Overhead Line Ground Clearance

The conductor's height above ground is the attachment height minus the sag AT THE WORST CONDITION, not at the condition it was strung in. That is the whole difficulty. NESC checks clearance at the maximum conductor temperature the line is designed to operate at, or at the final-sag ice condition, whichever gives the greater sag -- not at 60 degF on the day the crew strung it. A line sagged in spring with a comfortable margin can be out of compliance at design temperature, and the difference is routinely several feet: on a 600 ft ruling span, sixty degrees of heating adds about 3 ft of sag, and creep adds the equivalent of another forty-odd degrees permanently over the life of the line. One relation is run four ways. Given a sag it returns the clearance and the margin. Given a required clearance it returns the maximum sag the span may be strung to, which is the number a crew wants at the pole. Given a sag and a requirement it returns the minimum attachment height, which is what sizes the structure. And the margin is reported SIGNED, so a failing span reads as a negative number of feet rather than as a passing-looking small one. NO NESC TABLE IS SHIPPED. The required clearance depends on the voltage, on what is under the line -- road, driveway, pedestrian-only, water, rail -- and on the edition the jurisdiction has adopted, and it is taken as an input for the same reason the pole-embedment calculator takes lateral bearing as an input: a table copied into a calculator is a table that goes stale silently, and a wrong clearance is not a rounding error. This checks one point, midspan on a level span, against one entered requirement. It does not find the governing point on an inclined span or over uneven ground, where the low point of the curve and the high point of the ground are not at the same place; it does not evaluate clearance to buildings, other conductors, or communication lines, which have their own requirements; it does not address blowout, which is the horizontal question; and it does not determine which condition governs. The adopted NESC edition, the utility's construction standards, and the authority having jurisdiction govern.

height above ground = attachment height - sag at the maximum condition; margin = that less the required clearance; maximum allowable sag = attachment height - required clearance; minimum attachment height = required clearance + sag.

The ground-clearance subtraction with the NESC requirement CITED, NOT MIRRORED: no clearance table is shipped, because the requirement depends on voltage, on what is under the line, and on the edition the jurisdiction has adopted. NESC checks at the maximum operating temperature or the final-sag ice condition, whichever sags more. The adopted NESC edition, the utility's construction standards, and the authority having jurisdiction govern.

A subtraction on a height and a sag; the clearance requirement is entered by the user from their own adopted table.

Estimate. AHJ and licensed professional govern.

Field names used by the API: attachment_height_ft, max_condition_sag_ft, required_clearance_ft, clearance_ft, margin_ft, max_allowable_sag_ft, min_attachment_height_ft

• No NESC clearance table is shipped it is entered from the adopted editionthe adopted NESC edition
• Midspan on a level span the governing point on an inclined span or uneven ground is not foundoverhead line practice
• Which condition governs is not determined maximum temperature or final-sag ice, whichever sags morethe adopted NESC edition