Conductor Long-Term Creep and Sag Increase

Creep is permanent, non-elastic elongation under sustained load, and in aluminium conductor it is large enough to matter -- strains of a few times ten to the minus four over the life of a line are ordinary, with most of it spent in the first year or two and the rest accumulating slowly. Steel barely creeps at all, which is why ACSR creeps less than all-aluminium conductor and why the effect concentrates in the aluminium strands. The clean way to use it is the TEMPERATURE EQUIVALENT. Creep strain divided by the coefficient of thermal expansion is the temperature rise that would produce the same elongation, and since the change-of-state equation already knows how to move a conductor by a temperature, creep becomes just another temperature offset. An ordinary ten-year ACSR value of 5.0e-04 against an alpha of 1.06e-05 per degF is 47.2 degF equivalent -- which is not a correction, it is a bigger move than most seasonal swings, and it runs in the SAME direction as a hot day rather than against it. That is the whole reason utilities sag new conductor deliberately high. The practical instruction follows directly: sag the new line as though it were the equivalent temperature COLDER than it actually is, and it will arrive at the design sag once the creep is spent instead of sailing past it. A crew that sags to the design number on installation day has already spent the entire clearance margin before the line is a decade old, and the line will fail an inspection it passed at commissioning with nothing having gone wrong. The creep strain itself is an INPUT and it must come from the conductor manufacturer's creep data for the conductor, the tension, and the elapsed time in question. This does not predict it. Creep is a function of stress history, temperature history, and time, it differs by construction and by aluminium alloy, and a conductor that has been through a heavy ice event has had its creep partly displaced by that overload -- so a single strain figure is a working approximation to a path-dependent process. This treats the conductor as one homogeneous material with a single modulus, which understates the difference between the aluminium and steel components of ACSR at high temperature. It does not evaluate clearance, produce a stringing chart, or address the prestressing and overtensioning procedures that some utilities use to spend creep deliberately at installation. The conductor manufacturer's creep and stress-strain data, the utility's sag-tension program and stringing charts, and the applicable NESC edition govern.

equivalent temperature rise = creep strain / coefficient of thermal expansion; the resulting condition is evaluated through the same parabolic change-of-state relation the sag-at-temperature calculation uses, and the stringing sag is that relation run at the design temperature LESS the equivalent rise.

The creep temperature-equivalent method as standard overhead line practice, by name. The creep strain is an INPUT from the conductor manufacturer's creep data for the conductor, tension, and elapsed time; none is predicted here. The conductor manufacturer's creep and stress-strain data, the utility's sag-tension program and stringing charts, and the applicable NESC edition govern.

A division and a change-of-state run on a creep strain the user reads off the manufacturer's data; no creep curve is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: creep_strain, alpha_per_f, span_ft, area_in2, weight_lb_per_ft, modulus_psi, tension1_lb, design_temp_f, equivalent_temp_rise_f, sag_design_ft, sag_after_creep_ft, creep_sag_increase_ft, initial_stringing_sag_ft

• The creep strain is an input, not a prediction it is path-dependent on stress and temperature historythe conductor manufacturer's creep data
• One homogeneous material the aluminium and steel components of ACSR are not separatedconductor manufacturer stress-strain data
• Prestressing procedures not addressed some utilities spend creep deliberately at installationthe utility's construction standards