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 × L² / (8 × 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 × D² / (3 × L)

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

Worked example

A span of 300 feet using 336.4 kcmil ACSR (Drake) with a weight of 1.094 lb/ft, strung to a horizontal tension of 3,500 lb:

D = 1.094 × 300² / (8 × 3,500)
  = 1.094 × 90,000 / 28,000
  ≈ 3.52 feet midspan sag

Conductor length consumed:

S = 300 + 8 × (3.52)² / (3 × 300)
  = 300 + 8 × 12.39 / 900
  ≈ 300.11 feet

That 0.11-foot difference (about 1.3 inches) is the extra wire length you need to allow in the span. At this tension and sag, the sag-to-span ratio is about 1.17%, well within the parabolic model’s accuracy range.

Temperature correction and why it matters

Aluminium and copper both expand as they heat. A conductor at peak summer loading (conductor temperature can reach 75 C or higher during high-current periods plus solar gain) will have significantly more sag than the same conductor on a cool stringing day. If you string to final tension in winter without accounting for summer expansion, the conductor may sag into violation of NESC ground clearances on the hottest days.

The tool applies a first-order thermal correction using the conductor’s coefficient of linear expansion to estimate sag at a different temperature from the stringing condition. The key inputs are your stringing temperature and the design temperature (the worst-case hot day) you want to check.

For aluminium-based conductors (ACSR, AAC, ACAR), the coefficient of thermal expansion is higher than for copper. Mixed ACSR conductors have a composite expansion rate driven by both the steel core and the aluminium strands.

When the parabolic model breaks down

The parabola works well when sag is less than roughly 10% of the span. Beyond that point:

• The parabola underestimates sag compared to the true catenary.
• Wind and ice loading add transverse and vertical components that change the shape significantly.
• Conductor creep (permanent stretch over time in aluminium) increases real-world sag beyond the initial stringing figure and must be accounted for in final sag tables.

For long river crossings, high-voltage transmission spans, or any span where the sag-to-span ratio is large, use a full sag-tension program (such as SAG10 or PLS-CADD) with the manufacturer’s certified conductor data and NESC loading criteria.

NESC clearance requirements

Sag calculations only matter in context of the minimum ground clearance required by the National Electrical Safety Code (NESC) for the specific voltage level and line type over the terrain below the span. A calculated sag that clears a road by one foot is not acceptable — NESC specifies minimum clearances that must be met at the design temperature and loading condition. Confirm clearances with the applicable NESC table and any state or utility-specific amendments.

This calculator is a planning and estimation tool, not a substitute for a licensed engineer’s review of stringing charts and clearance calculations.