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