Download Aluminum Conductor Properties and Advantages

Section I Aluminum-the Metal
Chapter 2
Aluminum Conductor Properties and Advantages
The mechanical and electrical properties of bare alu­
minum wire and stranded conductor are tabulated in
Chapter 4 and of bus conductor in Chapter 13. Certaln
general properties related to the use of aluminum, as
distinct from other metals, in their application as electrical
conductors are discussed in this chapter. Principally,
these are:
I. Conductivity: More than twice that of copper, per
pound.
2. Light weight: Ease of handling, low installation
costs, longer spans, and mOre distance between
pull-ins.
3. Strength: A range of strengths from dead soft to
that of mild steel, depending on alloy. The high­
est strength alloys are employed in structural,
rather than electrical conductor, applications.
4. Workability: Permitting a wide range of processing
from wire drawing to extrusion or rolling. Excel­
lent bend quality.
5. Corrosion resilance: A tough, protective oxide coat­
ing quickly forms on freshly exposed aluminum
and it does not thicken significantly from con­
tinued exposure to alr. Most industrial, marine,
and chemical atmospheres do not cause corrosion,
providing the proper alloy is selected. The cor­
rosion resistance of all alloys can be improved
by anodizing.
6. Creep: Like all metals under sustained stress, there
is a gradual deformation over a term of years.
With aluminum, design factors take it into
account.
7. Compatibility with insulation: Does not adhere to
or combine with usual insulating materials. No
tin-coating required; clean stripping,
Other qualities of aluminum, such as thermal con­
ductivity and fatigue resistance, have a bearing on con­
ductor section. The high-rellectivity and non-magnetic
characteristics, as well as the properties under extremes
of temperature, are rarely associated with any commercial
use of electrical conductors; hence are not considered
herein.
The Effect of Alloying
A detailed study of aluminum applications usually
involves aluminum alloys that have properties markedly
different from those of the basic metal. Thus, less than 2.0
percent addition of other metals supplemented by a
specified heat treatment converts nearly pure aluminum
to 6101-T6 electrical bus conductor with an increase in
minimum yield strength from 3.5 ksi to 25.0 ks!. The
reduction of conductivity associated with this major
change of strength is only from 61.0 percent lACS to
55.0 percent lACS.
Merely adding the alloying elements to the mixture
is not sufficient to produce the desired results. The
strength of the non-heat-treatable alloys is brought to
the value specified by the -H temper of the alloy by cold
working and/or partial annealing, and the strength of the
heat-treatable-alloys is brought to that of the specified
-T temper by heat treatment as explained in greater
detail in Chapter I.
In the manufacture of heat-treatable aluminum alloy
conductor wire, the supplemental treatment (cold worlting
and heat treatment) usually is divided into two parts­
often at different locations: (I) that performed during
the production of redraw rod (0.375 inch diameter) and
(2) that performed during or after reduction of diameter
of the redraw rod to the finished wire size. Bus-conductor
shapes have most of the necessary heat treatment per­
formed during extrusion. Aging may be performed
subsequently.
Conductivity
The conductivity of pure aluminum is about 65.0
percent lACS, However, the conductivity of aluminum
1350 is 61.0 percent lACS minimum due to low level
impurities inherent to commerical processing (up to
62.4% lACS is available in 1350 on a special order basis).
The conductivity for bus conductor alloys is shown in
Table B-2. The conductivities of 6201 and the 8XXX
series alloys in the tempers, which are used in the pro­
duction of wires for cables, are also shown in Table I- L
A comparison of conductivities of metals sometimes
used for electrical conductors is shown in Table 2-1. The
2-1
aluminurn---the metal
TABLE 2-1
Relative Conductivities of Pure Metals(l)
Metal
Conductivity
: Percent lACS
i Vol. Basis(2)
Silver
Copper
!
Aluminum
Titanian
Magnesium i
Sodium
108.4
103.1
64.9
4.1
38.7
41.0
Conductivity
Specific Percent lACS
Gravity(3) Wgt. Basis(4)
10.49
8.93
2.70
4.51
1.74
0.97
91.9
102.6
213.7
8.1
197.7
376.2
Liallt Weiallt
The relative conductor weights required for equal con·
ductivity using various metals are listed in Table 2-2.
These were developed from Table 2·1 (percent lACS mass
conductivity and density values) applying conversion
methods described in ASTM Specification B 193.
The lighter weight aluminum provides obvious handling
cost reductions over heavier metals. Reduced capital and
installation costs are an added advantage of aluminum
conductors by reason of the long-span capability of ACSR
and ACAR, and the greater distance between pull-in
points in duct and conduit installation.
(1) Conductivities and densities taken from the ASM Metals Handbook.
Volume 2, Ninth Edition.
