Finding your hot side
Have you ever noticed that many device plugs have prongs with slightly different sizes? And if you try to plug them the wrong way into a modern outlet, they won’t fit. This design choice is all about polarity and the “hot” side of the wiring where the electrical current comes from.
Don’t worry: It’s not difficult to understand how the hot side of plugs and outlets work, and it will help develop your DIY skills when you want to replace an outlet. Let’s dive into exactly what all these electrical design choices mean.
Most of today’s North American receptacles or home power outlets (the standard 120V variety) are “polarized.” That means they have a neutral side and a hot side…which is very important regarding wiring and electrical safety.
The hot side is where live wiring enters the receptacle, ready to charge up any electrical device or appliance you plug in. North American plugs on standard devices like lamps, TVs, and power strips are all designed to plug into these outlets.
When connected, the electrical outlet completes a circuit: The electrical current moves from the hot side of the receptacle into the plug and passes through the device wiring (this is a bit simplified compared to real alternating current, but it works as a model). It then exits through the neutral side to rejoin the greater house circuit. Along the way, it imparts the necessary energy to power your device.
The name “polarized” refers to the positive and negative charges of the electrons that enable the flow of electrical. But the important part for homeowners is that one side of the outlet is hot, filled with electrical potential, and the other is neutral, or wires waiting for incoming current. There’s also usually a third slot for a ground connection, simply a safety feature that can discharge electrical current into the ground if something goes wrong.
The hot side makes the outlet work, so every outlet needs one (it’s also what makes sticking a fork in an outlet a bad time, so respect your electricity). It becomes even more critical in two everyday situations.
The first situation is when you are plugging in a standard AC (alternating current) device plug. Those plugs are typically designed for one prong to accept the current into the device and the other to pass the current along to the neutral side to complete the circuit. The prongs must match the hot and neutral sides, so designs often limit how plugs can plug into outlets. Understand this part, and you’ll have much less trouble dealing with power outlets in your home.
Second, if you are replacing a receptacle, such as with an outlet upgrade that includes childproofing or USB outlets, you must ensure you wire the new outlet correctly, or it could cause hazards that we’ll discuss below. When wiring an outlet, a simple rule will keep you on the right side:
The black (and occasionally red) wire is the hot wire. It’s channeling the electricity that’s originating from the electrical panel in your house.
The white or gray wire is the neutral wire. It’s waiting to accept the current once it passes through a device.
Along with the simple hot/neutral wiring rule, there’s another easy rule for finding out which side of a receptacle is hot when plugging in a device: Look at the slots.
A receptacle's top two slots, or “eyes,” are meant for the vertically aligned plug prongs, while the rounded bottom slot is intended for the ground prong. At a casual glance, these two slots look the same but look closer, and you’ll quickly see one slot is smaller than the other. This is the hot side.
The hot slot is the smaller slot to both quickly identify it and make it more difficult to slide in the wrong prong or other conductive material when working with the outlet. On modern North American outlets, it’s almost always on the right side of the receptacle—unless a receptacle has not been installed according to standard codes.
Now for the plug itself: When a plug has polarity, you can tell at a glance because one prong will also be noticeably smaller than the other, indicating that it should be connected to the live side of the outlet. That’s handy for identifying plugs, but it’s not that important in practice because if you flip a polarized plug the wrong way, it probably won’t fit into the outlet (and you should never try to force it).
With the correct wiring and hot side identified, you’re ready for a DIY outlet project. But there are a few other things you should know, too.
What happens if you get your wires crossed and mix up the hot and neutral sides when installing or wiring a new outlet? This reverses the polarity and causes problems for the device you plug in. Thanks to the AC current, devices still work, but they become more dangerous.
The mixup often causes the plug to become “hot” with electricity. This can create a considerable shock when accidentally touching the metal parts of the plug, such as when the plug is only partially inserted into the outlet but is live. It can also electrify other parts of the device, such as the metal socket of a light bulb or the metal innards of a toaster, allowing them to pass current through an unwilling human body and out through ground contact. Ultimately, all sorts of nasty shock hazards can arise.
