Bimetallic Wire

Copper Clad Steel and Aluminum Cables for
Lighter, Stronger and Lower Cost Wiring Harnesses

Wayne Perrard, Applications Engineer, LTV Copperweld
and
Alan Gibson, Product Applications Development, LTV Copperweld

INTRODUCTION
Bimetallic wire, especially copper-clad steel and copper-clad aluminum,
are long established products in industry. Copperweld Corporation
developed copper-clad steel in 1915, and later began manufacturing
copper-clad aluminum as well for applications where high performance
and lower cost were needed.

Copper-clad steel, or CCS, combines the strength of steel with the conductivity
and corrosion resistance of copper.  It is used where strengths higher than
copper are required, and some reduction in conductivity is acceptable.  It is
widely used in the telecommunications industry for telephone and coaxial
television drop wire, and by the electric utility industry for grounding conductors.

Copper-clad aluminum, or CCA, combines the light weight, high flexibility,
and low cost of aluminum with the conductivity and corrosion resistance of
copper.  It is used where these qualities are beneficial to a cable. Copper-clad
aluminum is used extensively in overhead coaxial cable for high frequency
television signals.  It is used to replace copper as grounding conductor on structures. 
CCA replaces aluminum in overhead power lines in corrosive environments.  It
also replaces copper in underground power lines to combat corrosion and theft
problems.

This article explains the properties and potential applications of copper-clad
steel and copper-clad aluminum wire. Through the use of bimetallic wire,
cable characteristics and performance can be improved over traditional designs
by providing greater strength, smaller size, reduced weight, and lower cost.

BIMETALLIC WIRE MANUFACTURING
Bimetallic wire is produced by cladding rod of steel or aluminum core
material with copper strip, and then drawing the clad rod to the desired
wire diameter.  Before cladding, both the copper and core material receive
special cleaning to help create a complete metallurgical bond.  Figure 1
shows the copper-clad steel process schematic.

Figure 1:  Schematic of the copper-clad steel bonding process.

The details of the processes for steel and aluminum differ slightly
due to the characteristics of the two metals. Figure 2 is a photograph
of the initial step of the copper-clad aluminum manufacturing process.

Figure 2: Copper cladding of aluminum rod.

After proprietary processing for bonding, the copper strip is formed
around the aluminum rod and sealed.  The resulting bond is inseparable,
just as it is for copper clad steel.  Subsequent drawing and processing
further enhance the metallurgical bonds present in copper-clad steel and
copper-clad aluminum.

Cross sections of finished bimetallic wire products after drawing are
shown in Figure 3.  The ratio of copper thickness to core diameter remains
constant throughout the drawing process. This means that conductivities
also remain constant.  The cross section photomicrographs show the same
product at two different stages of production. The section on the left was
taken at a diameter of 5.8 mm, while the one on the right was taken at
1.0 mm at a higher magnification.  Note that the relative thicknesses of 
copper and steel have remained constant.

5.8mm diameter                                               1.0mm diameter

Figure 3: Cross section photomicrographs of 40% conductivity
copper-clad steel

Properties of Copper-Clad Steel and
Copper-Clad Aluminum Wire
Copper clad steel is commonly produced to specifications of 21 percent,
30 percent, and 40 percent IACS composite conductivity,1 with copper
thickness amounts of 6 percent, 14 percent, and 20 percent of the wire radius,
respectively.  Grades of steel clad are SAE 1006, 1010, 1022, and 1055. 
Copper-clad steel conductivities of 50 percent, 60 percent, 70 percent, and
76 percent have also been made for specialty applications.  The steel cladding
process has also been used to produce copper clad stainless steel, as well as
several other metal combinations. Bimetallic wire is readily drawn to small sizes. 
There are commercial applications of copper clad steel at diameters down to 
0.05 mm, and of copper clad aluminum as small as 0.10 mm in diameter.

Many of the properties of bimetallic wire can be accurately estimated by
calculating a weighted average of the properties of the two metals present. 
Some of these properties are shown in Table 1.

The values shown in Table 1 are for electrical conductor grade aluminum
and SAE 1006 steel cores, respectively. Higher carbon grades of steel are
often used to develop higher strength levels. The copper claddings shown in
Table 1 are the common 10 percent of cross sectional area for aluminum and
40 percent conductivity for steel. Other cladding thicknesses are also produced commercially and also yield properties that are averages of those of the two
metals present.

The flexibility of copper-clad aluminum wire surpasses that of pure copper². 
It is more formable and easily installed, even at large sizes.  The fatigue
properties of copper-clad steel wire are derived from the properties of the
steel core.  Copper-clad steel wire exhibits an endurance limit and a high
fatigue strength related to its ultimate tensile strength.  When a conductor
with superior fatigue properties is needed, copper clad steel becomes a
strong candidate. 

