More Than You Ever Wanted to Know About Electrical Outlets!

There are electrical outlets all over your house, scattered along the walls, unnoticed and under-appreciated until the nearest one is too far away for your electrical cord to reach. As American life gets more complicated and higher-tech, we find ourselves increasingly dependent on these funny looking holes in the wall. Most of us have never unscrewed one to find out what's behind that decorative plastic shield. And well you shouldn't, unless you've shut off the circuit breaker first. You could get seriously hurt back there if you just go poking around blindly. If you are blind, get someone to help you with this.

The purpose of an electrical outlet (also called a receptacle) of course, is to provide a point to plug in your various electrical appliances, such as blenders, computers, and those stupid plastic AC adapter blocks with the big freakin' transformer inside that takes up three slots on your power strip. In my living room alone, I have two lamps, a DVD player, two computers with various and sundry peripherals, a battery charger, speakers, electric pencil sharpener, and a humidifier. On top of that I occasionally vacuum in here too. Most people would add a TV set to that list, but most people don't have massive superiority complexes that drive them to eschew the banality of the proletariat.

As modern life gets more and more high-tech, we find ourselves hungrier for electrical power and more irritated when the nearest outlet is farther away than the power cord can reach. For this reason modern homes are built with outlets every six feet along the wall. While this is safer and more convenient than using extension cords, we unfortunately still have to reset our VCR clocks, oven clocks, clock radios, wall clocks, coffee maker clocks, and alarm clocks every time a squirrel fries itself on the power lines. I predict that within fifty years, not only will every electrical appliance have a clock in it, but they will also have so many flashing LEDs that we will no longer need lamps.

A standard outlet in the US has spots for two plugs (called a duplex outlet), and is rated for 15 Amps at 125 Volts, alternating current. It is very important not to exceed these ratings, or the outlet could be damaged or start a fire inside the wall. Then your house will burn down and houses are very expensive and sometimes have family members inside them. Never power a 15A outlet from a circuit breaker larger than 15A, even if multiple outlets will be fed from the circuit breaker. It should be impossible to exceed the rated load.

Common Types of Outlets

There are several kinds of electrical outlets in the US, a new version coming out every time some bleeding-heart liberal decides things aren't safe enough and we need to work harder at preventing the weak and stupid from hurting themselves. The obsolete types can still be found in older buildings with cheap and lazy owners who haven't bothered to update their electrical system yet. According to US regulations, they must be replaced with the modern outlets the next time they are worked on for any reason. All outlets are backwards compatible with older plugs.

Non-Polarized outlets have two vertical slots side by side that are the same size. Which one is hot and which one is neutral? Keep your family entertained for hours guessing! Loser gets electrocuted sticking a knife into a toaster. These are no longer used, and modern polarized plugs will not fit in them. That is, unless you file down the wider prong so it will fit, in which case you deserve to be electrocuted.

Polarized outlets are different in that the slot for the neutral wire is wider than the slot for the hot wire. This makes it difficult to insert the electrical plug the wrong way, although I wouldn't put it past some people. The purpose for this is most easily seen in devices such as toasters and lamps, which have exposed parts that can have electrical current running through them. A lamp, for example, powers the bulb both through a button on the bottom and the body of the screw the light bulb fits into. Since the screw fitting is large and easy to accidentally touch, in a properly wired polarized outlet, the screw fitting will be connected to the neutral wire. This is much safer because the neutral conductor, also called the grounded conductor, should always be at zero volts with respect to the idiot changing the light bulb without turning off the power first. This makes it much less likely to deliver an electrical shock.
Polarized, non-grounded outlets are becoming increasingly rare these days.

Grounded outlets have a round hole for the grounding conductor in addition to the two vertical slots. In general, electronic devices such as computers require these to provide a solid ground for the case so the electronics will work properly. It is also used as a safety feature in certain higher-power appliances such as vacuum cleaners, ensuring that whatever happens to the wiring, the case will never be energized to wall voltage (which would, of course, also energize the vacuumer to wall voltage). Grounded outlets are always polarized. Never cut off the ground prong to make a grounded plug fit into a non-grounded outlet.

