How To Connect 4-Wire Appliance To 3-Wire Supply?

The easiest and safest way to connect a 4-wire appliance to a 3-wire supply is to simply buy a 4 to 3-prong adapter and use that as an intermediary between your appliance and the outlet.

Secondly, you can always choose to upgrade all your 3-wire slots to the newer 4-wire ones as that is the standard now, anyway. 

Conversely, you can also change the cords of your appliances to match the older 3-prong. This, however, may not be an ideal long-term solution since the 4-prong cords and outlets are the standards now, and good for a reason. 

Changes in the National Electric Code (NEC) recommend that all newer houses come with a standard 4-slot outlet. That being said, there are still older houses that have the older 3-wire outlet. 

Newer appliances come standard with a 4-prong or 4-wire cord that plugs right into 4-slot wall outlets. Previously, the 3-wire outlet had no provision for a grounding wire. 

Replacing a 3-Wire Outlet With A 4-Wire One 


Firstly, we’re going to look at the process of replacing an old 3-wire outlet with a newer 4-wire one. Before we get into the actual process, let’s brush up on our basics. 

In North America, in an electrical circuit, two hot wires carry 120 volts relative to the neutral. The voltage between the wires themselves is 240 volts. 

Now, replacing a 3-prong outlet with a 4-prong can be a potentially tricky process. The latter lacks a ground wire, which makes the fourth wire in a 4-prong outlet – the three being the two hot wires and a neutral one. 

wire hook up

This can be fixed by running a separate ground wire and connecting it to either a piece of metal stake semi-buried in the ground. Running it through your house’s existing electric panel ground bus is also a way to achieve this. 

1. Setup

Cut off the power to your outlets from your fuse box and use a tester to make sure it’s done properly. 

Once you’ve ensured the power is cut-off, unscrew the screws holding the outlet in place and pull it out from the junction box. 


2. Remove The Old 3-Slot Outlet 

You will discover three wires behind the outlet – red, black, and white. The newer, 4-prong outlet will have 4 wires, in contrast. 


The extra, fourth wire slot is for a grounding wire which was absent in the previous generation outlets. You can either use the white wire for both grounding and neutral or install a separate ground wire through your house’s electric panel. 


3. Installing The New 4-Slot Outlet

Screw the live wires i.e., red and black into the live slots, and the white one into the neutral slot. Do the same for the grounding wire, if you’ve installed one, into the ground slot. 

Push the now newly-connected 4-prong outlet back into the junction box.


4. Securing The Outlet And Testing 

Once the new outlet is in place, secure it tightly with the screws.

Turn the power back on and check if everything is working correctly by plugging in the appliance and testing it. 

Replacing A 4-Prong Cord With A 3-Prong One 

This is an alternative to replacing an outlet by instead switching from a 4-prong cord to a 3-prong for your appliance. 

Let’s take a dryer for example – 

1. Removing The Previous Connections 

Open up the panel box of the dryer it should expose four separate wires – two color-coded live ones, one white/ neutral, and a green chassis ground wire. 

panel box

Unscrew all four wires, remove the older chord, and place it in the newer 3-prong one. Since there’s one less wire here, the key is to use the middle slot i.e., where the white wire goes, as both neutral and ground. 

open panel box

2. Installing The New 3-Prong Cord

Screw the two live wires and the now neutral/ ground wire in the middle slot. Put the ground back to the chassis ground as it’s no longer going to be needed. 

The white wire now serves as both the neutral and the ground.

open panel box

3. Securing And Testing The New Cord 

Install a strain-relief guard where the wires exit the dryer panel to secure them in place. 


Finally, test whether the neutral wire and the chassis ground are mechanically bound to each other. 


Connecting A 3-Wire Appliance To A 4-Wire Outlet/Supply 

The easiest method is to simply buy a 3-prong to 4-prong adapter which will act as an intermediary between the outlet and the appliance. 

The second, the more permanent method is to simply replace the appliance cord. 

1. Undoing The Previous Connections

Remove the appliance’s electric panel and unscrew the wires inside. There should be three – one white neutral wire sandwiched between two live ones. 

electric panel

There is also a metal strip that connects the white wire to the frame of the appliance. Remove that piece of metal, as well. 

electric panel

2. Install The New 4-Prong Cord

Attach the two live wires to the black and red terminals, and the white one to neutral. Connect the additional green/ ground wire to the spot from where the metal strip was removed. 


Place the cover plate back onto the panel and screw it in place. Test the appliance using the new cord and you’re good to go. 


Final Remarks 

Connecting a 4-wire appliance to a 3-wire outlet or vice versa is a fairly simple process that can be done with a bit of studying and precaution. 

Replacing the outlet or the cord itself is a more time-consuming process but also safer. It is also more reliable and a real long-term solution, without breaking the bank.

The easiest method to do so is to get the necessary adapters and use them as intermediaries. They can run you anywhere between 30 and 60 dollars each, depending on the configuration.