts sag but raises tension, and pulling past about 60% of the cable's rated strength risks it over temperature and ice. Clearance minimums come from the NESC and your AHJ; confirm the governing edition.
How do you calculate cable sag between two poles? For a level span the mid-span sag is the cable weight per foot times the span squared, divided by eight times the tension. So a 150 foot span of a cable weighing 0.2 lb/ft pulled to 500 lbf sags about 1.1 feet. Sag grows with the square of the span, which is why long spans drop so fast.
Common questions
For a level span the mid-span sag is the cable weight per foot times the span squared, divided by eight times the tension. So a 150 foot span of a cable weighing 0.2 lb/ft pulled to 500 lbf sags about 1.1 feet. Sag grows with the square of the span, which is why long spans drop so fast.
Clearance is governed by the NESC and your local authority, and it depends on what is under the span. Communication cables are commonly around 15.5 feet over roads and driveways, less over pedestrian-only areas, and considerably more over a railroad. The calculator checks the clearance under the low point against the minimum you pick, but confirm the exact figure against the governing NESC edition.
They trade off. Pulling the cable tighter reduces sag, but doubling the tension only halves the sag, and higher tension pushes the cable closer to its limit. A common rule is to keep initial tension under about 60 percent of the rated break strength so the cable stays safe as it cools and loads with ice, when tension climbs.
Rearrange the sag formula: tension equals weight per foot times span squared divided by eight times the sag you want. The calculator has a helper that does this, so you can dial in a sag that clears the road and read off the tension to pull to.
This uses the parabolic approximation, which is standard and accurate for the small-sag spans typical of aerial fiber. A true catenary matters mostly for very long or very slack spans; for normal distribution spans the parabola is well within field tolerance.
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