How Circuits Work

Electricity is the flow of electrons through a conductor. In home electrical systems, wires, consisting of highly conductive copper wrapped in insulation for safety, are the conductors—the assigned pathway through which the electricity travels. A host of other items—a fork, a screwdriver, you—also can serve as conductors, sometimes with disastrous results. It’s the goal of a safe electrical system to prevent this from happening.

Electricity always flows in a loop, known as a circuit. When a circuit is interrupted at any point, the electricity shuts off. As soon as the circuit is reconnected, the flow begins again. Electricity is generated by your local electric company. Overhead and underground wires bring power from the utility company lines to a home’s service head, also called a weatherhead because it can withstand wind, heat, and ice. Although the utility company sends high-voltage electricity along some of its power lines, by the time it reaches your house, it is 120 volts per wire.

The electricity passes through an electric meter, which measures how much enters the house. It proceeds to a service panel (also called the breaker box or fuse box), which distributes electricity throughout the house along individual circuits. Each circuit flows out of the service panel, through a number of fixtures and receptacles, and back to the service panel. To make a circuit, electricity is carried out of the service panel on “hot” wires that usually have black insulation, although they sometimes may be red, and is returned to the panel on neutral wires that have white insulation.

The service panel contains circuit breakers or fuses—both are safety devices that shut off the power in case of a short circuit or other fault in the circuit. Each circuit has a number of outlets that lead to smaller circuits through which the electricity flows. These outlets might include receptacles, fixtures, and switches. For example, a wall switch interrupts (off) or completes (on) the circuit to one or more light fixtures. A heavy-use appliance— such as a dishwasher, garbage disposal, or microwave oven— may need a circuit to itself.

Most circuits carry current of 120 volts which will give most people quite a jolt but will not seriously harm them if they should accidentally come into contact with it. Most homes also have one or two 240-volt lines, which double the power by using two hot wires with one neutral wire. Because of the higher power, take special care when dealing with these. Every electrical system must be grounded for safety. Grounding allows excess current to travel harmlessly into the earth in case of overload or a short circuit. Usually this is done by connecting a wire to a cold water pipe, to a grounding rod sunk deep into the ground, or sometimes to both. Circuits in your home may be grounded with a grounding wire that is bare copper or green, or they may be grounded by means of the metal receptacle boxes and the metal sheathing that contains the wires.

The flow of electricity in typical household circuits starts with the two 120-volt lines and single neutral line that enter the top of the service panel. Both 120-volt lines are used to make the 240-volt circuit, though only one neutral line is needed for the electricity to complete its loop. The 120-volt circuit has one hot wire (black) and one neutral wire (white), plus a copper ground wire (green). In case of a short, the ground wire carries the current safely into the ground.

Service Panels

Electrical projects always begin at the service panel, which is either a breaker box or a fuse box. When a short or an overload shuts down power to a circuit, this is where you go to restore the flow. It’s also where you cut off power to a circuit before starting a project. Power arrives from the meter through two main power wires, each of which carry 120 volts of electricity into the house. Usually these are black and/or red. In addition there is a white main neutral wire that carries electricity back to the utility. The main hot wires are connected to a main power shutoff. When you turn this off, you don’t de-energize the hot wires, but you cut power to everything else in the box.

Breaker boxes - Emerging from a breaker box’s main shutoff are two hot bus bars. The 120-volt breakers are each attached to one of these bars. (This means that if one of the main hot wires gets damaged outside your house, you will lose power to about half of the circuits in your house.) Each 240-volt breaker gets twice the power by being attached to both bus bars. When a circuit is overloaded or a short occurs, its breaker trips and shuts off power before the wires heat up and become a danger.

