Wires as we define here are used for transmission of electricity or electrical signals.  Wires come in many forms and are made from many materials. They may seem simple but engineers are aware of two important points:

-Electricity in long wires used in transmission behaves very differently than in short wires used in design of devices
-The use of wires in AC circuits brings on all sorts of problems like skin effect and proximity effects. 

1. Resistivity/Impedance
2. Skin Effect
3. Types of Wire Design

4. More on Wire Materials
5. Wire Insulation

1.) Behavior of electricity in wires: Resistance and Impedance

It's important to know if you are dealing with DC or AC power in a given wire. AC power has some very complex physics which cause some strange effects. This was one of the reasons why AC power was developed in the 1890s, long after DC power. Engineers like C.P. Steinmetz had to figure out the mathematics and physics first.

AC Power:
In AC power current likes to travel near the surface of a wire (skin effect).  AC power in a wire also causes a magnetic field to form around it (inductance). This field effects other nearby wires (such as in a winding) causing proximity effect. All of these properties must be dealt with when designing an AC circuit.

DC Power:
In DC power current travels through the whole of a wire.

Size of the conductor and material (AC and DC power):

Electricity travels more easily in highly conductive elements like copper, silver or gold, the less conductive the material, the larger the diameter has to be to carry the same current load.

Engineers choose the right wire diameter for the job, raising current in a wire increases the resistivity and generates more heat. As you'll see in the diagram below copper can carry more current than aluminum while carrying the same load. 

Below: When Sir Humphrey Davy put lots of current through a thin platinum wire in 1802 it glowed and made the first incandescent light! but just a few seconds later the wire melted and vaporized due to the heat caused by resistance in the wire.

Quality of Material: Impurities and Crystals:

Most materials have impurities.  In copper the oxygen content and other materials in the copper effect the conductivity, so copper which will be made into an electrical wire is alloyed differently than copper which on it's way to becoming plumbing. 

Metals are crystalline (as you'll see in our Copper video). Monocrystalline copper or aluminum has better conductivity than polycrystalline metals, however large crystal copper is very expensive to produce and only used in high performance applications.


Resistance in a wire the describes the excitation of electrons in the wire's conductor material. This excitation results in the creation of heat, and loss of efficiency. In early DC power Thomas Edison couldn't send his power a long distance without using wide-diameter copper wires due to resistance over distance. This made DC power not cost effective and allowed for the growth of AC power.

Measurement Tools:
Engineers use Ohms Law to calculate how much resistance a given wire will have. This tells us how much energy we will loose over distance.

I = V / R Amps = Volts divided by Resistance

Formulas for Resistance and Conductance:

Resistance = resistivity / cross sectional area
Conductance = 1 / Resistance

When Resistance is Good:
Creation of heat in a wire is normally a sign of wasted energy, however in a tungsten or tantalum wire the heat makes the wire glow and produce light which may be desired. Tungsten is used to make filaments because it has a very high melting point. The wire can get very hot and glow brightly without melting. Tungsten would be very bad for power transmission since most of the energy put through is lost in the form of heat and light.

In power transmission we look for the lowest resistivity possible, we want to transmit power over long distances without losing energy through heat.  We measure resistance in a wire by ohms per 1000 feet or meters.  The longer electricity has to travel, the more energy it looses. 

Superconducting Wire and Resistance:

Above: Superconducting wire can be made into a metallic "tape"

Above: Carl Rosner , Mark Benz and others used special superconducting wire coils to produce the world's first 10 Tesla magnet. Niobium and tin are used instead of copper since materials work differently at different temperatures.

One great solution to power transmission is superconductors.  As a metal becomes super cold (approaching absolute zero) it obtains a conductivity of infinity.  At a certain point there is no resistivity at all. There have been experimental superconductive high voltage lines which were able to transmit power with almost no losses, however the technology is not developed enough to be cost effective.