Stnmgtll
(2) Conductivity on a volume basis compares conductivities of metals
for the same cross-sectional area and length.
(3) Specific gravity is density of a materia! compared to that of 'pure
water which has a density of one gmJcm~"
(4) Conductivity on a weight basis compares the conductivities of metals
The tables of mechanical properties in Chapter 4 show
rated fracture strengths of aluminum and aluminum-alloy
conductors as single wires or as stranded cables, or in
combination with steel reinforcing wires for ACSR (alumi­
num-conductor steel-reinforced) or with high-strength
aluminum-alloy reinforcement for ACAR (aluminum­
cable alloy-reinforced). Cables of other types similarly
are strength rated.
for the same weight.
metals listed are .those in almost pure form. As com­
mercially supplied, the conductivity values are slightly less.
The reduction of conductivity caused by individual
alloying agents in aluminum has been studied extensively.
Iron, zinc, and nickel cause but small reductions in con·
ductivity of aluminum. Copper, silicon. magnesium, and
vanadium produce greater reductions. Chrontium, titan­
ium, and manganese are alloying elements that cause the
greatest reduction of conductivity. Copper as an alloying
agent adds much to strength, but it is not used as a major
alloying element in electrical conductors because of a re­
duction in corrosion resistance. Aluminum alloy 2024· T4
bolts contain copper as an important alloying element,
but it is customary to anodize such bolts for corrosion
protection and to lubricate them to reduce friction and
prevent seizing.
The variation of conductivity (and its reciprocal, reo
sistivity) for usual applications is described in Chapter 3
where tables and formulas show the variation of co­
efficient of dc resistance with temperature and witb alloy
for the usual range of conductor temperatures, to I200C.
Temperature coefficients for bus·conductor alloys are
listed in Table 13-3.
Direct current (de) resistivity values for the usual
aluminum alloys used for conductors are shown in Table
3-5. The resistance under alternating current (ac) condi­
tions involves the concept of skin effect and R=!R"c ratio
as explained in Chapter 3.
2-2
Chapter 13 contains similar tables of sizes and structural
properties of usual bus-conductor shapes so that tbe
strength of a bus installation under normal or short-circuit
conditions may be readily computed, using the unit ksi
values of tensile strength for tbe various alloys as listed
in Table 13·1.
The reasons why alloying and associated cold-working
and/or heat·treatment increase the strength of the basic
metal are explained in texts on aluminum metallurgy.
WorUbility
This term has to do with the ability of the electrical con·
ductor to withstand single or repeated bending (the latter
TABLE 2-2 Relative Weights of Bare Conductor to Provide Equal Direct Current Conductance (20°C) (as Related to the Weight of a Conductor of Aluminum 1351l-61.0% lACS) Metal
Percent lACS
Percent lACS
Mass
Relative
Volume Conductivity , Conductivity Weight
Aluminum 1350 61.0% lACS
6201·T81 52.5% lACS
6101·T65 56.5% lACS
8017·H212 61.0% lACS
8030·H221 61.0% lACS
8176·H24 61.0% lACS
8177·H221 61.0% lACS
Copper
Comm'l. HD 96.0% lACS
Sodium
41.0% lACS
201
174
187
201
201
201
201
96
376
100
116
108
100
100
100
100
209
53
aluminum conductor properties and advantages
for portable cables), and for bus bars to be bent to a
specified radius either tlatwise or edgewise. Aluminum
compares favorably with other conductor metals in this
regard.
The bend radii for tlatwise and edgewise bending of
aluminum bus bars depends on alloy and temper. They are
listed in Tables 13-5 and -6 as a design guide to what can be
expected during fabrication of a bus-bar assembly.
The excellent workability of aluminum is also apparent
from noting the facility with which it may be extruded,
rolled, formed, and drawn. That bus conductors also
can be readily welded with only partial loss of rated
strength. compared with that of the unwelded alloy, is
further evidence of the workability of aluminum.
Corl1lSion Resistance
The inherent corrosion resistance of aluminum is due
to the thin. tough, oxide coating that forms directly
after a fresh surface of metallic aluminum is exposed to
air.
Another reason for the excellent corrosion resistance
of aluminum conductors in ordinary atmospheres is that
the alloy components are selected so as to minimize
corrosion. Thus. suitable alloys of the 600Q..series,
though not listed as "marine" alloys, are well suited for
oceanshore applications. as well as for usual industrial
and chemical atmospheres, as are the aluminum 1350
conductors. Instances where corrosion has appeared
are usually traceable to connections between dissimilar
metals subjected to moisture conditions. Protective
means should be employed to prevent this.
Present-day compression connectors act to break the
oxide layer on the wires of stranded cable connections.