An independent laboratory has recently determined the current carrying
capacity of several types of bimetallic cable ^3,4.  The results are
summarized in Table 2.

The clad layers on the surface of bimetallic wire are much thicker than
plated layers.  Because the outer layers are so massive, the corrosion
properties and other surface characteristics are the same as copper. 

Applications Case Study:
Automotive Wiring Components
Several example applications for bimetallic wire will now be presented,
as well as potential size, weight, and cost savings that can be achieved. 
The following applications include automotive harness wire, rear window
defogger power leads, and battery cable.

Automotive Harness Wire
Because of its greater strength, copper-clad steel is practical for use in
smaller sizes than copper.  The 7-strand conductor commonly used in
automotive wiring harnesses can provide an example.  For this type of
cable, 26 AWG (.48 mm diameter) hard-drawn copper-clad steel,
40 percent conductivity, using a 1006 steel core has the same strength
as 20 AWG (.96 mm diameter) copper cable. Copper-clad steel cable
of this type has the conductivity of 30 AWG (.30 mm diameter) copper. 
While many circuits in automobiles carry only signals or small low
currents that would require no more than 30 AWG copper conductors,
the use of such small copper cable is not practical because of its low strength. 
26 AWG copper clad steel, however, becomes an excellent choice for these
circuits because it has the strength of the widely used 20 AWG copper cable. 
The substitution of small copper clad steel cables for copper cables that are
sized on strength rather than conductivity reduces the size, weight, and cost
of components.  The breaking loads of various small cables are listed in Table 3.

The values in Table 3 all refer to copper-clad steel produced to 40 percent
conductivity.  Its small diameter gives the product adequate flexibility to
permit the use of hard-drawn wire, and results in significant bundle size
reduction when used in harnesses.  The strengths of wire clad to other
specifications would vary with the ratio of copper and steel in the cross
section.  Higher carbon steel grades could also be used to increase strength.

Table 4 compares properties of cable made with copper to cable made with
7-strand, 26 AWG, 40 percent conductivity, copper-clad steel.  For the
purposes of the comparison, insulation conforming to SAE specification
J1128 is assumed.  This specification calls for a nominal insulation thickness
of 0.406 mm for these cable sizes.

Use of copper clad steel cable will reduce the size, weight, and cost of an
actual wiring harness.  The wiring harness in Figure 4 was analyzed to
determine the benefits of substituting 26 AWG copper clad steel for copper
cable in low current circuits.  It is an engine harness from a large, US-produced
car with a V8 engine.  It was selected to represent a harness that was smaller
than a main vehicle wiring harness, yet complex enough to demonstrate the
potential benefits of bimetallic wire.

Figure 4: Large Car Engine Wiring Harness

This harness contains 92 leads with the distribution of cable sizes shown
in Table 5.

For the purpose of analysis, it was assumed that circuits with maximum
currents of 50 milliamps or less could be converted to 26 AWG copper-clad
steel cable.  This is a conservative assumption because the current carrying
capacity of 26 AWG 40 percent conductivity copper clad steel is much greater
than 50 milliamps.  Even with this conservative standard for substitution, the
effect of using copper clad steel for circuits where the wire is sized for strength
rather than current carrying ability can be illustrated.  The manufacturer of the
harness in Figure 4 identified 18 circuits with currents of 50 milliamps or less. 
Substitution of 26 AWG copper clad steel in these circuits yields the harness
configuration shown in Table 6.

Table 6 shows that the substitution of 26 AWG copper clad steel for
20 AWG copper cable in the eighteen circuits carrying 50 milliamps or
less would reduce the cross section of the harness by 8% at the point of
maximum cross section.  The calculated weight reduction is 15%, and
cost reduction would also be expected from this substitution.

Automotive Rear Window
Defogger Power Leads
A second potential application of bimetallic wire in the automotive
industry is the use of copper-clad aluminum power cables to reduce
weight and cost compared to copper.  As with copper-clad steel, copper-clad
aluminum can be produced at virtually any cladding thickness, but material
with the copper cladding being 10 percent of the overall volume is the most
common commercial product.  At this cladding ratio, copper-clad aluminum
wire has the resistance of solid copper wire that is two AWG sizes smaller⁵. 
For example, 12 AWG (2.05 mm diameter) 10 percent copper-clad aluminum
wire has the same resistance of an equal length of 14 AWG (1.63 mm diameter)
copper wire. 

The benefits of copper-clad aluminum in power cables can be illustrated by the
rear window defogger lead shown in Figure 5. This simple component consists
of 37 cm of 18 AWG (1.2 mm diameter) cable and connectors.

