Copper clad aluminum wire (C) Daniel FriedmanCopper-Clad Aluminum Wire Safety & History
Comments on the history, testing, & safety of Copper-Clad Aluminum electrical wiring

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Copper-Clad Aluminum Electrical Wiring identification, safety, production methods.

This article describes the history of copper-clad aluminum electrical wiring and its safe use in homes in the United States. Photo at page top: a #10 copper-clad aluminum electrical wire showing the stripped and cut end. Click to enlarge the image and you can see the thickness of the copper cladding around the solid aluminum conductor core of this wire.

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Copper-Clad Aluminum Electrical Wiring

Thomas J. D'Agostino, P.E., C.I., & Daniel Friedman updated 2016/09/26

Copper-clad aluminum electrical wiring markings (C) InspectApedia Roger Hankey

[Click to enlarge any image]

The photograph of copper-clad aluminum wiring shown above illustrates this product installed in a installed in a Minnesota home: photo provided by and used with permission of Roger Hankey, a Minnesota home inspector.

Article Contents

What is copper-clad aluminum wire?

Close-up of copper clad aluminum wiring showing the copper sheathing (C) Daniel Friedman

Copper-clad aluminum wire is made of a solid aluminum core covered with a copper skin in order to improve its performance in the field. This product was installed in homes in the United States between 1972 and 1975. Above you can see a close-up of a typical end-cut of a #10 copper clad aluminum wire. The image shows the thickness of the copper cladding (yellow arrow) as well as cut marks across the end of the wire's aluminum conductor (white arrow). In fact a small amount of the copper cladding was smeared across the cut wire-end by the wire-cutting process as you might notice at the lower edge of the cut edge above the "COR" in the word "CORE".

Some details about copper-clad aluminum wiring are provided by Tom D'Agostino:

Copper clad aluminum electrical wire is comprised of a solid aluminum core covered with a thin clad of copper. Approximately 10 percent of the cross-sectional area of the conductor is copper clad, thickness is not specified.

This product (only wire size Nos. 12-10 AWG), was designed by Texas Instruments for use with standard copper-only wiring devices, ie. switches, receptacles (binding head screw, back-wired, and pressure plate type terminals) and was treated as "aluminum" conductor for use in AL-CU pressure wire connectors in response to field failures of solid aluminum conductor branch circuit wiring. - D'Agostino (2014)

I would not assume that copper-clad AL wire might not appear in an older home in which electrical wiring was added or changed during that same interval, and it's also possible, though less likely, that copper clad aluminum wire might appear in new home built after 1975 if an electrician had and wished to use up old copper-clad aluminum wire stock.

Is copper-clad aluminum wire safe to leave in place?

Yes. Copper-clad aluminum electrical wire is described in REDUCE THE AL WIRE RISK: DETAILS, as well as in Aronstein (2011). There Aronstein states

Copper-clad aluminum wire (Nos. 12-10 AWG) has a thin copper outer skin and a core of aluminum. Therefore when installed and where visible in the main electrical service panel, it looks like "fat" copper wire (i.e., 10-12 AWG v. 12-14 AWG) and on the cut end, the copper skin looks to be "smeared" over the aluminum core. Markings on the cable jacket would include "Al" or "Aluminum". There is no known history of connection overheating problems associated with copper-clad aluminum wire. No corrective actions are required for copper-clad aluminum wire. - Aronstein (2011), D'Agostino (2014)

How to identify copper-clad aluminum wire

Examples of copper-clad aluminum wiring (C) Daniel Friedman

Recognizing that electrical wire is copper clad aluminum is useful for technical reasons. For example, the wire gauge or diameter sizes of copper clad aluminum wire will follow the same physical sizes as for solid core aluminum wiring. A 15 Amp electrical circuit would have been wired with #12 aluminum wire, #12 copper-clad aluminum wire, or with #14 solid copper wire. We summarize these wire sizes for aluminum and copper and copper-clad wire at SIZE of WIRE REQUIRED for common household branch circuit wiring, and also at SE CABLE SIZES vs AMPS including wire sizes from #14 AWG through 4/0 AWG.

