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Difference Between 3 and 4 Slot Dryer Outlets

If you are moving into a new house or replacing your dryer, the outlet may be confusing. However, understanding your outlet is important to understand what it can and possibly cannot support when it comes to these heat- and lint-generating appliances.

So, let’s take a look at what dryer outlets can tell you about the electrical wiring of your laundry room.


3-slot dryer outlets are 120 V and can only be used for gas dryers, which only require 120 V. 4-slot dryer outlets are 240 V and can be used for electric dryers, which require higher voltages. 3-slot outlets installed before 1996 may also be 240 V because the 4-slot requirement only came into effect in 1996.

Main Difference Lies in Neutral and Ground Wires

While relatively similar, 3-slot outlets and 4-slot outlets have differences in their wiring. A 3-prong outlet fits a 3-prong dryer plug. This plug has prongs for 2 hot wires and one prong for the neutral and ground wires.

4-slot outlets and plugs also have 2 hot wires. However, the 4-slot types have one prong/slot for a ground wire and another for a neutral wire.

Dryer Outlets: 3-Slot outlet and 4-slot outlet Illustration

Difference Between Ground and Neutral Wires

While ground and neutral wires are very closely related, they do have key differences, specifically in how they function.

Neutral wires carry current back to the source to complete a circuit.

Ground wires, on the other hand, are connected to the earth (which is why they are sometimes called earth wires) and provide a safety function.

Their function is to provide a low-resistance path to the ground in the event of an electrical malfunction, like a short circuit. Normally, they don’t have current running through them, but if there is excess power in the circuit, it will take the path of least resistance (the ground wire) and be conducted safely to the earth.

3-Slot Dryer Outlets

Pre-1996: 3-Slot Dryer Outlets Were 120 V or 240 V

If your dryer outlet was installed before 1996, then it could supply either 120 V or 240 V. Therefore, depending on the voltage, you could use it for both gas and electric dryers.

Gas dryers require 120 V to function. The electricity, while not used for heating purposes, is required to power the drum, lights, control panel, and fans. Because the heating is provided by gas, a smaller amount of electricity is needed to power these other functions, which is why 120 V is sufficient.

On the other hand, electric dryers require up to 240 V to function because they also use electricity to heat up the air in the drier on top of all the other functions (drum rotation, lights, control panel, and fans).

Post-1996: New 3-Slot Dryer Outlets Are Only 120 V

In 1996, the National Electrical Code required that 240 V outlets on new buildings utilize 4 slots as opposed to 3 slots. This was to promote safety since, if you were to get shocked, 240V would be easily deadly.

By requiring a separate ground wire, 4-slot outlets with 4-prong plugs eliminated the safety hazard that a combined neutral and ground wire could cause with higher voltage.

This does, however, mean that any 3-slot dryer outlets installed after 1996 are only compatible with gas dryers since electric dryers require a higher voltage than 3-slot outlets are allowed to carry.

You Still Get 3-Slot 240 V Dryer Outlets

If your house or laundry room was built after 1996, then your 3-slot outlets have to be only 120 V. 

On the other hand, if your dryer outlet was installed before then, then you can still have a 240 V outlet. This is because the regulation only applies to newer built constructions and installations; there is no requirement to change previous installations.

This means that you could theoretically run an electric dryer on a 3-slot dryer outlet if the outlet is old enough and has a high enough voltage.

How to Tell if Your 3-Slot Outlet Is 120 v or 240 V

How to Tell if the Outlet Is 120 v or 240 V

Thus, we come to the question of how to tell if your 3-slot outlet is 120 V or 240 V. There are a couple of ways to do this.

The first tell-tale sign that your outlet is 240 V is the size of it. Typically, a 240 V outlet will be larger than a 120 V outlet. Since the standard home outlet is 120 V, you can compare the size of your dryer outlet to another outlet in your house.

Another possible way to tell is by looking at the circuit breaker. An outlet that is only 120 V will use only one switch. A 240 V outlet, on the other hand, would typically use a double breaker.

If you still are unsure, an electrician would be able to figure it out quickly.

New 240 V Dryer Outlets Must Have 4 Slots

If your 240-volt dryer outlet is newer, then, by regulation, it must have four slots. This also means that all electric dryers must use a four-slot outlet if the electrical wiring was done after 1996.

If your dryer currently has a 3 prong cord, you are able to replace it with a 4 prong if necessary. This is helpful if you, for instance, have recently moved into a new house which has 4 prong outlets.

Leviton 278-S00 4-Wire, 30-Amp, 250V Flush Mount Dryer Receptacle,Black

While it is possible to change the cord yourself, you must make sure that the new cord is completely attached and secure to the dryer before plugging it in or it could potentially be deadly.

Reason for the Change

As stated before, 240 V dryer outlets were switched to 4 slots because of safety concerns.

With 3-slot outlets, the ground and neutral wires were grouped together. Because of this, in certain circumstances, the dryer itself could begin to conduct electricity.

This issue is exacerbated because moisture, a great conductor for electricity, is usually present in laundry rooms. If the dryer was conducting electricity then, in combination with moisture, it could shock whoever interacted with it.

A 240 V shock is extremely lethal. Therefore, the ground wire was separated from the neutral wire. This means that the dryer would have its own path to the ground. By giving it its own channel to the ground, the dryer itself would not be able to conduct electricity and become a hazard.

As such, it is safer to use a 4-slot over a 3-slot for higher voltages, which is why it is now required for new buildings.

Cords Sold Separately

If you are searching to replace your cord, or possibly buying a new dryer, then it is important to know that cords are sold separately from dryers.

This is because, since the code change did not require outlets to be changed, people use both 3-slot and 4-slot outlets. This makes it easy to get a new cord, as you just need to buy the one that matches your outlet type.

Sources

https://www.electronicshub.org/ground-vs-neutral/

https://fredsappliance.com/service/3-prong-vs-4-prong-dryer-outlets-whats-the-difference

https://www.dekooktips.com/advices/how-can-you-tell-120v-from-240v.html

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