Section 23/Clearances of Supply and Communication Lines
NESC Rule 23 establishes minimum vertical and horizontal clearances for overhead supply and communication lines from ground and structures.
Rule 23 establishes minimum vertical and horizontal clearances for overhead lines based on voltage, location, and the nature of the area crossed. Conductors crossing roads require greater clearance than those over land not normally accessible. At 0 to 750 volts, the minimum vertical clearance is 15.5 feet over streets and 12 feet over areas accessible to pedestrians only. Higher voltage lines require additional clearance based on the maximum operating voltage. Clearances to buildings must be maintained at the building surface and at any point where a person could reach from a window, fire escape, or roof.
Why this section exists
Overhead power lines are among the most dangerous elements of the electrical system. Contact with an energized conductor from a ladder, crane, vehicle, or building surface causes electrocution. The clearances in Rule 23 account for the maximum conductor sag under high-temperature conditions, vehicle heights, and the reach of a person standing on an accessible surface. Inadequate clearances are a leading cause of electrocution in the construction and utility industries.
What plan reviewers look for
Plan reviewers check utility coordination drawings and site plans for overhead line clearances to proposed structures. They verify that building heights and rooftop equipment do not encroach on the required clearances. For substation designs, they check conductor-to-ground and conductor-to-fence clearances. They verify clearances are based on the maximum operating voltage and maximum conductor sag conditions.