Photograph of ALUMINUM WIRE  - Recognizing aluminum wiring may be possible where nonmetallic wiring is visible such as in attics or basements. Copper-Clad Aluminum Wire Safety & History
Comments on the history, testing, & safety of Copper-Clad Aluminum electrical wiring

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Copper-Clad Aluminum Elecrical Wiring: this article describes the history of copper-clad aluminum electrical wiring and its safe use in homes in the United States.

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Copper-Clad Aluminum Elecrical Wiring

Thomas J. D'Agostino, P.E., C.I., & Daniel Friedman

What is copper-clad aluminum wire?

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 produc was installed in homes in the United States between 1972 and 1975. Details 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 devicves, ie. switches, receptacles (binding head screw, back-wired, and pressure plate thpe termionals) 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.

[Illustrations needed, CONTACT us]

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

Recognizing that electrical wire is copper clad aluminum is useful for technical reasons.

[Illustrations needed, CONTACT us]

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 adquate to identify its silver-colored aluminum core unless the scrape is vigorous enough to cut completely through the copper cladding.

Below is a cross-section cut of a solid aluminum electrical wire (with white insulation) and a solid copper aluminum conductor (with black insulation).

History of Copper Clad Aluminum Electrical Wiring

Manufacture of aluminum clad copper wire - from Carlson 1963Mr. Tom D'Agostino was employed at Underwriters Laboratories from 1969 to 1996 and spent a decade and a half studying and testing the safety of aluminum electrical wiring.

[Click to enlarge any image]

Early work by Carlson (shown at left) described a process "inside-out" of the later copper-clad aluminum wire. 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".

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).

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 tothe wire exterior.

Manufacture of copper clad aluminum wire - Dion (1963) InspectApedia.comThe patent included provisions to assure that the aluminum surface was free of contaminants as were the copper strips. [Abstracted from the patent.]

Illustrated at left 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 srcew from cutting through the copper, possibly leading to connector failures in field use.

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 precentages 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).

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.

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 terminals, 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 theU.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 alulminum wiring was reognize by UL/Industry and soon thereafter UL Listed NM Sheathed Cable with Nos. 10-12 AWG solid copper-clad aluminum conductor was distributed and installed in homes in the United States (and possibly Canada) between 1972 and 1975.

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 aluminu 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 Amerian Society of Home Inspectors.

Avoid confusing copper-clad wire with copper plated wire or with COALR or CU-AL Devices

Double tapped aluminum and copper wire in a CUAL breaker (C) InspectApedia At above left, even without any disassembly we can see a solid conductor aluminum wire (center breaker connection) double-tapped (two wires under one connector) into a circuit breaker along with a silver-colored solid conductor electrical wire that is probably aluminum wire. (The large multi-strand red-insulated wire is a multi-strand aluminum conductor.)

Double-tapping electrical wire connections is improper and unsafe unless the connector is specifically designed for that purpose and is so-indicated by its manufacturer.

But from just this photograph alone one cannot be sure if the silver colored wire is aluminum or plated copper.

Note: Tin plated copper wire was generally a practice used before 1965 on small-sized 14-12 copper wire. On second look, the size of the silver-colored black insulated wire shown above is likely to be a size 10 or 12 AWG aluminum. Further, tin-plated copper presents a "dirty silver" look. - D'Agostino (2014)

A closer look will be needed to confirm the opinions of our note above. Notice that the circuit breaker is marked CU-AL indicating that the manufacturer represented that the breaker could be used with either aluminum or copper wire, notwithstanding that this is not an accepted nor recommened repair for aluminum electrical wiring.

Watch out: In addition to the improper double-tapping in the center circuit breaker shown above, copper wire in contact with an aluminum conductor in an AL-CU rated circuit breaker is not covered by the UL-listing. The device is listed for use with either copper-only or aluminum only.


Solid conductor aluminum electical wire closeup for identification (C) Daniel FriedmanAn easy photograph for aluminum wire identification is shown above right. This is of course from a different electrical connection - a splice that used a twist-on connector.

We can see silver-colored (soft) metal that is aluminum at three locations:

  1. The wire surface, silver colored. If this were all we could see we might not know (unless aided by printing on the wire insulation) whether we're looking at aluminum wire or plated copper wire.

    But that white plastic insulation tells me this is not an older rubber-insulated wire so on that basis I'm already ruling out plated copper. But the next two observations confirm that conclusion.
  2. The interior of nicks into the wire that would have most likely cut through any plated skin - so we know this is not plated copper wire
  3. The square-cut end of the wire, silver colored throughout, so we know this is solid alumium wire

Fabric & rubber insulated copper wire (C) Daniel Friedman Paul GalowWatch out: plated copper wire is a completely different product that, because its conductors sport a thin plated silver colored surface, might be mistaken for unsafe alumium wire. It is not aluminum and it is safe unless, as with any electrical wiring, it has been damaged in some manner.

Not all rubber insulated copper wire is plated, but if you suspect that the silvery-colored wire is plated copper, it's easy enough to determine: with the wire disconnected from any power source, scratch the silver colored surface of the rubber-insulated wire and you'll see its red copper interior.

In REDUCE THE AL WIRE RISK: DETAILS, as well as in Aronstein (2011). There Aronstein describes plated-copper wire as:

Plated copper wire is relatively common in older homes, and it looks like aluminum wire. It was commonly used with rubber-based insulation. Identification can be made by careful inspection of a cut end of the wire. In general, plated copper wire would not be present in nonmetallic sheathed cable ("Romex"), it is most generally found in metallic sheathed cable ("BX"). Cable of the "BX" type is not likely to contain aluminum wire. - Aronstein (2011)

Cross section view of coppe wire and aluminum wire (C) Daniel Friedman

Above we show solid aluminum conductor wire and solid copper conductor electrical wire in cross section.

Watch out: don't confuse references to copper-clad aluminum wire or copper-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

Suggested citation for this web page

COPPER-CLAD ALUMINUM WIRE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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