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ELECTRICAL EQUIPMENT
Larry Pryor, P.E. – Sr. Specification Engineer, GE Consumer & Industrial Rick Schlobohm, P.E. – Sr. Specification Engineer, GE Consumer & Industrial Bill Brownell, P.E – Specification Engineer, GE Consumer & Industrial
ABSTRACT
The debate over the pros and cons of aluminum vs. copper conductors has been discussed for many years. Many of the concerns are based on old information and also misinformation. These concerns center on the very different properties of the two materials and their suitability for application within the Electrical Equipment Industry. Reliable performance from the conductors, the joints and terminations of the conductors is essential to the operation of the electrical system. With the recent increases in the cost of copper, customers are seeing these increases passed on to them by the equipment manufacturer. The variance in cost between equipment with copper versus aluminum conductors is now affecting the customer’s buying decision.
This paper provides a comparison of the mechanical and electrical properties of copper (Cu) and aluminum (Al) and their relevance as applied to electrical distribution products. The offerings and impact of Cu and Al on various pieces of equipment is also discussed. The intended purpose is to provide the user with the information necessary to make an informed decision on the selection of copper or aluminum conductors within electrical equipment.
INTRODUCTION
Cu and Al are the two most commonly used materials for conductors and bus bars in electrical equipment. Each has positive and negative characteristics that affect their use in various applications. Both materials have been in continuous use in the electrical industry for many years. While aluminum is the most abundantly available of the two metals, the demand and scarcity of Cu have caused its cost to fluctuate widely. Silver is generally considered the best electrical conductor, however its high cost and low
strength limits its use to special applications such as joint plating and sliding contact surfaces.
For the purposes of this paper, the areas of discussion are:
- Current Carrying Capacity, Conductivity
- Physical Properties a. Expansion b. Weight c. Tensile Strength
- Connections and Terminations
- Plating and Environmental Concerns
- Product Offerings
- Cost Comparison
- Impact on Various Pieces of Equipment
CURRENT CARRYING CAPACITY
The electrical and mechanical properties of a material are dependent on its alloy. For the comparison between copper and aluminum in this paper, the copper is cold-worked electrolytic tough pitch copper similar to ASTM B187, alloy UNC C11000. The copper used in electrical equipment is nominally pure 98% conductivity commercially hard based on the International Annealed Copper Standard (IACS). Pure Aluminum is not used as an electrical conductor in equipment since it is too soft for mechanical assemblies and is thus alloyed with other materials. The Al alloy 1350 used prior to 1975 was designated as EC (Electrical Conductor) grade aluminum with a 99.50% aluminum content. Even though it has 61% the conductivity of Cu it lacked in mechanical properties making it less than ideal as a conductor in the equipment. Al alloy 6101 is the predominant aluminum bus bar material being utilized and is stronger than 1350 Al because it has been hardened by heat treatment, but it only has 56% the conductivity of copper. The reduced conductivity of AL 6101 does not mean that the Al conductor will run hotter than the Cu
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conductor but does mean that the Al conductor for the same ampere rating must have a larger cross sectional area. To analyze the current carrying capacity, two design criteria must be taken into consideration: The temperature rise of the conductor within the equipment above a maximum allowable ambient temperature and the current density in amperes per square inch of the cross sectional area.
Temperature rise is the established method for determining the current rating of the conductors within the electrical equipment. The heat generated in a bus bar is dissipated by convection, radiation or conduction or a combination of these methods. Industry standards such as UL and ANSI provide design requirements for various electrical equipment products. Switchboards and panelboards are designed to conform to UL standards, which permit a 55 ° C rise for switchboards and a 50° C rise for panelboards. Switchgear conforms to ANSI standard C37. which permits a temperature rise of 65° C above a maximum ambient of 40° C, provided that silver- plated (or acceptable alternative) bolted terminations are used. If not a temperature rise of 30 ° C over the same ambient is allowed. Regardless of which conductor material is used, aluminum, or copper, equipment manufactures must apply the proper conductor size to stay within the design requirements so that the equipment will operate under the same allowable temperature rise. To achieve this, an Al conductor must have its cross section area increased inversely as a function of the conductivity of the alloy used.
When the density of Cu (559 lb/ft^3) is compared to that of Al (169 lb/ft^3) and taking into consideration the conductivity ratio of Al to Cu of 56%, the result shows that on a pound per pound basis, Al has an amperage capability that is approximately 1.85 times that of Cu. In other words, one pound of Al has the same electrical capability as 1.85 pounds of Cu. Cu has a greater conductivity on an equal volume, cross sectional area, basis.