Ground fault circuit interruptor outlets have two buttons on them (test and reset), and are always polarized and grounded. The purpose of the GFCI is to detect ground faults and shut off the power if they occur. They do this by monitoring the current in the hot and neutral wires and opening a relay contact if they do not match. Kirchhoff's Current Law states that the current in an electrical mesh (the simplest form of electrical circuit) is equal at all points in the mesh. Therefore, if the currents in the hot and neutral do not match, someone has dropped the hair dryer into the bathtub. GFCIs help stop this from killing you. This type can be found in the bathroom and near the kitchen sink.

Wiring an Outlet

NEMA 15 standard, grounded, polarized, 15A outlets are by far the most common in American homes. They generally have a single screw in the middle which holds the decorative plastic shield on. If you remove the shield, always turn the circuit breaker off first and never work on electricity alone. You can't call the hospital by yourself if your muscles have frozen up due to the current passing through your body. Looking behind the shield, you will see the outlet is connected to the junction box by two screws, one on top and one on the bottom.

The 14AWG wires powering the outlet will be connected to five screws in the sides of the outlet. The green one is the grounding conductor and is connected to a green-colored screw. Near the green screw are two regular colored screws, connected by a breakable tab, for the neutral wire which should have white or light grey insulation. This puts the wider, neutral slot on the left side of the outlet when viewed from the front with the ground on the bottom. On the other side of the outlet are two brass screws for the hot wire, also connected by a breakable tab. The hot wire insulation can be almost any color except white, light grey, or green. If you leave the breakable tabs on (see below), you only have to use one of the hot and one of the neutral screws.
Non-NEMA standard outlets have the ground screw on the hot screw (brass) side. Don't use non-NEMA standard outlets.

Always connect the wires to the proper screws, or you will have created an electrical hazard by defeating the polarization on the outlet. An incorrectly wired polarized outlet is more dangerous than a non-polarized outlet. Usually, there will be two wires on each screw. In a radial wiring setup, a single circuit breaker will be powering several outlets, wired in parallel. One of the wires on each screw will be going back to the circuit breaker or the previous outlet, and the other wire will be going to the next outlet.

If the outlet is switched, only the hot wire will go through the switch. The ground and neutral are continuous all the way back to the circuit breaker panel for safety reasons. Sometimes it is useful to switch one socket of a duplex outlet and not the other, for example if you have a floor lamp and a computer plugged into the same location and don't want the computer to lose power when you turn the lights off. This is what the breakable tabs, mentioned above, are for. Break off the tab on the hot side (the brass screws) and attach the switched hot to one of the brass screws and the unswitched hot to the other. The tab on the neutral side can stay. You only need separate neutrals if you are powering the two sockets in the outlet from different power sources (such as different transformers), which is not a concern in residential wiring.

Less Common Types of Outlets

Twist Lock Outlets: Apartment buildings sometimes have outlets in the hallways with the three slots arranged in a circle. This is for a special plug which is inserted and twisted to lock in place in the outlet. There are two reasons to do this. First, the twist lock plug cannot be accidentally pulled out of the outlet. This is handy for their intended purpose, which is vacuuming the hallways of the apartment. The second benefit is that the building tenants don't have electrical plugs that will fit into the twist lock outlets, so they can't steal electricity from the hallway.

20A Outlets: Some electrical outlets have a "T" shaped slot on the neutral instead of just a vertical slot. These outlets are rated for 20A, and can receive special plugs (with a sideways neutral prong) for appliances which draw more than 15A. This prevents people from tripping their circuit breakers by plugging them into 15A outlets by mistake. While 15A outlets are generally fed by 14AWG wire, 20A outlets are generally fed from 12AWG wire. There are exceptions for long runs or high temperature environments, which need thicker wire. A standard plug can still fit into this outlet because the T shaped slot will accept both kinds of neutral prongs.

Air Conditioner Outlets: Small air conditioners run on normal 120 Volt power, but larger ones use 240 Volt power. These outlets have a ground prong hole and two horizontal slots, instead of vertical slots, for the two hot wires. There is no space for a neutral.