Externally, if we eschew noticing printed labels and marking that may be present on copper clad aluminum wire, and if we simply look at the wire itself, stripped of insulation, the wire looks like a copper. However if we can take a closer look at a relatively square-cut end of the wire we should see an aluminum core surrounded by a copper skin. Without disassembly and possibly even careful cutting of the end of the wire this detail may not be easily visible, since typical wire nippers compress the end of the wire when making a cut.

Scraping copper clad aluminum wire may not be adequate to identify its silver-colored aluminum core unless the scrape is vigorous enough to cut completely through the copper cladding.

Copper clad multi-strand #12 aluminum wire (C) Daniel Friedman

Above: #12 multi-strand copper-clad aluminum electrical wire. Click to enlarge this photo and you'll see that when I [DF] stripped the wire my stripping tool produced a long horizontal gouge on one of the wire strands, exposing the aluminum through the copper cladding. Both Aronstein and D'Agostino suggest that nicks and scratches such as this won't prevent successful use of the wire as there is plenty of other copper-clad wire surface available to be in contact with the wire connector. Below, a close-up of the cut-ends of this wire product.

Cut-ends of copper-clad multi-strand #12 aluminum wire (C) Daniel Friedman

Below we illustrate the typical end of a copper-clad aluminum wire that was cut using a wiring tool, magnified about 120x. You can see that the copper "skin" on the aluminum wire surface is rather thick. (The entire field of view is about 1.5mm).

Microscopic view of cut end of copper-clad aluminum electrical wire 100x (C) Daniel Friedman

Let's compare the copper clad wire cross-cut image (above) with solid aluminum and solid copper wire. Below is a cross-section cut of a solid aluminum electrical wire with no copper cladding (with white insulation) and a solid copper conductor (with black insulation).

Comparing copper and aluminum wires in cross-section (C) Daniel Friedman

History of Copper Clad Aluminum Electrical Wiring

Early Efforts to Apply Aluminum, Tin, or other Coatings to Copper Wire Conductors: metal plating, metal tinning, vs metal cladding

Meyer Tinned Copper patent 1945 -

[Click to enlarge any image] Above: coatings on the copper wire to avoid interaction with the vulcanized rubber insulation. (Meyer 1945).

Wire coatings using plating or tinning

Early wire coating efforts and later wire cladding efforts addressed the effects on copper wire of sulphur in vulcanized rubber insulation.

Ideas for producing wire of one metal covered in another by covering the copper core wire with one or more layers of other metals had been around since at least the 1920's but it is interesting to learn that early wire coating attempts started with using aluminum (or other alloys) to coat copper. Tinning copper wire was employed to reduce the harmful interaction of rubber insulating coatings with the copper wire, including deterioration of the rubber insulation. (Meyer 1945) Note that tinning or plating coatings on wire does not produce the same coating thickness and these are different processes from copper cladding methods.

David's patent illustrated this early in the history of electrical wire production: the effort to coat the copper with aluminum (probably by plating then by tinning) was presumably to protect it from damage or from oxidation during manufacture. David had better results with a multi-layered approach of molten aluminum coating on nickel-coated copper.

The present invention relates to the manufacture of aluminum coated copper conductors, and as a consequence of my invention I have provided a conductor in which the core and coating are so well united that they will not separate, and in which the core is not injured or oxidized during the process of manufacture.

Heretofore, when attempt were made to coat copper wire with aluminum, it was found that the copper when brought into contact with molten aluminum became oxidized so that the coating would be imperfect. A tin coating on the copper also became oxidized. Furthermore, molten aluminum alloyed with the copper to such an extent that wire being coated became reduced in diameter, its strength and current carrying capacity were impaired and it was rendered brittle.
In accordance with my invention a nonferrous foundation metal such as copper is first coated with nickel and then aluminum is applied upon the nickel. No deleterious effects are produced by molten aluminum upon nickel-coated copper.
(David 1927)

Wire cladding with other metals

Attempts at coating the wire by using a metal cladding tape of a different metal were discussed along with other wire coating methods by Sprecht in 1934 who improved on earlier attempts to use silver solder or other coatings by using a metallic tape along with dies through which wire was drawn:

According to the present invention I propose to first produce a core of wire of the proper temper and diameter desired in the finished wire, and produce separately a strand of covering metal or other material in the form of a fiat ribbon-like strip, which when rolled about the core will cover it. The core wire and covering wire are placed in juxtaposition and are first drawn through a die which bends the covering into U-form about the core, and then through other dies which close the covering over the core. (Sprecht 1937).

