Engineering Design - The Aluminum Association

1 Section II Bare Aluminum Wire and Cable Chapter 3 Engineering Design This chapter describes lbe principal Design features of bare uninsulated conductors; however much lbat applies to hare conductors also pertains to lbe metaUic part of in sulated or covered conductors which are considered in Section III. Many types of bare conductors are in use depending on application requirements. They may differ in electrical and physical properties, configuration, method of assembly, and corrosion resistance. Certain general physical prop erties have been described in previous chapters. Detailed physical and electrical properties of lbe various com mercial sizes of bare conductors are listed in Chapter 4.

2 For many years it has been the practice to employ code words to identify and precisely define specific conductor constructions and designs (conductor size, stranding, insulation type, voltage rating, neutral configuration and size, number of phase conductors, type of assembly, etc.). In our text, code words are often used, as in the example under Table 3-6 wherein the code word "Blue bell" identifies a specific cable, in this case a 1,033,500 cmil, 37 strand, bare Aluminum 1350 conductor. These code words are tabulated in Aluminum Association pUblications "Code Words for Underground Distribution Cables" and "Code Words for Overhead Aluminum Electrical Cables.

3 " Symbols for types of Aluminum conductors: AAC all- Aluminum conductors (of 1350 Aluminum ); AAAC all- Aluminum alloy-conductors (of 6201-T81); ACSR Aluminum -conductor steel-reinforced (steel wire rein forcement); ACAR- Aluminum conductor Aluminum al loy-reinforced (high strength 6201-T81 wire reinforce ment). Except as otherwise referenced, ~raphs and data in tables are taken from Alcoa Aluminum Overhead Con ductor Engineering Series handbooks. Mechanical Design of Conduc:lors American Wire Gage (A WG) This wire system, formerly known as Brown & Sharpe (B&S) gage, was introduced by J. R. Brown in 1857, and is now standard for wire in the United States.

4 Successive AWG numbered sizes represent the approximate reduction in diameter associated with each successive step of wire drawing. Fig. 3-1 shows typical full-size cross-sections, and approximate relationships between the sizes. For wire sizes larger than 4/0 AWG, the size is desig nated in circular mils. Wire sizes of 4/0 AWG and smaller also are often designated in cir mils. One cir mil is the area of a circle I mil ( in.) diameter; that is, the area in cir mils equals diameter-in-mils squared. As one emil = "./4 sqmils ( ) Area in cir mils X 10' X area in sq. in. Expressing diameter of wire D, in inches D = 1O-'cmil"orcmil I()6D' ( ) Thus a solid round conductor of 1,000,000 cir mils has an area of n/4 sq.

5 In., and a diameter of in. Stranded Conductors Flexibility requirements for conductors vary widely. The conductors accordingly may be either lengths of single wires or a stranded group of smaller wires arranged in #30 #10 #1/0 #'10 Nominal AWG Awe AWG AWG Diameter, mils 10 Area, cmils 100 10,380 105,600 211,600 Approximate Relationships (I) An increase of three gage numbers doubles area and weight, and halves dc resistance. (2) An increase of six gage numbers doubles diameter. (3) An increase of ten gage numbers multiplies area and weight by 10 and divides de resistance by 10. Typical cross-sections ofsolid-round A WG-size wires and approximare relationships.

6 (Actual size.) 3 1 bare Aluminum wire and cable some regular manner. In ellber case, the total cross sectional area of all component conducting wires deter mines the A WG or emil size of the assembled conductor. Concentric-Lay Stranding Most bare power conductors are in concentric-lay stranded form; that is, a single straight core wire is sur rounded by one or more helically curved wires. The direc tion of twist of lay is usually reversed in adjacent layers. All wires of a given layer generally are of same diameter. The direction of lay is either right-or left-hand depending on whether the top wire of the helix extends to right or left as the conductor is viewed axially in the direction away from the observer.

7 The length of lay is the axial length parallel to the center line of the assembled conduc tor of one turn of the helix of a single wire. Bare alu minum conductors conventionally have a right-hand lay on outside layer. Ameriean practiee (ASTM) recognizes two classes of bare concentric-lay stranded conductors, AA and A, the former usually for bare-wire ov-erhead applications and the latter for covered overhead lines. Still greater flexibility of stranded conductors, mostly used for insulated conductors, are those with Class B, C, D, or even finer strandings. These have more wires for a given size of conductor than used for Class AA or A stranding.

8 Wires of softer temper than the usual hard drawn wires can be used. Added flexibility also may be obtained by using small braided wires or those in "bunched" arrangement.. The stranding arrangement of each class is also specified m ASTM Conductor Standards. Fig. 3-2 shows typical examples of concentric-lay stranded bare conductors for various degrees of flexibility. AAC/TW is a new Design of all Aluminum conductor composed of shaped wires (Trapezoidal) in a compact concentric-lay-stranded configuration. The Design is de scribed in ASTM B 778, and the properties are listed in Tables 4-10 and 4-11.

9 Single layer in. 00 7/w 7/.1953, (Class AA) Two layer, in. 00 19/.1185, (Class A) 19/w Three layer, in. 00 37/.0849, (Class B) 37/w Fig. 3-2. Typical Examples of Concentric Lay Conduc tors. All kcmil.(lllustrations are approximately to scale.} TABLE 3 1 Strand Lengths VS Solid Conductor Lengths for ASTM B 231 Incremental Increase for Weightandde Resistance of Stranded Over that of Solid Conductors Sizes ,000 to 3,000,001 emil 4% Sizes 3,000,000 to 2,000,001 cmil 3% Sizes 2,000,000 emil or under 2% DifJerences Between Stranded and Solid Conductors Because of the helical path of the strand layers there is more length of metal in a given length of stranded con ductor than in a solid round conductor of the same A WG size, hence both the weight and de resistance per unit length are increased.)

10 The amount of increase for ali-alumi num conductors may be computed according to a method described in ASTM B 231, or the standard increments of increase listed in Table 3 1 (also from ASTM B 231) may be used. The tensile load on a conductor is not always equally diVided among the strands. This effect can reduce the total load at which the first strand breaks as compared with that of a solid conductor of equal cross section. How ever, this effect is more than offset by the fact that the unit tensile strength of commercially cold-drawn wire generally increases as its diameter is reduced, as is evident by the comparison for H 19 stranded conductor in Table 3-2.