NEC Article 310 lists the allowable ampacities of Al and Cu conductors. As a comparison, table 310. shows that where a 500 MCM, 75° C, Cu cable has a rating of 380 A, a 750 MCM, 75° C, Al cable would be required. This is a 50% increase in cross section for the same current carrying capacity. This will
result in an increase in conduit size for aluminum conductors versus the copper conductors for the same current carrying capacity.
For applications where weight is a concern, Al may be the better choice. Depending on the equipment type and it’s application, and if space and size are a consideration, Cu may be the better choice. This comparison is examined later in this paper.
PHYSICAL PROPERTIES A comparison of some of the properties of Cu and Al are given in the following table. Properties will very depending on the alloy used.
CHARACTERISTICS COPPER ALUMINUM Tensile strength (lb/in2) (^) 50,000 32, Tensile strength for same conductivity (lb.)
Weight for same conductivity (lb.)
Cross section for same conductivity
Specific resistance (ohms- cir/mil ft) (20°C ref)
Coefficient of expansion (per deg. C x 10^-6)
Information in the above table was obtained from GE Properties and Material bulletins B11B for copper and B12H60 for aluminum.
The properties that need to be discussed are the tensile strength and thermal expansion of the conductors. Of particular concern is the ability of the conductor to withstand the forces resulting from short circuits and the effects of expansion from heat on joints and terminations.
Reviewing the information in the table above, you can see that the aluminum conductor will have a cross sectional area 56% larger than copper for the same current carrying capability. Even though aluminum does have a lower tensile strength than copper it can be seen that the AL has, essentially, the same tensile strength of Cu for the same ampacity (50,000 lb/in2). The main area where this would be of concern as stated previously would be strength to withstand the forces during short circuits.
Underwriters Laboratory (UL), the National Electrical Manufactures Association (NEMA), and the Institute of Electrical and Electronics
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need for more expensive compression connectors and the more laborious installations for these connectors. The substitution of aluminum wire for copper always involves size and can also impact quantity. The size increase is usually one or two wire sizes. It is more common to have compact stranding of aluminum wire than copper, which can reduce the conduit upsizing required. Even though physically larger the aluminum wire is lighter and easier to handle than the equivalent copper conductor. In most cases the same lug can accommodate either aluminum or copper and has adequate wire range. Any lug marked ALCU is suitable for use with either conductor.
Another factor with the use of aluminum wiring for the supply or load from a piece of the electrical equipment is the size of the conduits. As mentioned previously, the use of aluminum conductors will result in either larger conductor size or more quantity of conductors. Either way, more or larger conduits will be utilized. A design trend is always toward equipment with smaller footprints. Cost of the space in the structures housing the equipment is constantly increasing. However in many cases there might not be physical space in the equipment for the termination of the conduits using aluminum conductors while there is adequate space for the quantity and size of the conduits for the copper conductors.
PLATING AND ENVIRONMENTAL CONCERNS
Both Al and Cu will oxidize when exposed to the atmosphere. Oxides, chlorides, or sulfides of the base metal are much more conductive for copper than aluminum. For a low resistance aluminum joint, the aluminum bar conductors must be plated to minimize oxidation. Concern over the Al oxidation away from the joint is not an issue and will act to protect the conductor from further corrosion in most environments. Aluminum bus conductors depend upon the plating for the integrity of the electrical connection. Aluminum and copper conductors are typically plated with silver or tin. In general, bolted connection of unplated aluminum to copper bus bars is discouraged. The majority of Al to Cu connections are made by applying silver or tin plating to the joint areas of either or both of the conductors.
The presence of hydrogen sulfide (H2S) in the atmosphere is of main concern for base metal Cu
and silver plating. Both corrode heavily in a relatively low concentration of H2S and most intensely in locations usually having an elevated temperature while the equipment is energized. Two processes are active at the same time, general corrosion of the silver and creep corrosion of Cu. Silver plating is widely used on contacts and other conductive parts in electrical equipment due to its superior conductivity, abrasion resistance and longevity. Hydrogen sulfide is usually present at chemical plants, oil refineries, steel mills, pulp and paper mills, and wastewater treatment facilities.
