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 2.2.1.2 Classes of Conductors. Conductors are classified as solid or stranded. A solid
conductor is a single conductor of solid circular section. A stranded conductor is composed of a
group of small conductors in common contact. A stranded conductor is used where the solid
conductor is too large and not flexible enough to be handled readily. Large solid conductors are
also easily damaged by bending. The need for mechanical flexibility usually determines whether
a solid or a stranded conductor is used, and the degree of flexibility is a function of the total
number of strands. The strands in the stranded conductor are usually arranged in concentric
layers about a central core. The smallest number of wires in a stranded conductor is three. The
next number of strands are 7, 19, 37, 61, 91, 127, etc. Both copper and aluminum conductors
may be stranded.
2.2.1.3 Conductor Sizes. Conductor sizes are ordinarily expressed by two different
numbering methods: the AWG formerly known as the Browne and Sharpe gage, and the circular
mil.
(a) The AWG or conductor sizes are numbered from 30 to 1, then continuing with 0,
00, 000, and 0000 (or 1/0, 2/0, 3/0, and 4/0 respectively). Number 30 is the smallest size and 4/0
the largest in this system. As an example of the actual physical size of the conductors commonly
used in transmission and distribution work, the diameter of a number 8 AWG is 0.1285 inches
and for a 4/0 AWG it is 0.460 inches.
(b) The circular mil is the unit customarily used in designating the cross sectional area
of wires. A circular mil is defined as the area of a circle having a diameter of 1/1000 of an inch.
The circular mils of cross section in a wire are obtained by squaring the diameter expressed as
thousandths of an inch. For example, a wire with a diameter of 0.102 inches (102 thousandths of
an inch) has a circular mils cross section of 102 X 102 = 10,404. Conductors larger than 4/0
AWG are designated in circular mils. These range from 250,000 to 2,000,000 circular mils (250
MCM or 250 kcmil to 2,000 MCM or 2,000 kcmil).
2.2.2 Insulations. Insulations can be classified in broad categories as solid, taped or
special-purpose insulations. Basic insulating materials are either organic or inorganic. The
following is a list of insulations commonly used:
(a)
Thermosetting compounds (solid dielectric).
(b)
Thermoplastic compounds (solid dielectric).
(c)
Paper-laminated tapes.
(d)
Varnished cloth-laminated tapes.
(e)
Mineral inorganic insulation (solid dielectric granular).
Insulations in general use for voltages above 2 kV are listed below. Solid dielectrics of both
plastic and thermosetting types are being more and more commonly used, while the
laminated-type constructions, such as paper-lead cables are declining in popularity.
2-2
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