Dryer Outlets: A 240VAC clothes dryer outlet has four prongs. The top prong is round, and is for the ground connection. The bottom prong is shaped like an L and is for the neutral wire. The two vertical slots on the sides are for the two hot wires.

Oven or Range Outlets: A 240VAC oven outlet also has four prongs, but the neutral prong is straight and not L shaped. It is, however, narrower and thicker than the hot wire prongs. These two plugs have four prongs because they use two hot wires to provide the 240 Volt power.

240 Volt Plugs: Circuit breaker panels in the US are fed from 240 Volt center-tapped transformers. What this means is that the voltage from one hot wire to the other (called line to line voltage) is 240 Volts, and the voltage from either wire to the neutral center-tap (called line to neutral voltage) is half that, or 120 Volts. The reason for this is to provide safer, 120 Volt power to outlets that need to supply less than 2.4 kiloWatts. Larger appliances need more than that (a clothes dryer can pull 4 kiloWatts), and if they were fed with a 120 Volt supply they would need more than 20 Amps of current running through the wires. Higher currents require thicker wires, and at this point we're getting out of the range of "thick wire" and into "thin copper rod" — so we use higher voltage to get lower current and use smaller wires.

How to Kill a Baby with Scissors and an Electrical Outlet:
  1. Set baby on floor in front of electrical outlet.
  2. Give baby all-metal scissors with no plastic handle.
  3. Turn your back for five seconds.
  4. If baby is not dead, it is not curious enough. Spend a few weeks encouraging the baby to put things in other things, like buying those square peg and round hole boxes, whatever they're called. Repeat steps 1-3.

It has recently come to my attention that some people prefer their babies alive and not dead. In this case, I recommend small plastic outlet protectors, available at any hardware store.

benjya informs me that in the UK, the live and neutral pins of sockets are shuttered (and have been for 30+ years). The shutter releases either when the earth pin (always present) is inserted, or on newer sockets when both the live and neutral pins are inserted together. This makes it almost impossible for babies to stick things into them. This applies to wall sockets and also 2-, 3-, 4- etc extenders. Additionally, on all uk plugs, the "rear" half of the live and neutral pins (the half nearest to the plug) is shrouded in plastic. So even if the plug isn't fully pushed into the socket, or if you're plugging it in and your fingers wrap round a bit too far, you can't touch live metal.
Thanks for the info! The more I learn about British residential electric wiring, the more I envy it.

Questions about Outlets:

What's with those outlets with the T shaped slot?

See 20A Outlets, above.

What about the outlets on my computer that look similar to the outlets in the wall?

These are IEC 320 standard, as opposed to wall outlet NEMA 15 standard.

Why don't you mention the outlets with four receptacles?

These are actually two duplex outlets side by side with a large plastic shield that covers both of them, creating the illusion of a "quadruplex" outlet.

I hear electricity and water don't mix. What happens when the basement floods and the outlets get covered in water?

Current will flow out one slot, through the water, and back into the other slot. Water is a good conductor, but not that good, and the current will probably be less than 15A so the circuit breaker won't trip. However this wastes a lot of electricity and you should shut off the circuit breaker. Basement outlets should have GFCI protection, preferably a CFCI circuit breaker rather than individual CFCI outlets.
Wading through water with submerged live electrical connections is dangerous and foolish, people have been killed this way, usually when touching a metal object such as a water pipe to complete the circuit.

How many plugs is too many for one outlet?

If you're using a power strip, go crazy. You can keep adding electrical devices to it, plugging power strips into each other for even more outlets, until the load trips the circuit breaker. Power strips are generally rated for 20A, so you will be okay as long as the circuit breaker feeding the outlet is 15 or 20A.
However, if you're using those molded rubber three-way adapters, only use one per outlet. They can barely support their own weight with the prongs, and plugging them into each other will cause them to sag, exposing the prongs and causing a serious danger.

Is the government using my electrical outlets to broadcast mind-control rays at me?

Yes. It's the one with the TV plugged into it.

