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What is the Skin Effect on Wire

This application note discusses material effect on skin depth on an electrical conductor such as wire.

What is Skin Effect?

Skin effect can be defined as the tendency for alternating electric current (AC) to flow mostly near the outer surface of an electrical conductor, and not through the core. The term "skin" refers to the outer surface of the conductor. "Depth" is used to describe the depth of the skin where the current is flowing. Skin depth can also be thought of as a measurement of distance over which the current falls from its original value. The decline in current density (electrical current in a given area) versus
depth is known as the skin effect.

The skin effect causes the effective resistance of the electrical conductor to increase as the frequency of the current increases. This occurs because most of the conductor is carrying very little of the current, and because of the oscillation of the electrical AC current to the skin of the conductor. Therefore, at high frequencies of current skin depth is small. Analyzing skin depth helps to determine the material properties of a good conductor on a solid conductive wire.

The skin depth formula equation below can be utilized to determine skin depth effect on a solid conductive material:


Skin effect occurs when frequencies start to increase, and as conduction starts to move from the inner cross section of the conductor toward the outer surface of the cross sectional of the  conductor. Skin effect is mainly cause by circulating eddy currents. Eddy currents effectively oppose the original current that was flowing to the center of the conductor, thus pushing it to the outer surface of the conductor. Take for example a solid wire: When current flows it is mainly concentrated on the outer surface of the wire. In this case, skin depth is shallow and the material of the solid conductor can be replaced with a hollow conductor.

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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. 





    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.




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