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, in a word, is efficiency.

The model above depicts the advantages of high-voltage transmission wires. Alternating Current travels from the left side of the diagram to the right along 2 meters of circuitously routed Nichrome Wire. Move the Switch Position slider from the Off to the middle Low-Voltage position to see 110 volts of electricity travel through the wire. 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 of this set up, it has lost a lot of its power due to resistance in the wire, and glows at only about 70 percent of its normal brightness. That’s a lot of wasted energy.

Now let's introduce a couple of transformers to this arrangement. Move the switch position slider farther right to the High-Voltage setting. The low-voltage current is now redirected through a Step-Up Transformer that increases the voltage to about 750 volts. At this high voltage, the current travels through the 2 meters of wire before it meets a Step-Down Transformer, which returns the voltage in the lines to the original 110 volts. Now the lamp on far right glows as brightly as the one on the left.

Why does this happen?

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 juice that started out at the power plant is given off as heat and wasted, a phenomenon known as line loss. As it turns out, 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.

Current and voltage have a kind of cause-and-effect relationship. Voltage is the cause, current (the moving, charged electrons) the effect. Voltage is also called potential difference: The greater the difference of electrical charge between two points on a wire, the more push the current is given.

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.

Last modified on 17 June 2019
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Magnet Academy is a free resource on magnetism & electricity brought to you by the Center for Integrating Research + Learning at the National High Magnetic Field Laboratory.