Gas clothes dryers and electric clothes dryers have much different electrical needs. A gas dryer produces heat by burning either natural gas or liquid propane (LP) gas, and it uses 120-volt electrical current merely to spin the dryer compartment and run the controls. A gas dryer plugs into an ordinary 120-volt electrical outlet, and it has a cord fitted with an ordinary appliance plug. An electric dryer, on the other hand, heats the air with electric heating elements and runs on 240-volt current, which requires a much different outlet receptacle and a special heavy-duty appliance cord with a unique plug.
But there are two different outlet styles and plug-in cord styles used for these 240-volt electric dryers. Some have three slots, designed to accept appliance cords with three prongs, while other outlets have four slots, designed to accept four-prong cords. There are so many of both kinds of dryer outlets in homes throughout the U.S. that newly purchased clothes dryers usually come with no cord at all, which allows you to choose and install a cord that matches whatever kind of dryer outlet is found in your home.
Watch Now: The Difference Between 3-Slot and 4-Slot Dryer Outlets
What Is a Three-Slot Dryer Outlet?
Before 1996, it was standard practice for 240-volt clothes dryers to have three-prong cords that plugged into 240-volt outlets. These had three slots—two 120-volt "hot" slots and a combined "ground/neutral" slot. In this configuration, the ground connection on the dryer was bonded to the neutral connection, so that the single wire served both functions—as the neutral current pathway and as the grounding pathway. This didn't mean that the appliance wasn't grounded, but that the neutral wire also served as the grounding pathway. This is normally not a problem, since neutral wires are always grounded in the main service panel. Under some unique circumstances, though, there is a very small chance of shock with this configuration.
Therefore, since 1996, the NEC and standard local code practice have mandated that new installations must include four-slot dryer outlets for 240-volt dryers, and that dryers must be fitted with four-prong cords to match them. However, there is no mandate that requires homeowners to stop using or convert existing three-slot dryer outlets. The risks are so small that the code allows existing three-slot outlets to remain in place for homeowners to use. If you buy a new dryer but have only an older three-slot outlet, it's fine for you to install a three-prong cord to match that outlet.
What is a Four-Slot Dryer Outlet?
Since the 1990s, standard wiring practice and the NEC has directed that 240-volt dryer outlets should have a four-slot configuration, in which the neutral electrical pathway and the grounding pathway are carried by separate wires. This came about because it was gradually recognized that the frequent presence of water in the laundry area created the potential for shock unless a separate, dedicated ground pathway was also present in the dryer. A four-slot receptacle, four-prong plug configuration is therefore somewhat safer than the older three-slot, three-prong method, since it has a dedicated grounding pathway that serves no other function.
You are no longer allowed to install a three-slot dryer outlet, and if you move into a home with the newer four-slot outlet, your old three-prong dryer will need to be refitted with a four-prong cord. Again, though, you are not required to convert that old three-slot outlet to a new four-slot outlet
When to Choose a Four-Slot Outlet Over a Three-Slot Outlet
If installing an outlet for a 240-volt dryer, you will never choose a three-slot dryer outlet, since the electrical code no longer allows this. Even if you wanted to, you might find them hard to find. Instead, you should always install the proper four-slot outlet.
Remember that if you happen to have an older dryer with a three-prong cord, you will need to replace its cord with a four-prong cord to match your new four-slot outlet. This is an easy project that takes just a few minutes to complete.
Converting a Dryer Outlet or Dryer Cord Yourself
Changing a dryer cord from a three-prong to a four-prong (or vice versa) is an easy project for most people. The process is relatively simple, and this is usually the best way to deal with the situation when your dryer cord doesn't match the dryer outlet present in your home.
If you have very good skills as a DIY electrician, you can also consider wiring a new outlet yourself, but you need good skills and an understanding of electrical systems. For most people, it is better to have a certified electrician or appliance repairman to do this installation.
National electrical code changes for 1996 and USA participation in International Energy Agency activities related to photovoltaics safety and grid interconnection. Sandia National Laboratories with the United States Department of Energy.