The inverse of copper-clad aluminum wire, that is, aluminum-clad copper wire, was still attempted in the late 1950's and updated in the early 1960's when Carlson and Rosecrans described a method of making aluminum-clad copper wire - the inverse of the final and important product discussed just below. (Carlson 1963).

This invention relates to the preparation of composite flexible wire and the like comprising a core of copper and an external sheath of aluminum metallurgically bonded thereto, and processes for producing the same. This application is a division of our U.S. patent application Serial No. 558,699, filed January 12, 1956, now Patent 2,947,069, and assigned to the same assignee. It has long been desirable to produce wire, strip, rod and similar elongated members comprising a core of copper or copper base alloy with a relatively thin sheath of aluminum applied to the exterior surfaces thereof. It has been highly difficult to produce a satisfactory composite aluminum clad copper ,wire member heretofore.

... We have discovered processes for producing flexible, well bonded, composite elongated members comprising a copper core, a thin barrier layer of silver and a thin sheath of aluminum. By this process the composite member may be produced rapidly and economically. The composite members have proven highly satisfactory for use either as enameled wire or in other applications in the electrical industry. - (Carlson 1963)

That is, Carlson's patent described applying an an aluminum sheath as the exterior of a copper core. Significant was that the metallic sheath was "metallurgically bonded to the inner core".

Manufacture of aluminum clad copper wire - from Carlson 1963

[Click to enlarge any image]

Illustrated at above left is the production method for cladding one metal with another to produce a wire of one metal with a cladding of another metal. - Carlson et als. (1963).

Continuing by quotation:

When the aluminum wire and the strips enter the rolls from the plenum chamber, heat flows from the copper strips mostly to the aluminum core and some to the rolls. This rapidly brings the aluminum-copper composite to equilibrium temperature as the composite is being bonded by reduction in the solid phase.

Roll geometry copper thickness and aluminum diameter is such as to produce clad composites in the usually demanded three percentages of copper by volume, i.e. 10%, 15%, and 20%. ... The field of the invention is the manufacture of clad wire and the like as exemplified in U.S. Patents 3,220,106 and 3,220,106. - Dion (1969) This patent cites earlier work and patents by Dion (1966) and Thompson (1966, 1967).

Shift to Efforts to Apply Copper Coatings to Aluminum Wire: Copper-Clad Aluminum

Later work described by Ziemek (1974) reversed this process, placing the copper as the outer sheath and solid aluminum as the interior or main body of the wire as we discuss next. It seems likely that this reversal, placing copper on the exterior of solid-conductor aluminum electrical wiring, was in anticipation of or in response to the poor field performance of aluminum electrical wire in homes. By this time electrical wire insulation had been re-formulated from early vulcanized rubber coatings, allowing inventors to eschew the earlier concern with harmful interactions between the copper wire surface and the wire insulating material.

It was in his 1972 U.S. patent, that Gerhard Ziemek provided a succinct description of copper-clad aluminum electrical wire:

Aluminum is extruded, and the resulting core when still warm is lined with copper by deforming a copper tape around the core, welding the tape edges and drawing the resulting tube onto the core, and possibly further to obtain wire of desired diameter. (Ziemek 1972).

Dion (1966) and Dion and Thompson (1967) first described the successful manufacture of copper clad aluminum wire. Rolled aluminum wire is pulled through a lubricated draw to eliminate flat spots and to prepare the wire surface for a solid-bonding phase in which two copper cladding strips are added to the wire exterior.

Manufacture of copper clad aluminum wire - Dion (1963)

The patent included provisions to assure that the aluminum surface was free of contaminants as were the copper strips. [Abstracted from the patent.]