In a H2S environment metal filaments (whiskers) start to grow as soon as a thick enough layer of silver sulfide has been formed. This silver corrosion results in a high resistance producing more heat, which further stimulates tarnishing and growth of whiskers. This process if allowed to continue leads to failure due to over heating or short circuit. Tin plating displays good environmental protection and is a practical solution to the H2S corrosion problem of copper and silver-plated copper
OFFERINGS
GE currently offers aluminum conductors in a variety of products, but not in all products. Products lines such as Motor Control Centers and Switchgear have historically offered aluminum bus, but currently only offer copper bus. There are numerous reasons for the copper only offering, but primarily customer demand and in turn manufacturing efficiencies drove the decision. In the past Aluminum bus was prevalent in switchgear, but issues at the time with joint connections forced welding of the bus. This in turn limited flexibility of the equipment in the field and resulted in customer dissatisfaction. Also in the past copper bus was not offered with tin plating so aluminum bus with tin plating was the standard for certain environments such as H2S where the silver plating would react and turn black. When copper bus became available with tin plating and even though past issues with aluminum bus had been resolved, 99% of the customers requested copper bus. From a manufacturing perspective, it was not economical to support aluminum bus for this equipment.
The current offering from GE for both switchgear and motor control centers is either tin or silver
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plated copper bus. In the Spectra line of switchboards silver plated copper or tin plated aluminum is offered. In lighting panelboards, both aluminum and copper bus is offered with the copper available in either tin or silver plating. One unique item with lighting panels is that even though aluminum bus is furnished for the vertical bus, the breaker mounting straps are still furnished in copper. There is no issue with this mix since the aluminum is tin-plated and the copper is silver- plated. Copper and aluminum bus are also offered in the Spectra distribution panels.
Spectra busway is offered with either copper or aluminum conductors. The aluminum bus is provided with a multi-layer plating of tin, copper, bronze and silver to maximize conductivity, provide resistance to galling at plug-in locations and prevent any fretting corrosion at stab locations.
Dry type transformers are offered with either copper or aluminum conductors. Many specifications are still written not allowing aluminum foil conductors However for over 45 years, GE has utilized thermally stable insulated aluminum magnet wire terminated by solder or weld to lug pads for customer connections. At this time all general purpose transformers must meet the same NEMA TP-1 efficiency standards whether copper or aluminum wound.
Oil filled padmount or substation transformers are offered by GE with aluminum primary and secondary windings, copper primary and copper secondary winding or a combination with copper primary and aluminum secondary. The variety of offerings is provided to meet the customer’s electrical and budgetary needs.
COST COMPARISON
The cost difference between copper and aluminum varies with the fluctuating cost of the base metals on the commodities market. However this cost difference is many times the deciding factor when a customer is considering aluminum conductors in their equipment. All of the references on the cost differences are based on the effect that the current commodities market has on the components into electrical equipment.
The percentage difference in the cost between aluminum and copper also varies as the percentage that the conductor is a component of the overall equipment. For example a 1200A
distribution panel with no breakers installed would show a cost difference of 25-50% but when the panel is loaded with breakers, the percentage of the price of the bus bars is much less than the overall price of the panel. The cost difference between and aluminum and copper bussed panel drops to approx 7-8%. A contrasting example of the conductor being a large percentage of the overall assembly is with busway. The larger the amperage of the busway, the greater the base conductor is to the percentage of the overall equipment. An example would be comparing 3 phase, 4 wire busway at 1000A to the same busway at 4000A. At 1000A, the adder to go from aluminum to copper would be approximately 50%, but at 4000A, the adder for copper over aluminum is almost 100%.
Panelboards both distribution and lighting class can have a copper to aluminum price differential anywhere between 10-50%. This varies as explained above with the overall price of the breakers and other components installed in the panelboards. Switchboards on average will have a 25-30% premium for copper bus over aluminum bus.
With respect to percentage of conductor content within the equipment, transformers have a higher percentage of the conductor than a panelboard, but much less than busway. Typical dry type transformers used on commercial projects will vary anywhere from 45% to as much as 100% premium for the copper over the aluminum windings. Liquid filled transformers such as padmounts or SSTs, currently show a price differential of 12-15% for Cu-Cu vs. AL-AL, but this percentage drops to about 6-8% when a Cu-Cu is compared to a Cu-Al wound transformer.
Currently electrical equipment with aluminum conductors designed to perform identical to the same equipment with copper conductors will provide a dollars savings to the end user.
IMPACT OF VARIOUS PIECES OF EQUIPMENT
It is a common misconception that electrical equipment built using aluminum conductors will always be larger than the same equipment using copper conductors. While the actual conductor within the equipment will be larger with aluminum, many times the enclosure for the
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REFERENCES
- National Electrical Code
- Underwriters Laboratories Inc.
- National Electrical Manufacturers Association
- Institute of Electrical and Electronic Engineers
- American National Standards Institute
- Technical and Economic Considerations of Aluminum Conductors Yanniello, Pollak, and Rook
- Aluminum in Low-Voltage Distribution Equipment GE Publication GER - 2574
- Degradation of Power Contacts in Industrial Atmosphere: Silver Corrosion and Whiskers - Bella H. Chudnovsky
