TECHNICAL BULLETIN The Difference Between Copper and Aluminum First used almost a century ago, the aluminum conductor is no newcomer to the electrical field. Today, virtually all overhead transmission and distribution lines use aluminum conductors. The economy and performance record of aluminum, as well as its popularity in overhead use, has been a large factor in the rapid spread of its use in underground distribution and building wires. This expanding use has caused relatively few growing pains in Canada. Historically aluminum building wires have generally cost 25% to 35% less than equivalent copper wires. In addition to its attractive cost, the physical properties of aluminum make it far easier and more economical to install. The essential chararacteristics of aluminum, and the differences between aluminum and copper are described within. that connectors are visibly marked to indicate suitability for the conductor material. Most connectors available today are dual rated for both aluminum and copper conductors. DENSITY AND CONDUCTIVITY An aluminum conductor weighs about 70% less than a copper conductor of the same size and to offer the same current carrying capacity must be 50% larger incross-sectional area. Allowing for these two properties results in an aluminum conductor doing the same electrical work and weighing only half as much as the copper equivalent. Table 1 summarizes the comparative values for a 3/0 copper conductor and a 250 kcmil NUAL® Brand conductor. Table 1: Physical Property Comparison of a 3/0 RW90 Copper Conductor and a 250 kcmil RW90 NUAL Brand Conductor Units 3/0 AWG Copper 250 kcmil NUAL® Brand Ratio NUAL/ Copper Metal Weight kg/km 776.5 349.3 0.45 Total Weight kg/km 863.1 472.9 0.55 Metal O.D. mm 11.63 13.24 1.14 Overall O.D. mm 15.65 17.80 1.14 Area sq. mm 85.03 126.71 1.49 Current Rating Amps 330 330 1.00 This is in TThis indicated ndi dica c ted on the con connector o nector bo body itself by the indentations of A L7C L7 CU for or 775° 5° C cconnectors onnect on ctors and A AL7CU AL9CU on 90° C rated connectors. conn nectorss. Due to tthis hiss differen hi difference ence in th thermal exp expansion, aluminum conductors cond co nduc uctoors should shooul uldd ne nnever ve er be used wit with copper connectors. As thee connec ctition o heats up during oper connection operation, the outer copper body bo dy eexperiences xperie xp enc nces lless e s expansion tha es than the inner aluminum conductor. conduc ctor. Th This his results in pressure bbuild up within the connector and ps pssi pssible sibl ble extrusion of the aluminum to relieve the pressure to some degree. Repeated heat cycles can then result in higher resistance connections over time. MECHANICAL STRENGTH THERMAL EXPANSION The tensile strength of aluminum wires varies from 200 MPa (29 ksi) down to approximately 59 MPa (8.5 ksi). The difference between these values depends on the temper of the wire. For overhead lines, the desired property is strength, and wires must have a high temper (typically H19). Conversely, for building wires, tensile strength is not as important as flexibility and elongation. That is why the tensile strength of aluminum building wires is low. CSA requires NUAL Brand aluminum building wires to be between 104 MPa (15 ksi) to 138 MPa (20 ksi). Annealed copper is approximately 250 MPa (36 ksi). Copper, when heated expands about 26% less than aluminum. This difference in the linear coefficient of thermal expansion is taken into account by the manufacturers of connectors when designing their products. It is therefore necessary to ensure It has been said that because aluminum has a lower tensile strength compared with annealed copper, it must be treated more carefully when pulled. It is important to realize that NUAL Brand is General Cable's registered trade name for our aluminum conductor material (ACM) as recognized by CSA. 156 Parkshore Drive, Brampton, Ontario, Canada, L6T 5M1| GENERAL CABLE and NUAL BRAND are trademarks of General Cable Technologies Corporation. ©2014. General Cable Technologies Corporation. Highland Heights, KY 41076 All rights reserved. Printed in USA Phone: 1.800.561.0649 1.905.494.5300 Fax: 1.800.565.2529 www.nual.com Form No. BWWP-007-0414 aluminum is much lighter than copper and therefore the strengthto-weight ratio, which should govern the force of the pull, is a better indicator. Literally, this ratio indicates the length that a conductor can support itself before breaking. for aluminum, this ratio is 3900m, compared with 2800m for copper. Assuming friction and