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Section I Aluminum-the Metal

Chapter 2

Aluminum Conductor Properties and Advantages

The mechanical and electrical properties of bare aluminum wire and stranded conductor are tabulated inChapter 4 and of bus conductor in Chapter 13. Certalngeneral properties related to the use of aluminum, asdistinct from other metals, in their application as electricalconductors are discussed in this chapter. Principally,these are:I . Conductivity: More than twice that of copper, perpound.

2. Light weight: Ease of handling, low installationcosts, longer spans, and mOre distance betweenpull-ins.3. Strength: A range of strengths from dead soft tothat of mild steel, depending on alloy. The highest strength alloys are employed in structural,rather than electrical conductor, applications.4. Workability: Permitting a wide range of processingfrom wire drawing to extrusion or rolling. Excel

lent bend quality.5. Corrosion resilance: A tough, protective oxide coating quickly forms on freshly exposed aluminumand it does not thicken significantly from continued exposure to alr. Most industrial, marine,and chemical atmospheres do not cause corrosion,providing the proper alloy is selected. The corrosion resistance of all alloys can be improvedby anodizing.6. Creep: Like all metals under sustained stress, thereis a gradual deformation over a term of years.With aluminum, design factors take it intoaccount.7. Compatibility with insulation: Does not adhere toor combine with usual insulating materials. Notin-coating required; clean stripping,Other qualities of aluminum, such as thermal conductivity and fatigue resistance, have a bearing on conductor section. The high-rellectivity and non-magneticcharacteristics, as well as the properties under extremesof temperature, are rarely associated with any commercialuse of electrical conductors; hence are not consideredherein.

The Effect of AlloyingA detailed study of aluminum applications usuallyinvolves aluminum alloys that have properties markedlydifferent from those of the basic metal. Thus, less than 2.0percent addition of other metals supplemented by aspecified heat treatment converts nearly pure aluminum

to 6101-T6 electrical bus conductor with an increase inminimum yield strength from 3.5 ksi to 25.0 ks!. Thereduction of conductivity associated with this majorchange of strength is only from 61.0 percent lACS to55.0 percent lACS.Merely adding the alloying elements to the mixtureis not sufficient to produce the desired results. Thestrength of the non-heat-treatable alloys is brought tothe value specified by the -H temper of the alloy by coldworking and/or partial annealing, and the strength of theheat-treatable-alloys is brought to that of the specified-T temper by heat treatment as explained in greaterdetail in Chapter I.In the manufacture of heat-treatable aluminum alloyconductor wire, the supplemental treatment (cold worltingand heat treatment) usually is divided into two parts-often at different locations: (I) that performed duringthe production of redraw rod (0.375 inch diameter) and(2) that performed during or after reduction of diameterof the redraw rod to the finished wire size. Bus-conductorshapes have most of the necessary heat treatment performed during extrusion. Aging may be performedsubsequently.

ConductivityThe conductivity of pure aluminum is about 65.0

percent lACS, However, the conductivity of aluminum1350 is 61.0 percent lACS minimum due to low levelimpurities inherent to commerical processing (up to62.4% lACS is available in 1350 on a special order basis).The conductivity for bus conductor alloys is shown inTable B-2. The conductivities of 6201 and the 8XXXseries alloys in the tempers, which are used in the production of wires for cables, are also shown in Table I-LA comparison of conductivities of metals sometimesused for electrical conductors is shown in Table 2-1. The

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aluminurn---the metalTABLE 2-1Relative Conductivities of Pure Metals(l)

Conductivity Conductivity: Percent lACS Specific Percent lACSMetal i Vol. Basis(2) Gravity(3) Wgt. Basis(4)Silver 108.4 10.49 91.9

!opper 103.1 8.93 102.6Aluminum 64.9 2.70 213.7Titanian 4.1 4.51 8.1Magnesium i 38.7 1.74 197.7Sodium 41.0 0.97 376.2

(1) Conductivities and densities taken from the ASM Metals Handbook.Volume 2, Ninth Edition.(2) Conductivity on a volume basis compares conductivities of metalsfor 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 g m J c m ~ "(4) Conductivityon a weight basis compares the conductivities of metalsfor the same weight.

metals listed are .those in almost pure form. As commercially supplied, the conductivity values are slightly less.The reduction of conductivity caused by individualalloying agents in aluminum has been studied extensively.