Figure 5: Rear Window Defogger Lead

The replacement effects of the 18 AWG copper with 16 AWG
(1.4 mm diameter) copper-clad aluminum are shown in Table 7.

The values associated with such a small component are small, but the
percentages of improvement are significant.

Automotive Battery Cable
Another example that more dramatically shows the advantages of copper-clad
aluminum power cables is provided by comparing two cables that have been
proposed for a concept vehicle.  This vehicle requires battery cables that are
six meters long and insulated with TFE. The currents involved can be handled
by nominal 4 AWG (6.5 mm conductor diameter) copper or 2 AWG
(7.5 mm conductor diameter) copper-clad aluminum.  Table 8 is a comparison
of the two types of cables.  The copper-clad aluminum cable weighs
0.41 kilograms less than the copper cable. Since a pair of cables is required,
the use of copper clad aluminum would reduce vehicle weight by 0.82 kilograms,
and would reduce cost.

Optimization of a Wiring Harness
Using Bimetallic Wire
It is possible to incorporate both concepts into one wiring harness, using
both small diameter copper-clad steel for low current circuits, and copper-clad
aluminum for power cables.  This can be illustrated using a hypothetical wiring
harness for a large vehicle.  Unlike the engine harness discussed previously,
this is not a production component, but a representative hypothetical harness
created to demonstrate the potential for optimizing any harness by using
bimetallic wire.  Table 9 shows the benefit of using bimetallic wire in a
large harness.

The first column represents a base harness with an assumed distribution
of cable sizes.  The second column shows the weight and size benefits that
could be achieved by using copper-clad steel for low current circuits, given the
assumptions shown.  The final column adds substitution of copper-clad
aluminum for power cables to produce a minimum weight harness without
increasing bundle size.  Yet another substitution step could be taken to replace
the remaining 22 AWG copper leads with 20 AWG copper clad aluminum. 
This would result in a minimum weight harness, but with increased bundle size.

Other Harness Applications
The examples shown have been drawn from the automotive industry, but
the advantages of bimetallic wire apply to all types of wiring harnesses. 
The size, weight, and cost of harnesses for other types of vehicles,
appliances, and industrial equipment could all be reduced by application
of bimetallic wire.

High Frequency Applications
Copper clad aluminum is especially suitable for high frequency
applications.  Depending upon the signal frequency and the conductor
diameter, the impedance of copper clad aluminum is typically similar
or equal to that of copper.  This permits it to be substituted for copper
at the same gage, with significant savings in both cost and weight.

Summary
Bimetallic wire is an important commercial product historically used in
applications where it is beneficial to combine the conductivity of copper
with the strength of steel or the light weight of aluminum.  With the
wealth of information available in recent years, more cable designs can
be improved by using copper-clad steel or copper-clad aluminum in
place of solid copper.

Copper-clad aluminum wire has the potential to reduce the weight and
cost of power cables.  Copper-clad steel wire has the potential to reduce
the size, weight, and cost of harness wires and other signal cables. 
Together, CCA and CCS can optimize the properties and performance of
wiring harness designs. 

For reprints of this article or more detailed information about bimetallic
wire, please contact either of the authors:

Wayne Perrard
Applications Engineer
LTV Copperweld
888-284-9473
wperrard@ltv-copperweld.com

Alan Gibson
Product Applications Development
LTV Copperweld
888-726-9856
afgibson@aol.com

References
LTV Copperweld Fayetteville Division, The Power of Bimetallic Wire.

William Malone, Technical Manager, LTV Copperweld.

Test Report #99624, Current Carrying Capacity, Bimetallic Wire,
Copper Clad Steel (26-Gage, 7 Strand), Copper Clad Aluminum
(16-Gage, 19 Strand), Contech Research, Inc., December 2, 1999.

Test Report #99738, Current Carrying Capacity, Bulk Resistance,
Bimetallic Wire, 26 AWG (Copper Clad Steel), 16 AWG
(Copper Clad Aluminum), 2 AWG (Copper Clad Aluminum),
Contech Research, Inc., January 19, 2000.

"Copper-Clad Aluminum (CCA) Wire," LTV Copperweld
Corporation, May 1992, p. 1.

Definitions, Acronyms, Abbreviations
AWG: American Wire Gauge
CCA: Copper-Clad Aluminum
CCS: Copper-Clad Steel
ETP: Electrolytic Tough Pitch
GPT: Polyvinyl chloride insulation per SAE J1128
HDPE: High Density Polyethylene
IACS: International Annealed Copper Standard
TFE: Polytetrafluoroethylene insulation
XLPE: Cross-linked polyethylene insulation per SAE J1128