The Basics: Series and Parallel Circuits

View Of Fairy Lights On Ground Indoors
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Have you ever wondered what the difference was between a series and a parallel circuit? Both use current that flows through them, but that’s where the likeness ends.

Series Circuit

A series circuit starts with the hot wire from a power source that feeds one side of, let’s say, a light bulb. The other wire on the light then goes to feed the next light and so on. At the last light, the second wire comes back and connects to the other side of the power source.

The best example I can give you is a string of Christmas lights. You know the type. When you take out one bulb, they all go out. That is a series circuit. If you take a bulb out, you’re opening up the circuit and that means that current cannot flow.

Parallel Circuit

Let's say that you are installing six lights in a building. A parallel circuit starts with a hot wire and a neutral wire which feeds the first light. The remaining five lights are connected white to white (neutral) and black to black (hot) all the way to and including the last light. Unlike the series circuit, you can remove a bulb in any of the lights and it will not cause the other lights to go out. A great example is a hallway in your house where two or more lights come on with the same switch. When one burns out, the other remains lit.

Now let's look at a practical application in your home that may make sense to you. In your kitchen, bathroom or basement, there is likely a ground fault circuit interrupter, GFCI, that will show how this principle works.

As you may or may not know, a GFCI has a line and a load connection. One is for the incoming "hot" wires and the other is for the outgoing "load" connection wires. Let's look at the difference.

The line connection is the point where you will connect the incoming feeder wire, the line, which is fed from the home’s electrical panel.

You’ll connect the hot wire, often a black or red wire, to the brass-colored terminal screw or in the slot provided to accept the stripped wire.

The neutral connection, white wire, is then connected to the silver-colored terminal screw or slot provided to accept the stripped wire. Be sure to tighten the terminal screws and tug on the wire to check that the connection is tight. These terminal screws are uncovered and visible right out of the manufacturer’s packaging. This is different from the load connection’s appearance.

The load connection is visible from the time you pull it out of the package. There is a piece of tape that covers the terminal screws, often yellow in color. As with the line connection, when you look on the back of the GFCI, there is a labeled area designating the load connection. The load connection is available to feed addition regular outlets from the GFCI and have protection from the GFCI. In other words, any regular outlet that is fed from the load side of the GFCI is also protected from a ground fault because of the GFCI. The benefit of this is to reduce cost by only having to purchase one GFCI when running addition outlets, instead of buying a GFCI for every outlet.

The regular outlets are about 1/10th the cost. The downfall is that when there is a ​ground fault condition with any of these added outlets, the GFCI outlet then trips. 

All of the outlets down the line that are connected also trip, cutting off the power to them. This parallel connection works to kill the power to everything connected to the GFCI outlet.