Iron, zinc, and nickel cause but small reductions in conductivity of aluminum. Copper, silicon. magnesium, andvanadium produce greater reductions. Chrontium, titanium, and manganese are alloying elements that cause thegreatest reduction of conductivity. Copper as an alloyingagent adds much to strength, but it is not used as a majoralloying element in electrical conductors because of a reduction in corrosion resistance. Aluminum alloy 2024 T4bolts contain copper as an important alloying element,but it is customary to anodize such bolts for corrosionprotection and to lubricate them to reduce friction andprevent seizing.The variation of conductivity (and its reciprocal, reosistivity) for usual applications is described in Chapter 3

where tables and formulas show the variation of coefficient of dc resistance with temperature and witb alloyfor the usual range of conductor temperatures, to I200C.Temperature coefficients for busconductor alloys arelisted in Table 13-3.Direct current (de) resistivity values for the usualaluminum alloys used for conductors are shown in Table3-5. The resistance under alternating current (ac) conditions involves the concept of skin effect and R=!R"c ratioas explained in Chapter 3.

Liallt WeialltThe relative conductor weights required for equal conductivity using various metals are listed in Table 2-2These were developed from Table 21 (percent lACS masconductivity and density values) applying conversionmethods described in ASTM Specification B 193.The lighter weight aluminum provides obvious handling

cost reductions over heavier metals. Reduced capital andinstallation costs are an added advantage of aluminumconductors by reason of the long-span capability of ACSRand ACAR, and the greater distance between pull-inpoints in duct and condui t installation.Stnmgtll

The tables of mechanical properties in Chapter 4 showrated fracture strengths of aluminum and aluminum-alloconductors as single wires or as stranded cables, or incombination with steel reinforcing wires for ACSR (aluminum-conductor steel-reinforced) or with high-strengthaluminum-alloy reinforcement for ACAR (aluminumcable alloy-reinforced). Cables of other types similarlyare strength rated.Chapter 13 contains similar tables of sizes and structuraproperties of usual bus-conductor shapes so that tbstrength of a bus installation under normal or short-circuiconditions may be readily computed, using the unit ksvalues of tensile strength for tbe various alloys as listedin Table 131.The reasons why alloying and associated cold-working

and/or heattreatment increase the strength of the basimetal are explained in texts on aluminum metallurgyWorUbility

This term has to do with the ability of the electrical conductor to withstand single or repeated bending (the latte

TABLE 2-2 Relative Weights of Bare Conductor to Provide EquaDirect Current Conductance (20C) (as Related to the Weight of a Conductor of Aluminum 1351l-61.0% lACS)

Percent lACSPercent lACS Mass RelativMetal Volume Conductivi ty , Conductivi ty WeighAluminum 1350 61.0% lACS 201 100

6201T81 52.5% lACS 174 1166101T65 56.5% lACS 187 1088017H212 61.0% lACS 201 1008030H221 61.0% lACS 201 1008176H24 61.0% lACS 201 1008177H221 61.0% lACS 201 100Copper Comm'l. HD 96.0% lACS 96 209Sodium 41.0% lACS 376 53

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for portable cables), and for bus bars to be bent to aspecified radius either tlatwise or edgewise. Aluminumcompares favorably with other conductor metals in thisregard.The bend radii for tlatwise and edgewise bending ofaluminum bus bars depends on alloy and temper. They arelisted in Tables 13-5 and -6 as a design guide to what can beexpected during fabrication of a bus-bar assembly.The excellent workability of aluminum is also apparentfrom noting the facility with which it may be extruded,rolled, formed, and drawn. That bus conductors alsocan be readily welded with only partial loss of ratedstrength. compared with that of the unwelded alloy, isfurther evidence of the workability of aluminum.

Corl1lSion ResistanceThe inherent corrosion resistance of aluminum is dueto the thin. tough, oxide coating that forms directlyafter a fresh surface of metallic aluminum is exposed toair.Another reason for the excellent corrosion resistance

of aluminum conductors in ordinary atmospheres is thatthe alloy components are selected so as to minimizecorrosion. Thus. suitable alloys of the 600Q..series,though not listed as "marine" alloys, are well suited foroceanshore applications. as well as for usual industrialand chemical atmospheres, as are the aluminum 1350conductors. Instances where corrosion has appearedare usually traceable to connections between dissimilarmetals subjected to moisture conditions. Protectivemeans should be employed to prevent this.Present-day compression connectors act to break theoxide layer on the wires of stranded cable connections.Where unplated flat surfaces are joined. as with busconductors or terminal pads. scratch brushing and theaddition of oxide-inhibiting joint compound remove theoxide and prevent its further formation because thecompound excludes oxygen.

