When moving a newer electric dryer into an older house, it's not uncommon to find that the cord and plug included with the new dryer doesn't fit the 240-volt electric outlet. Before the mid-1990s, most electric clothes dryers operated with three-prong plugs that fit into three-slot outlets, but since 2000, the electric code has required four-slot outlets. The mismatch can work both ways: You may find that a newer dryer doesn't fit an older-style outlet, or that an older clothes dryer doesn't fit a newer outlet.
Three-Slot vs. Four-Slot Outlets
The change in electric dryer plugs and outlets was made in the 1990s because of safety concerns. In a traditional three-slot 240-volt outlet, there are two live (hot) slots and one ground slot. This is perfectly fine for providing power, but such circuits operate with each hot wire serving as the neutral pathway for the other hot wire. There is no dedicated neutral wire in a standard 240-volt circuit—because the cycles alternate between the two hot wires, they can effectively serve as the neutral pathway for the opposite hot wire. In these circuits, the grounding wire carries current only during ground-fault situations. This was the standard way in which electric clothes dryers were wired up until the mid-1990s.
However, clothes dryers are actually 120/240-volt appliances. This means that some elements of the dryer, such as the timer and light, operate on 120-volt current, while the heating element itself is powered by 240-volt current. The 120-volt, single-phase features of the machine require a true neutral pathway, so up until the 1990s, the solution was to jumper the neutral pathway into the ground wire. In other words, the circuit grounding wire served as the neutral pathway for the components that operated on 120-volt current. But it was soon recognized that this joining of the ground and the neutral opened the possibility that the metal frame of the dryer could become energized, posing a danger of shock.
In the 1990s, the solution enacted to remove this safety hazard was to require that dryer outlets be wired with true 120/240-volt circuits, thereby allowing the ground and the neutral pathways to be separated. These circuits contain two hot wires, a grounding wire, and a true dedicated neutral wire. Outlets for these circuits, therefore, have four slots: two hots, neutral, and a ground.
Dryers configured for older three-slot outlets, therefore, will not fit newer four-slot outlets.
Two Possible Solutions
If you are faced with the problem of a dryer plug and dryer outlet that don't match, there are two possible remedies.
You can rewire the old outlet and circuit to make it a true 120/240 volt circuit. This can be somewhat complicated since it requires adding another circuit wire, a dedicated neutral, from the circuit breaker panel to the outlet location, as well as installing a new four-slot outlet. However, once this improvement is made, you are up to code and any new dryer purchased by you or a future owner will fit just fine. Normally, this solution is performed by a licensed electrician, since it involves working in the main service panel and running new circuit wires.
Alternatively, you can replace the four-prong cord and plug with a three-prong plug. This is an allowable remedy under the electrical code, which "grandfathers in" older installations. It is only new installations where the four-slot dryer receptacles are required; existing three-slot receptacles are allowed to remain. Simply replacing the dryer cord is by far the easier solution and the one that most DIYers pursue when they are faced with the problem.
Installing a Three-Prong Dryer Cord to Match a Three-Slot Outlet
It is fairly easy and inexpensive to remove the four-prong cord on a new dryer and replace it with a three-prong cord to match your existing three-slot outlet. It requires only simple tools, and the replacement cord usually costs around $20. The most difficult part of the process is removing and reattaching the strain-relief collar that secures the cord to the dryer.
Tools and Supplies Needed
While you will be working on the dryer while it is unplugged, it is best to be cautious and shut off the circuit powering the dryer before you unplug it or plug it in.
Shut off the Power
In the main service panel, find the double-pole circuit breaker that controls the clothes dryer and flip the breaker to the OFF position. At the dryer outlet, check for power using a circuit tester.
Unplug the dryer and move it to a location where you have access to the back panel. You may need to disconnect the dryer vent while working on the dryer. Make sure you have plenty of light while working.
Remove the Dryer's Electrical Connection Cover Panel
Use a screwdriver to remove the safety cover on the electrical connection box. It is located just above where the power cord comes out of the dryer and is attached with one or two screws. Set the cover plate and screws aside.
Detach the Electrical Cord
Remove the strain-relief fitting holding the cord by removing the screws that hold the fitting onto the back panel. You may need to squeeze the fitting together with pliers to access the screws. A magnetic nut driver may be useful since it will prevent the screws from dropping into the chassis. Separate the two halves of the fitting and pull them out of the hole individually.
Using a magnetic nut driver or screwdriver, disconnect the black and red wires from the left-side and right-side posts. These are the hot wire connections. Now, disconnect the white wire from the center terminal. This serves the neutral connection when the dryer is configured for a four-prong cord. Finally, disconnect the green wire grounding wire from the machine case by unscrewing the green grounding screw.
Slide the four-prong cord out of the connection box.
Jumper the Neutral to the Ground
As noted earlier, a three-prong cord configuration requires that you connect the dryer's neutral connection to the ground connection.
Locate the white machine wire that is connected to the neutral (center) screw terminal. Disconnect it and reconnect it under the green grounding screw.
Insert and Connect the Three-Prong Cord
Thread the loose end of the three-prong cord through the hole in the electrical connection box. Connect the outer two wires (the hot wires) on the cord to the outer two screws terminals, one on each screw. Connect the remaining center wire to the center terminal. In this configuration, the center cord wire serves as the ground connection for the dryer's metal case.
Reattach the Strain-Relief Fitting
To attach the strain relief fitting, slide one half of the clamp into the connection hole under the cord wire. Install the other half of the clamp in the same way. Squeeze the two halves together with pliers and insert the screws into the holes. Tight the screws until tight.
Reattach the connection box cover plate with a screwdriver. Slide the dryer into place and plug the cord into the outlet. At the main service panel, turn the dryer's circuit breaker back to the ON position, then test the dryer for operation.
Converting a Three-Prong Cord to Fit a Four-Slot Outlet
If you have the related but opposite situation—an older dryer but a newer house—it is equally easy to replace an old cord with a new four-prong cord that will fit a four-slot outlet. Most of the steps described above will be the same, with one important difference: The neutral connection and ground connection in the box will be connected to their own wires.