structure. The structure should sustain the combination
of vertical and lateral loading that produces the
maximum shear and bending moments in the structure.
Manholes are necessary in a power distribution
system to permit the installation, removal, splicing,
and rearrangement of the cables. A manhole is merely
a subterranean vault or masonry chamber of sufficient
size to permit proper manipulation of the cables.
Arranged on the sides of the vault are devices that
support the cables.
The location of manholes is determined largely by
the layout of the base that is to be supplied with power.
Whenever a branch or lateral extends from the main
subway, there must be a manhole; and there must be
manholes at intersections of subways. In general,
cables are not made in lengths exceeding 400 to 600
feet; and as it is necessary to locate splices in
manholes, the distance between manholes cannot
exceed these values. Furthermore, it is not advisable to
pull in long lengths of cable because the mechanical
strain on the conductors and sheath may then become
too great during the pulling-in process. It is
recommended that manholes be located not more than
500 feet apart. The lines should preferably be run
straight between manholes.
Manholes are made in many shapes and sizes to
meet the ideas of the designer and to satisfy local
conditions. An example of a typical manhole is shown
in figure 4-39. If there are obstacles at the point where a
manhole is to be located, the form of the manhole must
be modified to avoid them. The form approximating an
ellipse (fig. 4-40) is used so that the cables will not be
abruptly bent in turning them around in the manhole.
When the rectangular type of manhole is used (fig.
4-41), care must be taken not to bend the cables too
The size of a manhole will vary with the
number of cables to be accommodated; but, in any
case, there must be sufficient room to work in the
manhole. A 5- by 7-foot manhole is probably as large
as will be required in isolated plant work, while a 3- by
4-foot manhole is about as small as should be used.
When transformers are located in a manhole, the size
should be increased to allow for working space around
the transformer and for ventilation. About 2 or 3 feet of
volume should be allowed per kilovoltampere of
Manholes are built of either brick or concrete or of
both of these materials. When many manholes are to be
built of one size and there are no subterranean
obstructions, concrete is usually the cheapest and the
best material. But when only a few are to be
constructed or when there are many obstructions, a
manhole with a concrete bottom, brick sides, and a
concrete top is probably the best. Such a manhole can
be constructed without having to wait for concrete to
set before you can remove the forms. There is a
growing use of precast concrete manholes that are
shipped directly to the project site.
A manhole with brick walls is built by first pouring
the concrete floor and then building up the brick walls
thereon. If the manhole is large, the roof can be either
of steel-reinforced concrete or of brick set between
rails. Probably for installations in which only a few
manholes are to be built, the brick-between-rails
method is the best. For a small manhole, no masonry
roof is necessary, as the cast-steel manhole cover
forms the roof.
Cement mortar for building brick manholes or for
conduit construction can be made by mixing together 1
part of cement, 3 parts of sand, and about 1/3 part of
water, all by volume.
A concrete manhole is built by first pouring the
concrete floor and then erecting the form for the sides.
In a self-supporting soil, the sides ofthe hole constitute
the form for the outside of the manhole. If the soil is not
self-supporting, there must be an outer form of rough
planks (plywood), which is usually left in the ground.
Steel reinforcing, such as old rails, must be placed in
the concrete top of a large manhole. All reinforcing
steel should be completely encased in concrete to
Manhole covers should always be made of cast
steel and covers should be round so that they cannot
drop into the hole accidentally.
So-called watertight covers are seldom used now,
as it is not feasible to make a satisfactory watertight
cover at reasonable expense. A cover should not be
fastened down because if it is and accumulated gas in a
manhole explodes, the vault and cover will be
shattered. A ventilated cover should be used to allow
the escape of gas. The newer types have ventilating
slots over approximately 50 percent of their area. Dirt
and water will get into the hole, but the dirt can be
cleaned out and the water will drain out and no harm
will result. If ventilation is not provided, an explosion
of gas may occur and do great damage.
When feasible, a sewer connection should lead
from the bottom of every manhole. The mouth of the
trap should be protected by a strainer made of