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practicable. The cable in some cases comprises three rubber-insulated, neoprene-insulated, or
polyethylene-insulated conductors lashed to a bare messenger which serves as the neutral for
3-phase circuits. In other cases, the phase conductors are supported from the messenger by
insulating spacers; this construction is commonly called spacer cable. Single-phase taps, in
either case, are usually one insulated conductor and messenger or an insulated conductor with
neutral strands spiralled around it.
2.5.2.2 Underground Systems. The conductor sizes most commonly used in underground
primary distribution vary from No. 6 AWG to 500 kcmil copper. Feeders are usually 3- or
4-conductor cable, while laterals may be single-conductor because of the requirement for
numerous transformer taps. On some systems, interconnected cable sheaths are used for the
neutral conductor; others use a separate bare neutral or fourth conductor in the cable. Stranded
aluminum conductors and aluminum sheaths have been used in some cases, but the increased
outside diameter usually requires excessive duct space. Recent improvements in polyethylene
insulation have extended the use of aluminum-conductor cable for direct-buried systems where
overall size is not detrimental.
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