Conductor Blowout and Horizontal Clearance

Ground clearance is checked straight down and nothing checks sideways, but a conductor in wind swings out of the plane of the poles like a hinged sheet, through the angle whose tangent is the wind load per foot over the weight per foot. The conductor does not stretch to do it -- the sag along the swung plane is the same sag, just tilted -- so the horizontal displacement at midspan is the sag times the sine of that angle. Two things fall out. The swing angle is independent of span and of tension: it depends only on the ratio of wind load to weight, so it is the same for a short span and a long one in the same wind. But the blowout DISTANCE is proportional to sag, so the long, slack spans blow out furthest, and they do it on exactly the hot, sagging days when vertical clearance is also at its worst. And a light conductor blows out far further than a heavy one in the same wind: ACSR Drake at 1.108 in and 1.094 lb/ft in a 9 psf wind swings 37.2 degrees and moves 7.26 ft, and at half that weight in the same wind it swings 56.7 degrees and moves 10.0 ft. That is why small distribution conductor near buildings and tree lines is the recurring problem rather than the transmission line overhead. The pressure at which a stated clearance is exhausted is reported so a span can be judged against a design wind rather than against one arbitrary gust. This is midspan blowout on a level span with the conductor treated as swinging rigidly about the chord between attachment points. It does not model the restraint a suspension insulator string imposes near the structures, which reduces blowout there and is why midspan is the governing point; it does not evaluate conductor-to-conductor clearance under differential swing, where adjacent phases swing by different amounts and can approach each other; and it does not address galloping, aeolian vibration, or the dynamic response of a conductor in gusty wind. It does not check vertical clearance. The applicable NESC edition, the utility's construction standards, and the right-of-way requirements govern.

wind load per foot = wind pressure x conductor diameter / 12; swing angle = atan(wind load per foot / weight per foot); blowout at midspan = sag x sin(swing angle); a wind speed entered instead of a pressure is converted as 0.00256 V^2.

The transverse blowout relation as standard overhead line practice, by name, with the ASCE 7 velocity-pressure constant 0.00256 -- the same relation the wind-pressure calculator uses, so the two cannot disagree. The applicable NESC edition, the utility's construction standards, and the right-of-way requirements govern.

Trigonometry on a wind pressure and a sag the user supplies; no clearance table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: conductor_diameter_in, weight_lb_per_ft, wind_pressure_psf, wind_speed_mph, sag_ft, still_air_clearance_ft, wind_load_lb_per_ft, swing_angle_deg, blowout_ft, remaining_clearance_ft

• Midspan on a level span insulator restraint near the structures is not modelledoverhead line practice
• Rigid swing about the chord differential swing between phases is not evaluatedoverhead line practice
• Galloping and vibration excluded dynamic response in gusty wind is a separate subjectoverhead line practice