CreepCreep is plastic deformation that occurs in metal atstresses below its yield strength. Normally, metal stressedbelow yield for a short time returns to its originalshape 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 tothe expected elastic deformation.The extent of creep is determined by the properties

of the metal involved, applied stress, temperature andtime under load. For example, hard-drawn 1350-H19aluminum wire in stranded cables under a steadilyapplied load of 14 ksi at 20'C (70 percent of minimumyield strength) will creep approximately 0.4 to 0.6 percentof initial length in 10 years.

aluminum conductor properties and advantagesCreep can be considerably reduced by proper choice

of metal, metal fabrication. shape and load, and theunwanted effects of creep may be nullified by properdeSign. Creep data have been incorporated in stress-straincurves for overhead conductors.Cable manufacturers supply sag and tension data thatinclude the effect of creep. From Fig. 5-11, the IO-yearcreep for a 1350-H19 cable at 10 ksi is estimated to be

0.23 percent: the horizontal distance between curves 2and 4 at 10 ksi. Similarly, by comparing Fig. 5-2 and5-3, a WOO-foot span of ACSR cable is estimated toincrease its sag from 22 feet to 26 reet in 10 years atWOP, and its tension drops from 5700 pounds to 5100pounds. From the catenary Table 5-4, the ratio of arclength increase for this change of sag is about 0.17percent; that is. the long time creep is about 1.7 feetof arc length for the WOO-foot span. Charts such asFig. 5-11 also are available for many ACSR sizes to provide better accuracy.

Bus bars creep in compression, and because the metalis not hard drawn, a 10-year creep of 1.0 percentgenerally is considered allowable. Design stresses to limitcreep to this amount in various alloys are in Table 134,Compatibility with Insulatillll

Aluminum does not have the sulphur-combining properties of copper; hence it has no effect on rubber or rubber-like compounds containing sulphur. Aluminum requires no tinning of the conductor metal before insulationis applied. Also, it does not produce stearates Or soapsby combining with oil content of an insulation. Usualinsulating materials do not adhere to the aluminum: henceremoval is easily performed by simple stripping.Thermal PropertiesThe variation of electrical dc resistance with temperature was covered in the preceding discussion of conductivity. Other thermal properties that require considerationin applications are the expansion or contraction withchanges in temperature and the thermal conductivity (therate at which heat is conducted).

The usual design coefficients of linear expansion for theprincipal conductor metals as well as those to which theconductor might be joined are as follows:Aluminum 0.0000230 in. lin.lOCCopper 0.0000169 in.lin.lOCSteel 0.0000115 in. lin.lOC

Slight differences occur for various aDoys and temperature ranges, but they are not significant in usual engineering design. The coefficient for the bronze alloys commonlyused for bolts is about the same as that listed for copper.Allowance must be made for differing rates of thermalexpansion when aluminum is joined by steel or bronzebolts, or when aluminum pads are bolted to copper pads.

For overhead cables, changes in sag due to temperaturechanges are discussed in Chapter 5. Actual movement of23

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aluminum-the metal

insulated conductors in duct, conduit, tray, or whenburied, is not proportional to increase in conductorlength with temperature. Tests show that lateral displacement (snaking) of the cables will absorb 3 to 5 times theincrease in length.The thermal conductivity of aluminum depends on alloyand 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 awayfrom a hot spot in aluminum as rapidly as with someother metals, a factor taken into account when planningwelding procedures. This subject is discussed in Chapter13. Heat dissipation from bare suspended cable is aboutthe same for aluminum and copper conductors of thesame ampacity rating.

The rate at which heat is conducted from a hot sp(the thermal-conductivity rate) affects the "burn-ofcharacteristic of a conductor, i. e., the amperage at whicthe conductor will melt and separate at a ground poinThis factor is important when locating undergrounfaults (see Chapter 12), and to some extent it is relateto short-circuit ampacity rating.

The preceding discussions of general properties oaluminum conductors provide background for the desigconsiderations described in the following chapters. Theserve to explain why aluminum is such a satisfactormetal for electrical conductors, as proved by its excellenlong-time operating-experience record.

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