On Seinfeld, Kramer tripped the circuit breaker in an apartment by putting a paper clip in the outlet. Would this really work?

It might, if the paperclip was clean, with no corrosion, and made of good conducting steel. On the other hand, if it has too much resistance, the current will make the paper clip heat up, quickly becoming red hot and probably starting a fire. Even if it does work, remember that Kramer electrocuted himself doing it. Anyway, I'm not about to try it.
littlerubberfeet says that he's successfully tripped many a circuit breaker with a large paper clip bent into a U, and a handle duct taped on for insulation. The paper clip would get damaged but it was otherwise effective. Still, you try this at your own risk.

How are electrical outlets different in the UK?

They use 240 Volt, 50Hz power over there, compared to our 120 Volt, 60Hz power. Their outlets also have three slots (one vertical, two horizontal) arranged in an isosceles triangle instead of a round hole and two slots, and they are typically connected in ring main instead of a radial fashion. Their Residual Current Device is similar to our GFCI protection. They also call their outlets "sockets", their trucks "lorries" and their president "Prime Minister". They're weird over there.

How dangerous is wall voltage?

To repeat the tired cliche, it's not the Volts, it's the amps that kills you. But while every electrician will tell you this, it doesn't really answer your question. Current is voltage divided by resistance, so the lower your resistance, the more dangerous a given voltage is. Wall voltage can easily kill you if the conditions are right, although it usually will just hurt you very badly. Your body resistance can change significantly depending on several things, which means a voltage that hurt you one time might kill you another time. Electricity can cause a serious burn if the arm that comes in contact with the hot wire is also touching a piece of metal, such as the junction box. Respect electricity and shut off the power at the circuit breaker before dong any electrical work. The circuit breaker will not protect you, it takes less than one Amp to kill you. By the time you're pulling over 15 Amps through your body, you're a crispy critter. GFCI outlets are designed to protect you, but don't go tempting fate.
Current levels and the danger they represent can be found in the electrocution writeup.

Who's asking all these questions?

I did, over the last few years. It all started when I got curious about the T-shaped outlets and came to its logical conclusion when I went to Iowa State University to study electrical engineering. True story.
The section below is for questions other people asked:

PLAYER1 asks: Why when you are at a LAN party you can have 20 computers hooked to one outlet, but as soon as some smart aleck turns on the microwave everyone loses power?

Big CRT monitors can use up to an Amp of current, and LCD displays use about half what a comparably sized CRT uses. The computer itself can use two or three Amps, more during operations that use a lot of processing power and stress your video cards (like gaming).
The kicker is when the microwave is turned on. Microwaves can pull up to ten Amps all by themselves! Putting a dozen or so computers together on one circuit is already putting quite a load on the circuit breaker, so when a 1200 Watt microwave is suddenly added to it, well, the circuit breaker is just doing its job. Put the microwave in a different room so it will be on a different circuit.

oakling asks: When I do turn on too many things at once, how can I tell the difference between blowing a circuit and blowing a fuse, and where are likely places to find the fuse box and circuit-breakers?

It is very unlikely that your house has both fuses and circuit breakers. When a fuse blows or when a circuit breaker trips, the effects are exactly the same - your outlets lose power. The only difference is circuit breakers can be reset, and fuses must be replaced (always keep spares handy). A good fuse has a strip of metal across the window. When the fuse blows, this metal strip breaks. You will probably need a flashlight to tell. When a circuit breaker trips, the switch will flip halfway into the off position, revealing an orange marker. The circuit breaker must be turned completely off, then back on, to reset it.
The circuit breaker panel (or fusebox) itself is usually in one of a handful of places. Be aware, though, that some larger houses have a main circuit breaker panel which feeds one or more sub-panels. Main circuit breaker panels can usually be located by looking outside to see where the service drop from the power utility comes into the house. Sometimes that doesn't work, either because the service drop doesn't go directly to the panel or because the service drop is underground. Likely locations are the workroom, the basement, and the utility closet. Sub-panels can sometimes be found in the utility closet, the laundry room, and the basement.

mr100percent asks: Why do electrical cords have holes in the prongs?

He also answers the question.

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