Copper was the metal first used for conductors during the great development of
electrical engineering in the early 1880’s. However, the simultaneous rapid
growth of the aluminum industry soon reduced the price of aluminum so that it could
compete in electrical conductor applications.
For overhead transmission lines, aluminum is now used almost to the exclusion of copper.
Although the increasing use of aluminum bus conductor is credited largely to economics,
advances in joining techniques and general experience have prompted its use in many
manufacturing, chemical and utility installations. The most recent aluminum conductor
development, magnet strip, owes its success to savings in coil manufacturing costs as
well as to the lower initial cost of aluminum.
Many other electrical, mechanical and structural applications of aluminum in electrical
industries developed along with the growth of aluminum wire and cable. Non-current-carrying
applications of aluminum are numerous in transformers, capacitors, motors and other types
of electrical equipment.
Wire and Cable
The favorable attributes of aluminum for electrical applications are: relatively high
electrical and thermal conductivities, low density, nonmagnetic properties, ease of
drawing down to the smaller wire sizes and high resistance to weathering.
Because of technological advances and favorable price as compared to copper,
there has been continuous significant growth in the volume, sizes and varieties
of aluminum conductors.
Aluminum Conductor Materials. It was early learned that the electrical
resistivity of aluminum is markedly increased by impurities; electrical conductor
grade (EC) metal, containing approximately 99.5% Al, was established
for most conductor uses. At present, aluminum producers offer EC with a minimum of
99.6% Al and conductivity of 62.0% IACS on a volume basis, although
ASTM specifications permit 99.45% Al minimum and 61.0% conductivity.
EC wire has a tensile strength range of (83-200 MPa), depending on temper.
Transmission Conductor. All-aluminum stranded conductors, although having the advantage
of light weight, had to be strung with large sags, because of their low yield strength
compared to stranded copper.
Experiments were conducted with several alloys of aluminum, and with combinations of
aluminum with other metals, to obtain higher mechanical strength combined with suitable
Aluminized and aluminum-clad core wires recently have been employed to reinforce
aluminum conductors. For many years, it was believed that a superior acsr could be
produced if the zinc coating of the steel wire were replaced with aluminum, in 1957,
hot-dipped aluminum-coated (aluminized) steel wire was produced as an acceptable core
wire for acsr. The excellent resistance to atmospheric corrosion of aluminized wire
resulted in its acceptance for service in coastal and severely corrosive industrial
atmospheres. Aluminized steel core wire is covered by ASTM B 341.
Another method of covering steel wire incorporates an aluminum cladding that is about
10% of the composite diameter, representing approximately 25% of the total cross section,
bonded to a high-strength steel core. It is utilized as a highly corrosion-resistant core
for acsr, and as an overhead ground wire for transmission and distribution lines.
In recent years, both heat treatable alloy 6201-T81 (0.7% Si, 0.75%
Mg) and non-heat-treatable 5005-H19 (0.8% Mg) have
been employed as stranded aluminum conductors for transmission and distribution lines.
Material selection involves comparing the estimated installed cost of these and other
conductors with expected service requirements, considering current-carrying capacity,
thermal overload characteristics, line electric-energy losses, and atmospheric exposure
Distribution Conductor. The use of aluminum conductors for urban
distribution has increased rapidly since World War II, stimulated by the change in the
aluminum-copper price relationship. Aluminum has over twice the conductivity of copper
on a weight basis, and aluminum conductor is used for virtually all overhead
subtransmission and distribution lines.
For many years, predating even the introduction of acsr, weatherproof distribution
conductors were covered with cotton braid and impregnated with asphalt. After World
War II, the cotton braided coverings were displaced by neoprene and, later, by
polyethylene. In addition to excellent weathering characteristics, neoprene and
(especially) polyethylene have high electrical insulating properties.
Utilizing these properties, multiplex cable, a new secondary and service-drop multiple
conductor, was developed. These cables are either duplex, triplex, or quadruplex,
consisting of one, two or three insulated conductors wrapped around a bare aluminum
or acsr neutral. The bare neutral act as the messenger, supporting the entire cable
when strung is an aerial line.
The aluminum alloys employed for conductor accessories, including drawn, extruded,
and cast products, vary with the specific application and with the preferences of the
individual manufacturer. However, the alloys are generally selected to provide suitable
conductivity, high resistance to atmospheric corrosion, galvanic compatibility with
conductor grade (EC) aluminum, and satisfactory mechanical properties. Typical suitable
alloys are 6061-TG for wrought forms and 356-T6 for castings.
Aluminum clamps, frequently 356-T6, are preferred to other metals for supporting
stranded aluminum conductors. They eliminate heating and power waste from hysteresis
losses that occur in ferrous materials. Because the use of dissimilar metals is
avoided, the possibility of galvanic corrosion is minimized. The body and keeper
components of suspension clamps and dead-end clamps are aluminum castings or
Aluminum armor rods are spiraled around the conductors at points of support. They
provide some vibration damping and reinforce the conductors against the effect of
vibration. Two general types of armor rods are available. One is straight rods that
are applied by winding them on the conductor with special wrenches. The other is
pre-formed during manufacture, and usually can be worked into place on the conductor
by hand. The pre-formed type is generally easier to install; however, the straight
rods applied with wrenches have considerably better damping characteristics.
Insulated Power Cable. Today the choice between aluminum and copper
for the metal in insulated conductors is based primarily on cost considerations for the
particular application. The cost comparison should be based on the final installed cost
of the circuits involved. For a specific application, the construction of insulated cable
with aluminum conductor is essentially the same as that with copper conductor. However,
there is usually a difference in conductor size, compensating for aluminum’s lower
The minimum conductivity of EC grade aluminum is 62.0% that of copper. A convenient
and reasonably accurate guide derived from this relationship is that, for equal voltage
drop, an aluminum conductor should be two average sizes larger than a given copper
If, however, circuit design is based primarily on ampacity (current-carrying capacity),
conductor size is determined by the temperature rating of the cable and its rate of
heat dissipation. The ampacity of a cable of specific size depends on the details of
construction and on the metal used for the conductor. For insulated cables of identical
size and construction, those with aluminum conductors have 78 to 84% the ampacity of
those with copper conductors.
The aluminum conductors normally used in insulated cable are EC grade, in either the
H24 or H26 temper. Where unusually high strength is required, 5005 or 6201 alloy, or
sometimes acsr, is used. The most common specifications for conductors in insulated cables
are B 323 for solid conductors and B 231 for stranded conductors.
Specialty Wire and Cable. There are several specialty wire and cable
constructions where aluminum may be advantageous for one or more components.
For example, portable power cables and welding cables having aluminum conductors usually
cost appreciably less than equivalent copper-conductor cables, Conductor weight is reduced
almost half by using aluminum instead of copper. The aluminum cable is larger in diameter
when the lower conductivity of aluminum makes this necessary.
Applications of aluminum for wire and cable exist where the diameter difference is
small or unimportant and the lighter weight of the aluminum cable is of major significance.
If ultimate fatigue failure of the cable is expected, as a result of repeated flexing,
somewhat shorter life is anticipated.
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