Most of my training in college was in
electronics. When I graduated in 1971, most of the electronics
companies were not hiring so I took a job with the local
electric utility until I could find a job in electronics.
Thirty-three years later, I’m still with the utility. Over
the years working for an electric utility, I have become
accustomed to referring to bare wire as wire and insulated
wire as cable. In general, most of our aerial or overhead
construction involves wire and most of our underground
construction involves cable. These are the definitions that
are commonly used in many electric utilities around the
country. Unfortunately, the published definitions for wire,
cable, and conductor vary with the source. The definitions in
the 2005 National Electrical Code (NEC) are
different from those in the 2002 National Electrical Safety
Code (NESC), which are also very different from slang some
industries use. To reduce confusion, I recommend that if you
are dealing with the requirements of the NEC, use the
terms as defined in the NEC. Likewise with the NESC or
other industry standards. Be aware that there are differences.
Since most of the readers of this series are familiar with the NEC, I will try to use the NEC terms and
definitions in this segment.
While working for an electric utility, it
didn’t take me long to realize that the main difference
between electronics and electric power is a few zeroes.
Electronic equipment usually deals with very small currents
and voltages, milliamps and millivolts. One milliamp is
0.001amp. Electric power distribution equipment usually deals
with high currents and voltages, kilo-amps and kilo-volts. One
kilo-volt is 1000 volts. The same is true for conductor sizes
relative to electronics and electric power. Since the currents
in electronic equipment are small, the conductors are usually
small, 14 AWG to 24 AWG. Since the currents in electric power
supply facilities are large, the conductors are usually large,
from #4/0 AWG to 2000 kcmil.
Photo
1. An example of a large conductor: 2505 kcmil
Aluminum Alloy Conductor Steel Reinforced AACSR 84/19,
1.972" in diameter
Conductor
area
The current-carrying capacity or ampacity of a conductor is
directly proportional to the cross-sectional area of the
conducting material. We usually measure area in square inches,
or square feet, or square yards, etc. In the late 1800s, the
U.S. electric industry standardized on using circular mils for
measuring the cross-sectional area of conductors. A circular
mil is the area of a circle with a diameter of one mil (0.001
inch). The abbreviation for circular mils is cmil. There are
1,273,200 circular mils in one square inch. For conductors
having a cross-sectional area smaller than or equal to 211,600
circular mils, the industry elected to adopt the Brown and
Sharpe wire gage designation of 1857, what we now call
American Wire Gage (AWG). The area of large conductors is
often designated in kcmil (thousand circular mils) rather than
cmil. For example, the cross-sectional area of 4/0 AWG is
211,200 cmil or 211.2 kcmil. Prior to 1972, the abbreviation
for thousand circular mils was MCM.
Stranding
The conductor most commonly used in houses for receptacle
circuit wiring is two conductor 14 AWG copper with ground. The
phase and neutral conductors are insulated conductors. The
ground conductor is bare. Each conductor is a single solid
wire. The cross-sectional area of the 14 AWG solid wire is
4,110 cmil. The 14 AWG power cord on a circular saw is very
different. On the circular saw cord, each conductor is made up
of numerous small wires or strands all twisted together. This
is often referred to as multiple strand conductor. The
manufacturer uses multiple strand because multiple strand
conductor is flexible and can be flexed hundreds of times
before breaking. Single strand conductor is usually used for
installation where the conductor is not subject to flexing. If
you measure the diameter of each strand of the saw cord,
calculate the cross-sectional area of the strand and multiply
it times the number of strands, you will find the
cross-sectional area is 4,110 cmil. I cut open the power cord
of an old electric drill. The cord was not marked as to the
wire size. Using a micrometer, I measured the diameter of one
of the strands. The diameter was 0.010" or 10 mils. The
area of that strand is the diameter squared times p divided by 4 (p is 3.14159). The area of each strand is 0.000078539 square
inches or 100 cmils. To convert square inches to cmil we
multiply by 1,273,200. If we then multiply the area by the
number of strands (24), we find the total cross-sectional area
of the conductor is 2,400 cmil, just short of 16 AWG (2,580
cmil). 17 AWG is 2,050 cmil. It doesn’t matter if the
conductor is 17 AWG single strand or 17 AWG 24 strand, the
cross-sectional area is the same. With multi-strand
conductors, the only way to positively identify the wire size
is to use a micrometer to measure the diameter of the strands,
count the number of strands, and work through the calculation.
I have found that the marking on the jacket isn’t always
correct.
Number of strands
Conductor manufacturers make conductors in a wide variety of
stranding. A few examples of stranding are shown in diagram 1.
Diagram 1. A few examples of concentric lay
conductors, 7 strand, 19 strand, and 37 strand.
Compressed and
compact conductors
Insulated conductors are often compressed or compacted to
reduce the diameter. To create compressed or compact
conductors, the manufacturer draws the full strand conductors
through a series of funnel shaped dies to reduce the diameter.
