U.S. Department of Transportation
Federal Aviation Administration
Advisory Circular AC 70/7460-1K
March 1, 2007

34. Markers
Markers used to highlight structures when it is impractical to make them conspicuous by painting.  Markers may also be used in addition to aviation orange and white paint when additional conspicuity is necessary for aviation safety.  They should be displayed in conspicuous positions on or adjacent to the structures so as to retain the general definition of the structure.  They should be recognizable in clear air from a distance of at least 4,000 feet (1219m) and in all directions from which aircraft are likely to approach.  Markers should be distinctively shaped, i.e. spherical, cylindrical, so they are not mistaken for items that are used to convey other information.  They should be replaced when faded or otherwise deteriorated.

a.  Spherical Markers.  Spherical markers are used to identify overhead wires.  Markers may be of another shape, i.e., cylindrical, provided the projected area of such markers will not be less than that presented by a spherical marker.

1.  Size and Color.
     (a)  The diameter of the markers used on extensive catenary wires across canyons, lakes, rivers, etc., should be not less than 36 inches (91cm).  

Smaller 20-inch (51cm) spheres are permitted on less extensive power lines or on power lines below 50 feet (15m) above the ground and within 1,500 feet (458m) of an airport runway end.  Each marker should be a solid color such as aviation orange, white, or yellow.

2.  Installations.
     (a)  Spacing.  Markers should be spaced equally along the wire at intervals of approximately 200 feet (61m) or fraction thereof.  Intervals between markers should be less in critical areas near runway ends (i.e., 30 to 50 feet).  They should be displayed on the highest wire or by another means at the same height as the highest wire.  Where there is more than one wire at the highest point, the markers may be installed alternately along each wire if the distance between adjacent markers meets the spacing standard.  This method allows the weight and wind loading factors to be distributed.

     (b)  Pattern.  An alternating color scheme provides the most conspicuity against all backgrounds.  Mark overhead wires by alternating solid colored markers of aviation orange, white, and yellow.  Normally, an orange sphere is placed at each end of a line and the spacing is adjusted (not to exceed 200 feet) to accommodate the rest of the markers.  When less than four markers are used, they should all be aviation orange.