Magnetic Fields (inductance and impedance):

Every wire used to transmit AC power creates a magnetic field while current flows through it.  The magnetic field is visualized by concentric rings around the cross section of the wire, each ring closer to the wire has a stronger
magnetic power. Magnetic fields are useful for making very strong magnets (when in a coil) i.e. making motors and generators, however these magnetic fields are unwanted in power transmission lines. 

While resistivity of a wire can impede the flow of current and make heat, the inductance of a wire/transmission line can also impede the flow of current but this impedance does not create heat since the energy is 'lost' in creating a magnetic field rather than exciting electrons in the material. This impedance is called Reactive impedance in AC Circuits. We used the word 'lost' however the power is not truly lost, it is used to create the magnetic field and it returns when the magnetic field collapses.

2.) Skin Effect:

In AC power electrons like to flow on the outside of a wire. This is because the changing of current back and forth causes eddy currents that result in current crowding toward the surface.

Skin Depth

Skin depth is a fixed number for given frequency, resistivity and permittivity. The higher the frequency of AC power in system, the more current is compressed on the outside of the wire, so a wire that is used at 60 Hz at a given voltage will not be ok at 200 MHz.  Engineers must always have the skin effect in mind when designing circuits. See the wikipedia site for the formula used to calculate the skin depth.
Above: engineers overcome skin effect by by using insulated stranded wire. If you make the individual strands equal to one skin depth, most of the current flows in the entire cross section and you use all of the copper. The downside is your wire must have a larger diameter as you need all the extra space for insulation. As the wire strands get smaller in diameter, and the insulation stays the same thickness, the ration of copper area to insulation can become less than one, then you will have more insulation than copper in the winding or cable.

Below: Higher frequency AC = less skin depth. The 'faster' current alternates back and forth the more eddy currents it creates. This high frequency power supply operates in the MHz range, notice the special wire used on the right.  The wire appears to be stranded and bare, but it is not, it has a clear enamel coating insulating it, so each small strand of wire carries it's own part of the current, with current traveling on the outside of each strand. This gives more surface area as a whole and allows for a large amount of current to travel through. 

Above: Compact fluorescent light electronics, the transformer is very small and is designed very cheaply. These parts often fail before the end of the typical life cycle of the unit.`

Engineers and Costs Savings Design:

Engineers use mathematics to calculate the 'skin depth' to find out how much of the wire is being used to conduct electricity. This is a critical part of the electrical engineers work in design of power systems. This work is also related to cost savings as engineers can find out what gauge wire and what wire type to use and compare that with other materials and configurations. Older electric motors and generators from the beginning of the 20th century were known to last a long time because at that time engineers could design the windings and type of wire for best performance since the costs of appliances and machines were higher. Today many motors burn out because engineers are forced to use the cheapest option - the least amount of material which can handle the current, however when the motor begins to overheat thinner wires of cheaper material will burn out faster. Ballasts (transformers) in modern lighting systems have a notoriously short lifespan in the effort to keep the cost per unit down.

Hands-on Exercise: How Cost Effects Design

You can see and feel the work of engineers in wire design around your house. Simply find older power supplies or professional power supplies used with high cost machines or tools. Feel the weight of these wall-worts or power supplies. Now find a kids toy or mobile phone charger. Feel how light the transformers feel in comparison.
If you are lucky you can find two transformers which convert power from the wall (120 or 220 V) to the same DC voltage for a device. If you open up the casing you can see the difference in the size of the gauge of windings, and whether they use copper or aluminum. You will clearly see how cost of the overall item effects design.

3.) Types of Wire:

Below: Types of wire used by utilities in power transmission:

Below: fixed wiring used in houses along with cords used in speakers, appliances and telephone systems. The graphic below shows old wires once used in houses (SJTWA and Type SE cable) and the modern standard romex.

