MIL-HDBK-1004/2A
Section 3:
UNDERGROUND DISTRIBUTION SYSTEMS
3.1
Circuit Design. Follow the circuit design procedure outlined in
Section 2 of this handbook for overhead distribution systems. For additional
criteria, refer to MIL-HDBK-1190.
3.2
Direct Burial. Install direct-burial cables only in areas that are
rarely disturbed. After first considering economic, maintenance, and
reliability effects, restrict direct burial to light loads, to roadway
lighting systems, and to long untapped runs in low density areas. In some
instances, a minimal amount of taps may be acceptable.
3.2.1
Protection. For protection against mechanical injury, medium-
voltage direct-burial cables can be provided with a protective covering of
metal armor. Consider the need for such protection, such as against dig-ins
or because of possible termite or rodent attack, on a case-by-case basis.
Possibly other protective means are more economical. Where corrosion
considerations are of importance, provide armored cables with a plastic or
synthetic rubber jacket. For cable specifications, refer to NFGS-16301.
Provide a colored warning tape 6 inches (52.4 mm) above the direct-burial
cable.
3.2.2
Installation
3.2.2.1 Trench Dimensions. Provide trenches in accordance with the
requirements of NFGS-16301 and NFGS-02225 Excavation, Backfilling, and
Compacting for Utilities.
3.2.2.2 Cable Protection. General installations shall be in accordance with
requirements of NFGS-16301. Where additional protection of buried cable
against dig-ins is necessary, provide a continuous 1-inch (25.4 mm) thick
treated wood plank or a concrete slab, not less than 2 inches (50.8 mm) thick,
located directly above a top layer of sand in lieu of or in addition to a
protective covering. Accommodate protection against termites or rodents by
using a chemical treatment. Obtain approval of the treatment by the facility
prior to use.
3.3
Draw-In Systems. Draw-in systems consist of duct systems (which may
include access points such as manholes and handholes) in which cable is drawn
after the duct system has been installed. Provide a draw-in system where
overhead distribution is not feasible (refer to MIL-HDBK-1004/1). Provide a
draw-in system for distribution of large blocks of electric power, where many
circuits follow the same route or are run under permanent hard pavements, or
where service reliability is paramount.
3.3.1
3.3.1.1 Routes. Select duct line routes to balance maximum flexibility with
minimum cost and to avoid foundations for future buildings and other
structures.
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