Last Updated on January 18, 2023 by Dave Farquhar
Building inspectors get cranky when they find reverse polarity outlets, because it causes a dangerous situation. Let’s look at the danger of a reverse polarity outlet and how to fix it.
Reverse polarity on AC outlets is a code violation. It poses a danger because it can cause wiring to always be electrically live, even when the device is turned off.
I was in the process of buying a house. The house had a sunroom that had been recently renovated, including adding grounded outlets where they used to be ungrounded. My inspector was happy to see that. He wasn’t so happy when he saw they all had reverse polarity on them, just like he was the time he found a bootleg ground. But if the houses hadn’t had problems that scared people, I wouldn’t have been able to afford either of them.
It’s not hard to remember how to wire an electrical outlet. In the United States, the black wire is hot and the white wire is the common or neutral. On the electrical terminals, the gold or brass screws are hot, and the white metal screws are common. Just remember black and gold go together and white and white go together.
Does polarity matter in AC?
The whole idea with AC is that polarity doesn’t matter, the electricity just flows regardless. So the device will function even with reverse polarity. And on devices that don’t have any kind of on-off switch, the plug may go in either way. That’s called a non-polarized plug.
But on most never devices, the plug will only go in one way because one prong is slightly thicker than the other. That’s called a polarized plug. That ensures that the live wire always goes to the device’s power switch. That’s important, because the power switch needs to not just shut off the flow of electricity but it also needs to ensure the device is electrically dead.
You do have to go with the size of the plug rather than the plug being on the left or right, since you can install outlets with ground either up or down.
The danger in a reverse polarity outlet
If your electrical outlet’s polarity is wrong, current can still flow into the device, it just can’t get back to the panel. This means the device is off, but it’s not electrically dead. There are live wires in it, and if you somehow touch it, the power will find its way to ground through you. If we’re talking the full 120 volts from the outlet, you’ll feel that.
I’ve been shocked with 120 a few times, and while it’s not the worst thing that’s ever happened to me, let’s just say I still remember every time that happened. The last time was in 2013 and I don’t remember anything else that happened that month, but I remember that jolt.
It’s not pleasant and under the right conditions it can be fatal. So we’re talking edge cases here, but it’s 100 percent preventable. It takes 30 seconds to get it right when wiring the outlet. If the outlet is already wired, it’s worth the five minutes fix it. Either way we’re talking a tiny investment in time to avoid causing harm to a fellow human being.
How to test for reverse polarity
You can test for reverse polarity with a cheap electrical outlet tester available at any hardware store or home center. It’s a small plastic box that plugs into an outlet and has three lights on it. If the red and center light are lit, you have reverse polarity. A proper outlet will show two yellow lights.
How to correct a reverse polarity outlet
It’s easy to correct a reverse polarity outlet, and it only takes about five minutes. Less once you get the hang of wiring an outlet. Turn the power off at the breaker box. Then remove the screw holding the wall plate. Remove the two screws holding the outlet in place and pull the outlet out. Then move the wires. The black wires go on the gold-colored screws and the white wires go on the white metal screws. Loop the wire to make it easier to go on, and make sure there’s no insulation under the screw so you get a good electrical connection. Then screw the outlet back into the box, replace the wall plate, and test the outlet.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.