WIRING 1880s to Today:

Above: 3 conductor underground copper wire (now rare)

Below: Flat 'tape' wire used in superconducting magnets

The best wire for the job:

All electrical engineers must know about wires and think about using the right design and material for the task at hand. Here are the factors for determining wire design:

-Durability (ability to flex repeatedly or be subject to crushing weights)
-Voltage and Current level
-Suspension strength (ability to hold its own weight over long spans between support)
-Underground or underwater
-Temperature of operation (like superconducting wire)

Solid Wire:

Less surface area to corrode
Can be rigid and strong
Not good if flexed repeatedly, can break if flexed in the same spot
Not practical for high voltage

Stranded Wire:

Above: Stranded speaker wire found in every household
Below: Specialized use super-thick stranded copper wire

-Stranded wire - lots of smaller wires in parallel, can be twisted together
Great conductor for its size
You may think this would be good for high frequency use because it has lots of surface area on all the little strands of wire, however it is worse than solid wire because the strands touch each other, shorting, and therefore the wire acts as one larger wire, and it has lots of air spaces which makes for more resistance for the size

Braided Wire:

-Great for durability compared with solid wire
-Better conductivity than solid wire (lots of surface area)
-Can act as an electromagnetic shield in noise-reduction wires
-The more strands in the wire, the more bendable and strong it is, but it costs more

Special Wires:

Solid with braided exterior or some combination of this, these wires are used for all kinds of special applications.

Coax cable is used for radio or cable television transmission because in its design braided and foil conductors on the outside keep frequencies trapped inside.  The shielding prevents stray electromagnetic energy from tainting the area around sensitive receivers.

Below: Video on types of wire used by electric utilities:

Hands-on Exercise: Wire Guessing Game

Gather bits of scrap wire from around you house or school workshop, collect short samples of different types. Now use the charts above to figure out what kind of wire it is, what it is made of, and list the uses for each. Show this to your teacher and see if you guessed right. Wire comes in so many exotic types that you may find yourself with a real mystery on your hands. Use internet searches to try to identify all of your samples.

4.) Wire Materials:

The most common material for electrical wire is copper and aluminum, these are not the best conductors however they are abundant and low cost. Gold is also used in applications because it is corrosion resistant.  Gold is used in automobile airbag electronics to guarantee that the device will function many years later despite exposure to harmful elements.

Above: gold used in connectors for Motorola chips

Gold is usually used in contact areas because this point in the system is more exposed to corrosion and has more potential for oxidization.

Aluminum wrapped around a steel center wire is used in power transmission because the aluminum is cheaper than copper and doesn't corrode. The steel center is used simply for strength, to hold the wire over long spans. Above is a typical ACSR cable used in overhead powerlines around the world.

Good conductors which are a solid at room temperature:

Platinum, Silver, Gold, Copper, Aluminum


Left: To make an efficient motor or generator windings have to be packed tight together, minimizing air spaces. Wire used in motors and generators is generally coated in enamel to allow the windings to be packed tight together. Traditional rubber or polymer insulation would make the wire diameter thicker, this is one reason why old electric motors were bigger and heavier than modern motors of the same horsepower.

See how motor wire is packed and wound into modern induction motors in our video here.

Learn more about the whole field of electrical insulation on our page here.

Hands-on Exercise: Burn up a motor!

Have you noticed that when a toy's motor gets very hot it smells? This is the insulation vaporizing. Heat breaks down all forms of insulation eventually, and in a motor winding when the insulation gets weak enough two wires side by side will short, this causes and arc and the device burns out.

If you take a small motor that you don't care about you can intentionally burn it out to see what happens to the windings. You can do this by putting more than the recommended voltage through the device, or by running the motor hot for a long period of time. Consult with an electrician or engineer to do this exercise safely.

Article, photos and videos by M.Whelan and W.Kornrumpf

Georgia State University
Wizards of Schenectady Carl Rosner. Edison Tech Center. 2008
Interview with Rudy Dehn. Edison Tech Center. 2012
Video with Denver Electric Motor. Edison Tech Center. 2012
Video with San Miguel Power Association. Edison Tech Center. 2014
William Kornrumpf, Electrical Engineer

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