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Alternating Current (AC) Home Wiring

by Ron Kurtus (updated 30 May 2023)

Homes in North America typically are powered with both 120-volt and 240-volt alternating current (AC) electricity. (Because of voltage variations, they are also listed as 110-120 VAC and 220-240 VAC).

Modern outlets have three different shaped holes to assure plugs can only be inserted in one way. Two of the holes are considered grounds, for reasons of safety. Proper grounding and the use of fuses are important to maintain electric safety in the home.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion

Home wiring

Typically, homes in North America receive 240 volts of AC electricity. Certain high-power devices, such as an electric stove, use the full 240 volts. The rest of the outlets in the house use 120 volts.

Wires into home

Usually, three copper wires come into the home. Two are covered in black insulation and one has white insulation. Sometimes one wire is red instead of black. Each black or red wire is called a "hot" wire and has 120-volt AC. The white wire is called the "common" and is grounded at the power station. Measuring across the two hot wires results in 240 volts. Measuring the voltage between a black (or red) and white wire, results in 120-volt AC.

Wiring configuration

Wiring configuration

Copper wire

Copper wire is used because it is a good conductor of electricity. Materials that do not conduct electricity as good usually have a higher resistance. This results in wasted energy and the tendency to get hot, which could be a safety hazard.

In the 1960s many electric contractors started to use aluminum wire instead of copper. Aluminum is almost as good of a conductor as copper, but it is much less expensive. After a number of years, it was found that this type of wiring caused a potential fire hazard. Problems due to expansion caused overheating at connections between the wire and switches, outlets, or splices. Many homes had to be re-wired, although there still are many that still have aluminum wiring but have never had problems.

Wall outlets

The wall outlets usually have a one wide slot, one narrow slot and one round-with-flat-bottom hole. This is to assure that each part of the plug will be used as it is supposed to and to increase safety. Older outlets have both slots the same size and no round hole.

Typical wall outlet

Typical wall outlet

Outlet slots

The narrow slot is considered "hot" and is where the alternating current power comes out. The wiring behind the outlet to this slot is usually black in the U.S. The wide slot is considered the "common" and is supposed to be grounded. Using the white wire as a common grounded wire, means that everyone is working from the same zero voltage position.

Round hole

The hole that is round on the top and flat on the bottom is an extra ground. Usually the wire behind the wall outlet has green insulation. Sometimes it is a bare wire. This extra ground is to make sure your utensils are properly grounded in the situation that someone had improperly wired the house. It is an extra safety measure.

Common wire

Although the white wire is not supposed to be a "hot" wire, in some cases it is used that way, especially in older homes that have the old style outlets. In general, this is acceptable, but it can result in problems. If you touch a common wire that is properly grounded, you should not get a shock. But if that wiring has made it hot, you can get a shock. Also, by using the white wire where the black should be used, you may cause a short circuit.


Proper grounding and the use of fuses are important for protection against shock, as well as to prevent electric overheating and fire hazards


Correct grounding is very important. Often ground wires are connected to water pipes that normally go into the ground. Connecting to a hot water pipe means that the water heater is between the connection and the ground. The water heater may have plastic parts that would insulate the connection to ground. Thus, using a hot water pipe is not a good idea.

Another consideration in using water pipes to ground the circuit is that plastic piping is often being used in plumbing. You must make sure there are no plastic pipes between your connection and the outside earth or ground.


Fuses and circuit breakers are used as a safety measure in case of short circuits. A fuse or circuit breaker will break the connection if more current is passing through the wire than is considered safe. This will prevent the house wiring to overheat and start a fire.

Most homes now use circuit breakers instead of fuses. One reason is because people with bad wiring in their homes that constantly blow out fuses, would then force pennies in the fuse receptacles, thus bypassing the requirement for a fuse. This removed the aggravation, as well as the expense of buying new fuses, but it also often resulted in serious electric fires in the house.


Most homes use both 240- and 120-volt AC electricity. Wires have black, red, white or green insulation, depending on their use. The holes in modern outlets assure plugs can only be inserted in one way. Proper grounding and the use of fuses are important to maintain electric safety in the home.

Be curious about things you may take for granted

Resources and references

Ron Kurtus' Credentials

Web sites

DC and AC Electricity Resources

Physics Resources


(Notice: The School for Champions may earn commissions from book purchases)

Top-rated books on AC Home Wiring

Basic Electricity by Bureau of Naval Personnel (1970) - Provides thorough coverage of the basic theory of electricity and its applications

Schaum's Outline of Basic Electricity - Popular outline

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