Full strand conductors have air space between the strands. The
compression process squishes the air space out thus reducing
the diameter without reducing the cross-sectional area of the
conductor. An example of a compact conductor is shown in
diagram 2. Note that following compression, the strands are no
longer round. The diameter of compressed conductors is reduced
by approximately three percent. The diameter of compact
conductors is reduced by approximately nine percent. If you
elect to purchase compressed or compact conductors, the
compression connectors that are designed to fit full strand
conductors may not work on compressed or compact conductors.
Compact conductors are almost as small as solid conductors.
Check with the connector manufacturer before purchasing the
connectors (see diagram 2).
Diagram 2. A compact concentric strand
conductor.
Alloys and temper
Conductor manufacturers use many different alloys of aluminum
to make aluminum conductors. The alloy of a metal defines what
elements and what proportions of elements are used to make the
metal. The alloy is like a recipe for the metal. Alloys are
identified by number. The temper of a wire defines how the
wire is thermally treated after it is manufactured. If you
have ever seen a blacksmith making something out of iron, you
might have noticed the blacksmith putting the item into a
bucket of water while the item is still very hot. By cooling
the item very quickly, the metal becomes very hard and
brittle. Drill bits and other tools are tempered in this
manner. Items that are cooled slowly stay soft and easy to
bend. The temper of copper wire is identified by three terms,
hard drawn, medium hard drawn and soft drawn. Copper wire that
is cooled quickly to make it hard is referred to as hard drawn
(HD). If copper wire is cooled slowly to make it soft and easy
to bend, it is referred to as soft drawn (SD). The temper of
aluminum is usually identified by a combination of letters and
numbers. For example, 1350-H19, 1350 is the aluminum alloy,
H19 is the temper. This is aluminum that is commonly used to
make what the industry calls all aluminum conductor (AAC).
6201-T81, 6201 is the aluminum alloy, T81 is the temper. This
is a high strength aluminum alloy commonly used to make what
the industry calls all aluminum alloy conductor (AAAC). The
resistance, strength, and flexibility of the conductor is a
function of the alloy, temper and stranding.
Composite
conductors
Composite conductors are conductors made up of strands of
different alloys or materials. One example is the use of
1350-H19 aluminum and 6201-T81 high strength aluminum alloy.
This conductor is called aluminum conductor alloy reinforced (ACAR).
Some common stranding for ACAR is four strands of 1350 and
three strands of 6201, or 15 strands of 1350 and four strands
of 6201. Another example of composite conductors is the use of
1350-H19 aluminum and steel. This conductor is called aluminum
conductor steel reinforced (ACSR). Some common stranding of
ACSR is six strands of aluminum around one strand of steel or
twelve strands of aluminum around seven strands of steel. Note
that the cross-sectional area of aluminum in a 4/0 AWG ACSR
conductor is the same as the cross-sectional area of the
aluminum in a 4/0 AWG AAC conductor. That is why the diameter
of an ACSR conductor is much greater than the same wire size
AAC conductor. This is particularly important when ordering
connectors. Some connectors designed to work on AAC conductors
will not fit on the same conductor size ACSR conductors.
Code words
As you can see, if we consider the number of standard
conductor sizes, the number of different stranding options,
the variations in alloy, temper, insulation and composite
conductors, there are hundreds of different conductors
available from the manufacturers. Rather than trying to
identify each conductor with a lengthy description, the
conductor manufacturers decided to assign code words to each
conductor. Some of the code words are plant names and animal
names. Some are just common words like Razor, Fly, and Vienna.
It is so much easier to call up a supplier and ask for 10,000
feet of Celtic, rather than asking for aerial quadruplex
consisting of 3-1C, 2/0 AWG, 7 strand, AAC phase with 60 mils
of PE Insulation, 1-1C, 123.3 kcmil (1350 equivalent 1/0 AWG),
7 strand, 6201-T81 AAAC, bare neutral. The index for aluminum
aerial conductors is published in, Code Words for Overhead
Aluminum Electrical Conductors. The index for aluminum
underground conductors is published in Code Words for
Underground Distribution Cables. Both standards are
published by the Aluminum Association.
If you want to learn more about aluminum
conductors, I suggest you consult the Aluminum Electrical
Conductor Handbook, a publication of the Aluminum
Association.
Please send me your comments on this
series. If you have questions about basic electricity or
general questions about the NESC, please e-mail me at dave.young@conectiv.com .
National Electrical Safety Code and NESC
are registered trademarks of the Institute of Electrical and
Electronics Engineers. National Electrical Code and NEC
are registered trademarks of the National Fire Protection
Association.
Dave is a consulting engineer with Conectiv Power
Delivery of Wilmington, Delaware, where he has been working with and
teaching all aspects of the NESC for over 33 years. He is a member of
the NESC Interpretations Subcommittee and represents the Edison Electric
Institute on the NESC Overhead Line Clearances Subcommittee 4. Dave is
also an inspector member of the IAEI. |
