of underground raceways installed should be the
pletely below-grade structures. In addition to
markers, 3-inch wide 5-mil brightly colored plas-
nominal 4-inch, 3-inch, 2-inch, or 1-inch size,
tic tape placed approximately 12 inches below
except where large numbers of secondary ducts
make this uneconomical, such as on tank farms.
finished grade may be used.
d. Spare capacity. A sufficient number of spare
7-4. Manholes, Handholes, and Pullboxes.
ducts will be provided in duct systems between
In traffic areas, design will be for a H20 wheel
manholes to provide for at least a 25 percent
loading as defined by AASHTO HB-13.
increase in the number of cables. The number of
a. Manholes. Manholes will normally be located
spare ducts will be increased as required for future
at street intersections and will be spaced otherwise
service to planned expansion. Such spare provi-
to meet facility needs; to conform to the facility
sions do not apply to building service ducts, unless
master plan; or as required by the cable pulling
there is a definite planned expansion or a planned
(1) Criteria for construction. Manholes will not
cate feeders. Spare ducts should be plugged to
be less than six feet in depth, by six feet in length,
prevent entry of debris and rodents.
e. Installation. Installation requirements for
by four feet in width with an access opening to the
concrete-encased duct lines will meet requirements
surface above (outer air) of not less than 30 inches
in diameter. Manholes will provide a minimum
of ANSI C2. Figure 7-2 shows typical contigura-
wall space of six feet on all sides where splices will
tions, however, spacing and arrangement will be
be racked. Duct entrances into the manhole can be
adapted to ensure adequate conductor ampacities
in accordance with the NEC and IEEE/ICEA
located near one end of long walls so that sharp
bends of cables at the duct mouth are avoided, or
publications. Additional duct configurations are
else sufficient space will be provided for a reverse
depicted in the NEC. Figure 7-3 indicates drain-
bend before the cable straightens out on the wall
age requirements for underground ducts.
(1) Maximum number of conduits in a duct
on which the cable is racked. Manhole elevations
run. Electric power cables generate heat depen-
and elevations of duct lines entering manholes will
be shown. For nonmetallic-jacketed cables, the
dent upon the cable loading and resistance. Dissi-
minimum bending radius will be 12 times the
pation of this heat is no problem because of
overall cable diameter. Metallic-jacketed cable
diversity of cable loading, as previously noted.
bends will be in accordance with ICEA/NEMA
More than eight ducts entering at any one point in
requirements or 12 times the overall cable diame-
a manhole provide a cable congestion which makes
ter, whichever is greater. Figure 7-4 shows details
maintenance time-consuming and costly. Where
of factors which affect lengths of manholes. A scale
the use of more than eight ducts in a single run is
example of a specific cable size installed in a
necessary, the minimum manhole size required, as
manhole is shown on figure 7-5.
noted later, will be increased. More than two duct
(2) Types of manholes. A combination electric
entrances may require larger manhole sizes.
(2) Configurations. Arrangements for electric
use with most electric power and communication
(E) and communication (C) ducts are given below:
or 4C . . . . .
. . . . . 2 ducts wide by 2
duct arrangements is shown on figure 7-6. Other
and 4C . . . . .
. . . . . . 4 ducts wide by 2
arrangements are acceptable, but minimum inside
or 6C . . . . .
. . . . . 3 ducts wide by 2
dimensions and reinforcing will match require-
and 6C . . .
. . . 6 ducts wide by 2
ments shown on figure 7-6. Generally, manhole
drawings indicate the requirements for a cast-in-
(a) Jacking. Where ducts are jacked under
place concrete manhole. Precast manholes may be
existing pavement or used for exposed installa-
specified as a Contractor's option, when they pro-
tions, rigid steel conduit will be installed because
vide the same inside dimensions, strength, and
of its strength. To protect the corrosion-resistant
sealed joints comparable to the monolithic con-
conduit coating, predrilling or installing conduit
struction of case-in-place manholes. Prefabricated
inside a larger iron pipe sleeve (jack-and-sleeve) is
vaults of other than concrete construction will be
required when conduits are jacked. For crossings
restricted to direct-burial cable systems.
of existing railroads and airfield pavements
(3) Prohibited devices. Electrical equipment
greater than 50 feet in length, the predrilling
method or the jack-and-sleeve method will be used.
installed in manholes or underground vaults, ex-
(b) Duct line markers. Duct line markers
cept in manholes adjacent to airfields where such
will be provided only for duct line stubouts or for
installations may be necessary to meet airfield
other ducts whose locations are indeterminate
clearance requirements, or as specifically required
because of duct curvature or terminations at com-