MIL-HDBK-1004/2A
3.3.1.2 Multipurpose Conditions. Where it may be necessary to run
communication lines along with electric power distribution lines, provide two
isolated systems in separate manhole compartments. Where possible, run ducts
in the same concrete envelope.
3.3.1.3 Clearance. Keep electric and communication ducts clear of all other
underground utilities, especially high-temperature water or steam pipes.
3.3.1.4 Materials. Acceptable standard materials include the various types
of plastic as specified in NFGS-16301. Rigid steel conduit may also be
installed below grade and provided with field or factory applied coatings for
corrosion protection where required.
3.3.1.5 Size of Ducts. Base the size of conduits in a duct bank shall be
based on consideration of the following factors:
a) for general electric power distribution, do not use less than 5
inch (127 mm) ducts;
b) for communication duct banks, normally use 4 in. (101.6 mm)
ducts although 3 inch (76.2 mm) ducts may be acceptable in some cases;
c) special cases may require use of larger sizes, but such sizes
shall be functionally justified.
3.3.1.6 Arrangement of Duct Banks. For best heat dissipation, use an
arrangement of two conduits wide or high. This may be impossible where a
large number of ducts are involved. The vertical, two-conduit-wide
arrangement enables the cables to be more easily racked on manhole walls but
may not be as economical as the horizontal two-conduit-high arrangement. For
dimensions and arrangement of duct banks see Figure 1. Encase conduits in
concrete in accordance with NFGS-16301.
3.3.1.7 Drainage. Drain all ducts to manholes with a constant slope in
accordance with NFGS-16301. Where two manholes are at different elevations, a
single slope following the general slope of the terrain may be the most
economical. Where grades are flat or crest between manholes, a single slope
usually requires too much depth in one of the manholes. In this event,
generally slope the duct from the crest area to both manholes, keeping a
minimum earth coverage on the highest elevation.
3.3.1.8 Spare Capacity. Include ducts for planned future expansion, plus 25
percent additional spare ducts for unplanned expansion.
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