Be Sure Your Outlets Are Wired Correctly: How To Test For Crossed Ground And Neutral Wiring

If you are planning to purchase a new electronic device, such as a television, computer or other sophisticated equipment, then it's important to make sure its electrical outlet is properly wired. Incorrectly wired outlets can ruin your sensitive electronics, but you can test them quickly and easily if you follow the directions below. Here is what you need to know:

What you will need

  • Digital multimeter

  • Small appliance with three-pronged plug

  • Pen and paper

Understanding how outlets are wired

In the United States, electrical outlets are connected to three wires: the "hot" wire, neutral wire and ground wire. The hot wire and neutral wire are connected to the vertical slots that lie parallel to each other. In most cases, the hot wire is attached to the smaller slot. The ground wire is attached to the round hole that lies below or above the slots, depending on whether the outlet is installed right side-up or upside-down.

A crossed hot wire and neutral wire usually doesn't cause any harm and is not a cause for concern. However, there can be a substantial negative impact on devices if the ground wire and neutral wire are crossed. Sensitive electronics can be severely damaged when exposed to the voltage differentials that exist in a ground/neutral mixup. Unfortunately, it is often an invisible problem until the damage is already done; many other ordinary appliances can operate without a hitch on crossed connections. It's only after you have plugged in certain electronic devices that trouble ensues.

That's why it is wise to verify your electrical outlets are not cross-wired for ground and neutral. You can do do this yourself by using a multimeter. Below is more information about multimeters and how to test for faulty connections inside your outlets:

How to prepare your multimeter for testing AC voltage

You should get acquainted with your multimeter before attempting to use it. Read the instruction manual thoroughly and keep a good electronics reference handy to look up terms you don't understand. It's important to remember that most multimeters share some common features: red and black test leads, digital readouts, and selector dials for indicating what type of measurement you wish to make. Here is how to get your multimeter ready to use:

  1. On the selector dial, locate "VAC", which is the abbreviation for "Volts Alternating Current". You may see the VAC section on the dial divided into more than one range segment. Since standard electrical outlet in the United States provides 120 VAC, choose whatever range includes this value. For example, if your multimeter has a "0-500 VAC" range setting, then select that. If your multimeter is multi-ranging, then it will automatically select the appropriate range; your instruction manual will let you know if your device has that feature.

  2. Plug the red test lead into the red jack and the black lead into the black jack, which is also marked "COM" on some multimeters.

  3. Your multimeter is now ready to test your outlets.

How to check for a reversed ground wire and neutral wire

Testing for a mixed-up ground and neutral connection is safe as long as you use caution and follow directions. Keep your hands and fingers away from bare metal probes on the test leads, and illuminate your work area well so you can see what you are doing. Finally, remember that even a household current of 120 VAC can be fatal, so never attempt to measure any higher voltages.

Here is the step-by-step procedure for checking outlets:

1. Unplug any devices from the outlet you wish to test, but have a small appliance that uses a three-pronged plug handy for testing.

2. Once your multimeter is set up for measuring AC current, insert the red test lead probe into the smaller slot on the outlet face. Insert the black test lead probe into the ground plug hole.

3. Take a look at the digital readout, and write down the number that appears. If the hot wire is attached to the small slot, as it should be, then your meter will read at or very close to 120 VAC.

4. Remove the red test lead probe from the outlet, but leave the black test lead probe inserted into the ground plug. Insert the red test lead probe into the opposite, larger slot.

5. Look again at the digital display, and write down the number that you see. The measured voltage should be something less than 120 VAC. However, if the voltage displayed is higher than what you found in step 3, then the hot wire is attached to the larger slot.

6. Once you have determined which slot is wired for the hot connection, then you can proceed to the next step in testing for crossed neutral/ground connection.

7. Now, remove the test lead probes from the outlet, and plug in a small appliance that has a three-pronged cord. Many electrical devices are acceptable, such as a fan, space heater, vacuum cleaner or anything that draws a fair amount of current. Don't use a clock radio or other similar small electronic; these don't draw enough current for the upcoming reading. For this reading to be accurate, you must be pulling current from the outlet.

8. Once the appliance is plugged in and turned on, insert the red test lead probe into the hot connection you verified above. Insert the black test lead probe into the opposite slot.

9. Write down the reading that appears on your multimeter display. As long as the reading is less than the highest measured voltage you found in steps 3 or 5, then your ground and neutral wires are properly connected. However, if the reading is higher than the highest measured voltage, then you likely have a crossed neutral wire and ground wire.

If you discover that your outlet is wired incorrectly, do not use the outlet until it has been repaired. Shut off the power at the breaker panel, remove the outlet cover and properly rewire the connection. If you are not comfortable with that, contact a licensed electrician for assistance.