Illustrated above is Dion's method of cladding aluminum wire with a layer of copper. It is significant to note that the copper thickness in this method is much greater than the typical thickness imparted by plating one metal on another. It is likely that it's that thickness that prevents a device connector screw from cutting through the copper, possibly leading to connector failures in field use.

Quoting from a later description of this process, a copper band is formed around an aluminum core wire and the single seam in the sheath material is welded without bonding of the sheath and core, care being taken that all surfaces are clean and maintained free of oxides. - Ziemek (1974)

Mr. D'Agostino relates that in 1971 as project engineer at UL, he conducted tests evaluating the performance of copper-clad aluminum conductor wiring using a number of types of copper-only receptacles (i.e. binding head screw terminals, back-wired push-in terminals, sid-wired pressure plate screw terminals) and a number of types of AL-CU pressure wire connectors. That research, conducted under the auspices of U.L., demonstrated that performance of copper-clad wire was

  1. similar to that of solid copper-only wiring devices (with binding head screw terminals
  2. satisfactory for use in AL-CU pressure wire connectors and
  3. along with copper wire performed poorly, less than adequate in "back-wired push-in" wiring devices.

The findings of this research was published in a 1971 UL bulletin that was distributed / reviewed by industry and government (i.e. David Rabinov of the U.S. National Bureau of Standards).

The copper clad aluminum wire product produced by Texas Instruments is attributed by D'Agostino to John Fan (see patent citations below).

Following successful testing by D'Agostino at U.L., copper-clad aluminum electrical conductor ["copper clad electrical wire" in lay terminology] was recognized by UL/Industry and soon thereafter Nos. 10-12 AWG solid copper-clad aluminum conductor in THHN wire was used UL Listed NM Sheathed Cable. NM Cable using copper-clad aluminum electrical conductors was subsequently distributed and installed in homes in the United States (and possibly Canada) between 1972 and 1976.

Original patents on the production of copper-clad aluminum wire were awarded to a variety of parties (cited below) with early patent applications in 1956 (Carlson) and key patents (Carlson, Roserans, Westinghouse Electric) dating from 1963.

Research into the production and use of copper-clad aluminum wire continues to the present, with work by Kwon (2004) and Rhee (2004) and others on fabrication methods and by Sasaki (2010) on the metallurgy of copper-clad aluminum.

- Special thanks to Tom D'Agostino for recapping the history of aluminum electrical wiring and copper clad aluminum electrical wiring. Personal communication 5/3/2014. Mr. D'Agostino presently lives on Long Island, NY. and continues to write and research electrical hazards. He has also worked as a professional home inspector and is a member of ASHI, the American Society of Home Inspectors.

Don't confuse copper-clad wire with plated wire nor with tinned-copper plated wire nor with COALR or CU-AL Devices

Tinned copper multi-strand wire ca 1960 (C) Daniel Friedman

Technical note: copper-clad wire or any metal "cladding" surface on wire refers to the physical application of a thin metal strip (or strips) of cladding metal that is forced-against or wrapped around the wire. This will be a thicker coating of metal skin on the wire than that typically afforded by other wire coating methods such as tin-plating. Above: tinned copper multi-strand #12 copper wire used in wiring a fluorescent light ballast in a Minnesota home constructed in the 1960's.

This discussion has moved to TINNED COPPER ELECTRICAL WIRE

Watch out: tin-plated copper wire is a completely different product that, because its conductors sport a thin plated silver colored surface, might be mistaken for unsafe aluminum wire. It is not aluminum and it is safe unless, as with any electrical wiring, it has been damaged in some manner.

Watch out: don't confuse references to copper-clad aluminum wire or tin-plated electrical wire with devices such as electrical receptacles and switches marked COALR or CU-AL. The latter two are device labels not wire types and are not a recommended repair for aluminum electrical wire. COALR and CU-AL and their field performance and safety warnings are described in more detail at ALUMINUM WIRING REPAIR COALR & CU-AL

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References for copper clad aluminum wire & related research

Aluminum Electrical Wiring Articles


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