Where unplated flat surfaces are joined. as with bus
conductors or terminal pads. scratch brushing and the
addition of oxide-inhibiting joint compound remove the
oxide and prevent its further formation because the
compound excludes oxygen.
Creep
Creep is plastic deformation that occurs in metal at
stresses below its yield strength. Normally, metal stressed
below yield for a short time returns to its original
shape and size by virtue of its elasticity. However, when
the time period is sufficiently long, plastic deformation.
called creep, occurs. This deformation is in addition to
the expected elastic deformation.
The extent of creep is determined by the properties
of the metal involved, applied stress, temperature and
time under load. For example, hard-drawn 1350-H19
aluminum wire in stranded cables under a steadily
applied load of 14 ksi at 20'C (70 percent of minimum
yield strength) will creep approximately 0.4 to 0.6 percent
of initial length in 10 years.
Creep can be considerably reduced by proper choice
of metal, metal fabrication. shape and load, and the
unwanted effects of creep may be nullified by proper
deSign. Creep data have been incorporated in stress-strain
curves for overhead conductors.
Cable manufacturers supply sag and tension data that
include the effect of creep. From Fig. 5-11, the IO-year
creep for a 1350-H19 cable at 10 ksi is estimated to be
0.23 percent: the horizontal distance between curves 2
and 4 at 10 ksi. Similarly, by comparing Fig. 5-2 and
5-3, a WOO-foot span of ACSR cable is estimated to
increase its sag from 22 feet to 26 reet in 10 years at
WOP, and its tension drops from 5700 pounds to 5100
pounds. From the catenary Table 5-4, the ratio of arc
length increase for this change of sag is about 0.17
percent; that is. the long time creep is about 1.7 feet
of arc length for the WOO-foot span. Charts such as
Fig. 5-11 also are available for many ACSR sizes to pro­
vide better accuracy.
Bus bars creep in compression, and because the metal
is not hard drawn, a 10-year creep of 1.0 percent
generally is considered allowable. Design stresses to limit
creep to this amount in various alloys are in Table 134,
Compatibility with Insulatillll
Aluminum does not have the sulphur-combining prop­
erties of copper; hence it has no effect on rubber or rub­
ber-like compounds containing sulphur. Aluminum re­
quires no tinning of the conductor metal before insulation
is applied. Also, it does not produce stearates Or soaps
by combining with oil content of an insulation. Usual
insulating materials do not adhere to the aluminum: hence
removal is easily performed by simple stripping.
Thermal Properties
The variation of electrical dc resistance with tempera·
ture was covered in the preceding discussion of conduc­
tivity. Other thermal properties that require consideration
in applications are the expansion or contraction with
changes in temperature and the thermal conductivity (the
rate at which heat is conducted).
The usual design coefficients of linear expansion for the
principal conductor metals as well as those to which the
conductor might be joined are as follows:
Aluminum
0.0000230 in.lin.lOC
Copper
0.0000169 in.lin.lOC
Steel
0.0000115 in.lin.lOC
Slight differences occur for various aDoys and tempera­
ture ranges, but they are not significant in usual engineer­
ing design. The coefficient for the bronze alloys commonly
used for bolts is about the same as that listed for copper.
Allowance must be made for differing rates of thermal
expansion when aluminum is joined by steel or bronze
bolts, or when aluminum pads are bolted to copper pads.
For overhead cables, changes in sag due to temperature
changes are discussed in Chapter 5. Actual movement of
2·3
aluminum-the metal
insulated conductors in duct, conduit, tray, or when
buried, is not proportional to increase in conductor
length with temperature. Tests show that lateral displace­
ment (snaking) of the cables will absorb 3 to 5 times the
increase in length.
The thermal conductivity of aluminum depends on alloy
and temper. For 1350-H19, it is about 0.56 callcm2/cml
OC/sec. whereas for alloys of lower electrical conductivity,
it is less. For 6063-T6, it is about 0.48. For copper,
it is about 0.98, hence heat is not conducted away
from a hot spot in aluminum as rapidly as with some
other metals, a factor taken into account when planning
welding procedures. This subject is discussed in Chapter
13. Heat dissipation from bare suspended cable is about
the same for aluminum and copper conductors of the
same ampacity rating.
2-4
The rate at which heat is conducted from a hot spot
(the thermal-conductivity rate) affects the "burn-off"
characteristic of a conductor, i.e., the amperage at which
the conductor will melt and separate at a ground point.
This factor is important when locating underground
faults (see Chapter 12), and to some extent it is related
to short-circuit ampacity rating.
•••••••••
The preceding discussions of general properties of
aluminum conductors provide background for the design
considerations described in the following chapters. They
serve to explain why aluminum is such a satisfactory
metal for electrical conductors, as proved by its excellent
long-time operating-experience record.