Skip to main content

The National MagLab is funded by the National Science Foundation and the State of Florida.

Transmission Lines

Electricity goes through some ups and downs on its way from the power plant to your house. Here's how it works.

More than half a million miles of high–voltage transmission lines criss-cross the United States. If you’ve ever wondered about those looming towers and the wires draped between them, the following tutorial will fill you in.

The amount of power running along those high-voltage lines is awesome. Compared to your humble 12-volt car battery, easily hundreds of thousands of volts of electricity are transported across these wires. Why is electricity distributed this way, rather than through the same low-voltage lines that run through our homes?

The answer is efficiency. Higher voltage is needed to decrease resistance as the current moves along the transmission line. Take a look:


  1. Our example represents two meters of transmission wire carrying Alternating Current.
  2. Move the switch to low voltage and see what happens when 110 volts of electricity travels through the wire.
  3. Notice the 60-Watt Lamp nearest the power source glows at its normal brightness. But by the time the electricity has traveled through the 2 meters of wire to the lamp on the far right, it has lost a lot of its power because of resistance in the wire. The second lamp glows at only about 70 percent of normal brightness.
  4. Move the switch to high voltage to see how use of transformers can improve the flow of electricity. The low-voltage current is now redirected through a transformer that increases the voltage to about 750 volts. At the other end of the wire, another transformer returns the voltage to the original 110 volts. Now the lamp on the right glows as brightly as the one on the left.

Electrical current is carried by electrons in a wire. Unless the wire is superconducting, the electrons encounter resistance, bumping into each other as well as the cable or wire through which they’re traveling. When that happens, some of the electricity is given off as heat and wasted, a phenomenon known as line loss. But increasing the voltage along the line allows you to decrease the current (and hence the line loss) without sacrificing how much power you’re transporting.

There’s a simple equation for calculating power loss: the current squared multiplied by the resistance. So if you lower the current, you lower the resistance. And if you lower the current while raising the voltage, you can keep the resistance low without sacrificing the amount of power you’re transmitting.