Getting a wallop of 120 volts at 15 to 20 amps (standard U.S. home power) can be unpleasant and dangerous. It’s not usually life-threatening, but in the wrong circumstances, it can be, so it’s crucial to always get the wiring correct for safety. You can test an outlet with a plug-in meter, a multimeter, or a similar device when working on wiring to ensure the polarity is correct.
“I thought these kinds of shocks were always a possibility,” you may be thinking. It was a greater danger years ago. One reason polarized outlets and plugs became the standard was to reduce these kinds of risks.
It varies by device. Polarized plugs aren’t required for many devices but are included as a simple safety feature, so the plug only works one way. Other devices where the risk of shock is higher may be required to have polarized plugs depending on North American standards.
It’s not uncommon. Many devices don’t use plugs with polarized prongs, including larger appliances like computers and TVs. The power outlet still functions the same way, and the device will not suffer as long as it can fit into the outlet, which is rarely an issue.
In some cases, polarized plugs aren’t necessary because the device uses shielded, completely contained wiring, and there’s no enhanced risk of a shock. In other cases, the device may be old or made by a manufacturer that doesn’t offer a polarized design. The device should still be safe to use, but you’ll want to take additional care to avoid shocks when handling the plug.
You don’t see it often, but non-polarized North American outlets exist, where both slots are the same size. It was more common decades ago, along with outlets with no ground slot. Today, you don’t find them very often. Still, there are exceptions, and there’s even a NEMA (National Electrical Manufacturers Association) standard for a simplified, non-polarized outlet that can be used in certain circumstances.
Non-polarized outlets create two issues. First, it’s harder to spot wiring mistakes quickly since the hot side isn’t immediately identified when working on the outlet. Second, in some cases, a polarized plug will not fit into a nonpolarized outlet, in which case the outlet needs to be replaced. Since these outlets are often very old, it’s a good idea to inspect the condition of the wiring behind them for wear, too.
Some large appliances need extra power, like ovens and clothes dryers. That’s why, in U.S. homes, these appliances have separate 240V outlets to provide the necessary electricity. This outlet design looks very different, and none of the above rules apply.
The design of these outlets may incorporate three slots, but the shape is notably different, as are their wiring needs. That makes it more challenging to change these outlets as a DIY project, but it also means that you don’t have to worry about polarity. Large appliance receptacles typically have two hot connections, and the appliance plugs will only fit in a certain way. If you aren’t sure about your wiring, you can use a voltmeter to measure the outlet: Both right and left slot wiring should measure 120V for the combined 240V needed.
Converting to a polarized outlet is mostly about visual information: Behind the receptacle, the wiring should remain the same, with its hot and neutral sides. A polarized outlet simply shows which is which, preventing polarized plugs from plugging in incorrectly and creating extra shock risks. Ensure your wiring is lined up correctly, and any new standard receptacle and faceplate you purchase should have the polarized design. This is a relatively simple DIY project, but if working with wires makes you nervous, contact an electrician near you and ask about their prices. Installing a new outlet costs around $75 and rises depending on your wiring needs.
They can vary—a lot. For example, while the EU continues to move toward broader standardization, many different types of outlets still exist from country to country. Some local standards cared about polarization, and some didn’t. It’s similar to other countries around the world.
Generally, you’re less likely to find polarized outlets outside North America. However, alternative plugs and outlets often incorporate precautions so that accidentally touching live prongs on a plug is hard or impossible. You may also find interesting regulations overseas that limit the use of outlets in bathrooms or near sinks to decrease the chance of shock.
A duplex outlet simply means that the receptacle has two plug-in outlets for two plugs. It’s a common design choice for homes. The two outlets should be wired identically during installation so the hot side is the same for both.
GFCI stands for Ground Fault Circuit Interrupter, a standard safety addition to home power outlets. In rooms where moisture is more likely, like the bathroom and kitchen, GFCI outlets stop the current when certain kinds of shorts are detected, preventing shocks. Both GFCI and the similar AFCI (Arc Fault Circuit Interrupter) outlets are polarized and have clear indicators on the receptacle where hot wiring should go.
Outlets with only two slots have no ground connection. That’s more common on very old outlets, which means it’s more likely the outlet won’t be polarized. You’ll have to closely examine the slots to see if there’s a size difference indicating polarization. Keep in mind outlets this old are usually out of code and should be replaced if you’re doing any electrical work.
