At least two down conductors are required on all structures except on slender objects like flag poles,
antenna masts (not substantial towers), light poles, and the like.
Provide one additional down conductor for each additional 30 meters (100 feet) or fraction thereof
on structures having a perimeter exceeding 75 meters (250 feet). On structures having flat or gently sloping
roofs and on irregular-shaped structures, the number of down conductors should be such that the length of the
average roof conductor joining them does not exceed 30 meters (100 feet). On structures higher than 18 meters
(60 feet) where down conductors are required, install at least one additional down conductor for each 18 meters
(SO feet) of height or fraction thereof; however, the spacing between down conductors need not be less than 15
meters (50 feet).
Down conductors are to be provided or located appropriately to avoid dead ends in excess of 4.8
meters (16 feet) in length. See Figure 1-22, Note 1.
Maintain down conductors in a downward course with routing around or through any obstruction
which may lie in the path. Sharp bends or turns are to be avoided with necessary turns limited to not less than
90 degrees and not less than 20 cm (8 inches) in radius.
Where large re-entrant loops (i.e., those with greater than 90-degree turns) cannot be avoided, e.g.,
around cornices or over parapets, the conductor should be routed to ensure that the open side of the loop is
greater than one-eighth the length of the remaining sides of the loop. It is advised, however, to course the
conductor through holes or troughs through the obstacles and avoid the loop completely (as shown in
Figure 1-23(e)) whenever possible.
On structures with overhangs such as antenna towers with extended platforms or buildings utilizing
cantilevered construction, run the down conductors vertically through the interior of the structure (l-5).
Internally routed conductors must be enclosed in nonmetallic, noncombustible ducts.
Substantial metal structural elements of buildings may be substituted for regular lightning
conductors where, inherently or by suitable electrical bonding, they are electrically continuous from the air
terminal to the earth electrode connection. The structural elements must have a conducting cross-sectional
area, including that in joints, at least twice that of the lightning conductor that would otherwise be used. There
need be no difference whether such conductors are on the interior or exterior of the structure when used for
down conductors. Steel frame buildings encased in bricks or other masonry products must have external air
terminals and roof conductors installed and bonded directly to the structural members to keep the lightning
discharge from having to penetrate the masonry shell to reach the frame members.
Securely attach air terminals and roof and down conductors to the building or other object upon
which they are placed.
Fasteners (including nails, screws, or other means by which they are attached) should be substantial
in construction, not subject to breakage, and should be of the same material as the conductor or of a material
that will preclude serious tendency towards electrolytic corrosion in the presence of moisture because of
contact between the different metals. (For further information on corrosion, see Volume I, Section 7.8.)