What formula does this sag calculator use?

It uses the parabolic sag approximation, D = w x L squared / (8 x T), where w is conductor weight per foot, L is span length, and T is horizontal tension. This is the standard field formula and is accurate when sag stays below roughly ten percent of the span.

Why does sag change with temperature?

Aluminium and copper conductors expand when heated by current and sun, so the wire lengthens and tension drops, which increases sag. The tool applies a first-order thermal correction using each metal's coefficient of expansion to estimate hot-day sag from your stringing-day tension.

When does the parabolic formula stop being accurate?

The parabola assumes a uniform horizontal load and small sag. Once the sag-to-span ratio passes about ten percent, or for very long crossings, you should switch to the exact catenary equation or a change-of-state sag-tension program that accounts for ice, wind, and creep.

Does this replace a sag-tension program?

No. It is a quick estimate for inspection and planning. Final stringing charts, NESC clearance checks, and ice and wind loading require a full program such as SAG10 or PLS-CADD with the manufacturer's conductor data.

What is the sag-to-span ratio used for?

It is a quick sanity check on the result and on the validity of the parabolic model. Typical distribution spans run a few percent. A high ratio warns you that the wire is loose or lightly tensioned and that you should verify ground clearance carefully.

Overhead Wire Sag & Tension Calculator

Overhead line workers and utility designers need to know how far a conductor will droop at the middle of a span so the wire keeps legal clearance over roads, driveways, and other circuits. This calculator returns midspan sag and the horizontal tension that produces it, for common ACSR, all-aluminium, and copper conductors.

How it works

A conductor hung between two poles takes the shape of a catenary, but for ordinary distribution spans a parabola is an excellent and far simpler approximation. The midspan sag is:

D = w x L^2 / (8 x T)

where w is the conductor weight per foot, L is the horizontal span, and T is the horizontal tension. The length of wire actually consumed by the span is slightly more than the straight-line distance:

S = L + 8 x D^2 / (3 x L)

The difference, S - L, is the slack you must pull in when sagging the line.

Temperature, clearance, and notes

Conductors expand as they heat, so tension falls and sag grows on a hot summer afternoon. The tool applies a first-order correction using the metal’s coefficient of linear expansion to estimate service-temperature sag from your stringing-temperature tension. Always size sag for the worst case: maximum operating temperature for clearance, and maximum ice and wind for structural loading. Verify the final numbers against NESC clearance rules and a full sag-tension program before stringing.