MIL-HDBK-1004/2A
3.4
Underground Cables. Cable installations make up a large portion of
the initial distribution system investment, contribute to a lesser extent to
the annual maintenance and operating costs, and affect system reliability.
Therefore, underground cables and their accompanying protective and operating
devices should be selected in accordance with criteria set forth in the
following paragraphs. The joint specifications of the Insulated Cable
Engineers Association-National Electrical Manufacturers Association (ICEA-
NEMA) and the specifications of the Association of Edison Illuminating
Companies (AEIC) should be used as covered by NFGS-16301. ICEA-NEMA
specifications cover medium-voltage cables which are manufactured as stock
items. Requiring medium-voltage cable to meet AEIC specifications should be
limited to medium-voltage cables which are not stock items (35-kV rating) or
where the footage installed is large enough for manufacturers to make a
special run.
3.4.1
Single- or Multiple-Conductor Cables
3.4.1.1 Single-Conductor Cables. Single-conductor cables are usually used
in distribution systems because the installed cost is less than that of
multiple-conductor cables.
3.4.1.2 Multiple-Conductor Cables. Select multiple-conductor cables where
justified by special considerations such as installation in cable trays,
twisted to provide lower inductance for 400-Hz distribution systems, or for
High-altitude Electromagnetic Pulse (HEMP) hardened systems.
3.4.2
Conductor Materials
3.4.2.1 Annealed Copper. Select annealed copper for high conductivity,
flexibility, and ease of handling; it is used in all forms of insulated
conductors.
3.4.2.2 Medium-Hard-Drawn Copper. Medium-hard-drawn copper has greater
tensile strength than annealed copper but may not be available as a stock
item. Its use in long pulls and unsupported vertical risers is acceptable;
however, procurement difficulties make other designs more advisable.
3.4.2.3 Aluminum. Generally, aluminum conductors are permitted as a
contractor's option to copper subject to the restrictions of NFGS-16301,
except where corrosive conditions limit usage.
Preferred Cable Insulations. Insulation material to be used in a
3.4.3
specific design depends on the system voltage and the thermal, mechanical, and
chemical effects from the installation involved. Use crosslinked-Polyethylene
(XLP) or ethylene-propylene rubber (EPR) whenever possible. These insulations
provide the maximum rated conductor temperatures for operating, overload, and
short-circuit conditions for cables rated up to a maximum of 35 kV.
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