flexibility being equal, aluminum conductors will perform better in pulls. CREEP Creep is a permanent deformation caused by applying a stress, at a temperature, for a given time. A common example is a piece of lumber leaning up on a wall. With time, it will acquire a permanent curve. This is due to a light bending stress caused by its own weight and applied for a long time. An extreme example of creep is a tensile rupture — in this case, a high stress is applied for a short time. Creep is a complicated phenomenon. All materials experience creep, albeit to different degrees. For metals, it is related to grain size, grain shape, residual stresses due to manufacturing methods, alloying elements, ambient temperature, etc. As it pertains to conductors, it is sometimes said that aluminum connections become loose in service or that current cycling forces them to loosen. This alleged movement from fully tight to fully loose has, in fact, no basis. Extensive testing in General Cable laboratories has shown no deterioration of the contact resistance. wire brushing with immediate application of joint compound to the conductor surface will remove it and prevent its reformation. THE CASE FOR ALUMINUM As mentioned previsously, a pound of aluminum does the work of two pounds of copper. It may seem irrelevant to mention weight for a product that is usually thought of in units of lengths. However, raw material is bought by weight, and although commodity prices tend to fluctuate, historically copper has been a far more expensive commodity. In fact, savings of up to 35% have been achieved by replacing copper conductors with their equivalent aluminum conductors. Aluminum conductors have been used for almost a century to carry power from the generating plant to end users and have proven their reliability and effectiveness. With over 50 years of use in building wire applications, aluminum conductors also offer greater reliability and longer life due to greater corrosion resistance. NUAL® Brand conductors can be installed more easily than copper conductors because of their lower weight and significant flexibility advantage over copper. These properties lower handling and installation costs. Copper and NUAL® Brand - Comparison Table Units 3 NUAL® Brand Copper CORROSION AND SURFACE OXIDE Density kg/m 2700 8930 Tensile Strength* MPa 103 - 138 248 Corrosion of aluminum connectors and wire is non-existent in dry locations. For corrosion to occur in an aluminum connector/ conductor system, an electrolyte must be present. Under certain conditions of moisture exposure, combined with contaminants, a small amount of electrolytic corrosion is possible. Tensile Strength ksi 15 - 20 36 Strength/Weight km 3.9 2.8 Elongation** % 10 15 - 35 m/m/°C 23.0 x 10-6 16.9 x 10-6 % IACS (vol.) 64.9 103.1 Thermal Expansion Conductivity*** Conductivity The oxide coating on aluminum is particularly resistant to most types of corrosion and is a great protection for the aluminum conductor in such situations. The coating on the aluminum conductor is, in itself, a minor corrosion product which builds up on the surface of bare aluminum components due to air exposure. This oxide coating is extremely thin (10-6 cm), is brittle like glass and is ruptured when only minimal force is applied. In fact, recommended tightening torque are far in excess of what is required to break the oxide layer. % IACS (wt.) 213.7 102.6 Resistivity Ohm • mm2 / m 0.02656 0.01673 Resistivity Ohm • kg2 / m 0.12255 0.23926 *Tensile strength for NUAL® Brand corresponds to H14 temper of AA-1350. There is no tensile requirement for annealed copper. The value of 248 MPa is used as typical. **Elongation for NUAL® Brand is minimum. For copper, CSA requirements vary with the size of the wire. ***Conductivity on a volume basis compares conductivities of metals for the same srosssectional area and length. Conductivity on a weight basis compares the conductivities of metals for the same weight. The coating has some unique and important characteristics in that the thickness of oxide is limited and does not continue to grow forever, like oxide would do on steel or copper. If removed in air, aluminum oxide will start to reform immediately, and simple 156 Parkshore Drive, Brampton, Ontario, Canada, L6T 5M1| GENERAL CABLE and NUAL BRAND are trademarks of General Cable Technologies Corporation. ©2014. General Cable Technologies Corporation. Highland Heights, KY 41076 All rights reserved. Printed in USA Phone: 1.800.561.0649 1.905.494.5300 Fax: 1.800.565.2529 www.nual.com Form No. BWWP-007-0414