TM
5-811-1/AFJMAN
32-1080
adequate cable protection. Special shielding or
minum will be justified based upon an analysis
coverings will not be specified, unless the designer
using life, environmental, and cost factors. The
has checked that the footage installed for each
need for mechanical flexibility requires that con-
different cable diameter is large enough for manu-
ducts be stranded, and the NEC makes this man-
facturers to make the special runs required.
datory for cables larger than No. 8 AWG installed
in raceways. The installation of conductors larger
(b) Metallic-armored cable. Armored cable is
than 500 kcmil is not economical, and such large
justified only when cable is installed under water
cables should be used only under exceptional
(submarine cables) and sometimes when installed
circumstances. Large ampacities can be served by
in cable trays or trenches. Armored cable will
parallel or multiple circuits. Three 15 kV, single-
have XLP or EPR insulation covered with a
conductor, nonmetallic-jacketed cables larger than
thermoplastic core covering and then provided
No. 4/0 AWG will require use of ducts larger than
with an interlocked-metal tape armor. A nonmetal-
the standard four-inch size (i.e. three single-
lic jacket is required for underground installations,
conductor cables making up a three-phase circuit
where corrosion and moisture protection is re-
and each having individual overall diameters
quired, for installations in outdoor cable trays, or
greater than 1.25 inches will need to be installed
for submarine cables. Submarine cable may also
in a duct larger than four inches). One three-
require a lead covering. Cable having a steel
conductor cable is more costly than three single-
armor will be three-conductor type to avoid the
conductor cables, and use of multiple-conductor
high hysteresis and eddy current losses which can
cable will be restricted to special conditions.
result when single-conductor cable is used.
Metallic-armored cable is such a special condition.
(c) Lead-covered cable. Lead-covered cables
b. Insulation and jacket material. The type of
will not be used, unless extenuating circumstances
insulation used will be dependent upon the voltage
prevail such as for submarine cable. The lead
level and type of service required. Factors affect-
covering is both more costly and more difficult to
ing selection will be the effects of the surrounding
handle. The use of laminated insulation such as
environment, the importance of the load in regard
for paper-insulated-lead-covered (PILC) or for
to operation of the installation, and whether peak
varnished-cambric-lead-covered (VCLC) instead of
loading is continuous or intermittent.
the solid or extruded dielectrics such as
(1) Medium-voltage cable. Cable will be speci-
crosslinked-polyethylene (XLP) or ethylenepropy-
tied as 133 percent insulation level (ungrounded)
lene-rubber (EPR) is not approved. In addition,
which allows greater margin for voltage surges,
these cables have lower temperature ratings.
insulation deterioration, and fault clearing time
(2) Low-voltage cables. Cables suitable for be-
than does the use of the 100 percent insulation
low grade installations are listed in the NEC.
level (grounded). When marking guide specifica-
Insulation will be either XLP (NEMA WC 7) or
tions, refer to NFPA 70, which currently limits the
EPR (NEMA WC 8) and jackets or other protection
minimum size to No. 1 AWG at 133 percent
will be in accordance with the applicable Under-
insulation for 15 kV to 28 kV systems and No. 2
writer's Laboratories (UL) specification covering
AWG at 133 percent insulation for 8 kV to 15 kV
that NEC type. Use of metal-clad (MC) cable will
systems. Medium-voltage cable above 3 kV will be
be limited as previously discussed for metallic-
shielded.
armored cable. The use of the less expensive
(a) Nonmetallic-jacketed cable. Nonmetallic-
M o i s t u r e - a n d - H e a t - R e s i s t a n t Thermoplastic
jacketed cable will be used, except where circum-
(THWN) or Moisture-and-Heat-Resistant Cross-
stances warrant other coverings. Insulation will be
Linked Synthetic Polymer (XHHW) is not recom-
either crosslinked-polyethylene (XLP) for short life
mended for underground work as their thinner
requirements, or ethylene-propylene-rubber (EPR)
insulation has been designed for interior usage.
for long life requirements, in accordance with
Moisture-and-Heat Resistant Thermoplastic (THW)
NEMA WC-7 and WC-8. Comparisons of various
wiring does have the same thickness of insulation
cable insulations, as shown in table 7-1, indicate
as Heat-Resistant Rubber (RHH)/Moisture-and-
the advantages of these two insulations over other
H e a t Resistant Rubber (RHW)/Underground
types. Coverings (jackets) will be any of the rubber
Service-Entrance (USE) wire, but polyvinyl-
or plastic options covered by NEMA specifications.
chloride insulation is considered to have only fair
This option allows the use of cables which are
electrical and mechanical insulation properties as
available as stock items in small quantities. In
compared to the excellent properties exhibited by
some environments, however, selection of other
XLP and EPR insulation. UF cable may have a
jacket materials may be necessary because proper-
greater insulation thickness, but some sizes have a
ties of some jacket materials may not provide
lower ampacity rating than does USE cable.
7-2
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