The main neutral wire is connected to the neutral bus bar. This bar is connected to a system ground wire, which leads to a grounding rod. White wires for every circuit, and possibly bare or green ground wires, also connect to the neutral bus bar. As a result, each 120-volt circuit has a black or colored wire leading from a circuit breaker, a white wire leading to the neutral bus bar, and possibly a bare copper or green-covered ground wire also connected to the neutral bar. Each 240-volt circuit has two wires leading to a circuit breaker. In addition, the 240-volt circuit has a neutral and, possibly, a ground wire connected to the neutral bus bar. Systems with conduit or armored cable do not need separate ground wires—the conduit or metal sheathing act as ground conductors.

Caution! Leave Incoming Wires for the Utility Company - If you suspect that the wires entering your house may be damaged in any way, do not attempt to work on them yourself. Have the utility company inspect them. Usually it will inspect and repair them for free.

Fuse boxes - If you have an older home that has not been rewired in the last 25 or 30 years, chances are that its electrical heart is a fuse box rather than a breaker box. Fuse boxes are wired and work the same way as breaker boxes, but instead of tripping as a breaker does, a fuse “blows” when there’s too much current in its circuit. When this happens, you must eliminate the short or the overload, remove the blown fuse, and screw in or plug in a new one.

Power comes into a fuse box through two main power wires. (In a house with no 240-volt equipment, there may be only one of these.) Current flows through a main disconnect, in this case a pullout block that holds a pair of cartridge fuses. Next in line are a series of plug fuses that protect the black hot wires of the individual circuits, often called branch circuits. Unscrewing a fuse disconnects its circuit. A neutral bus bar receives the main neutral wire and a neutral wire for each of the branch circuits. A system ground wire leads from the neutral bus bar to a grounding rod outside the house.

Caution! Never "Upgrade" a Fuse - If you have a chronically overloaded circuit, you might be tempted to install a bigger fuse—replace a 15-amp fuse with a 20-amp fuse, for example. Don't do it. Wiring that gets more current than it was designed to handle heats up and can catch fire. Always replace a blown fuse with one of the same amperage rating.

Typical plug fuse - A plug fuse is threaded and screws into the fuse box. Handle only the rim. Do not touch the threads while removing or replacing the fuse. For information on identifying and replacing a blown fuse.

Handling pullout blocks - Larger 240-volt circuits, as well as main shutoff fuses, often are protected by pullout blocks that contain cartridge fuses. If you need to pull out a cartridge fuse that is not in a pullout block, do not use your fingers. Get a fuse puller.

Grounding and Polarization

Older homes often have receptacles and fixtures that are ungrounded, and many local codes do not require that they be rewired so they’re grounded. Still, grounding is worth adding to your system because it adds protection against electrical shock. Grounding provides a third path for electricity to travel along, so if there is a leak of any sort, it will flow into the earth rather than into the body of a person who touches a defective fixture, appliance, or tool.

An electrical system is grounded with a grounding rod driven at least 8 feet into the ground outside the house or by connecting to a cold water pipe. Each individual branch circuit must be grounded as well, either with a separate wire that leads to the neutral bar of the service panel or with metal sheathing that runs without a break from each outlet to the panel. (In theory, electrical outlets can be grounded individually, but this is impractical.)

Some locations in your house— especially where the outlet and/or appliances may become wet— require ground-fault circuit-interrupter (GFCI) receptacles. Older, ungrounded circuits usually are protected by polarization, which is less effective than grounding but better than nothing. Grounded and polarized receptacles work only if they are wired correctly.

A polarized receptacle - A polarized outlet has one slot that is longer than the other. This is to ensure that the plug is inserted so that its hot current flows through black or red wires, and neutral current flows through white wires. Although not as safe as a grounded system, polarization is the next best thing.

A grounded receptacle - The grounding circuit must follow an unbroken path to the earth. A third, rounded prong on a modern plug fits into the round slot in the receptacle. This slot connects to a wire—-or to metal conduit or sheathing—that leads without interruption to the neutral bus bar of the breaker box. The system ground wire then leads from the bus bar to the earth. Instead of a grounding rod, a cold water pipe often is used for grounding because it is connected to water supply pipes that go deep under the ground.

Caution! Don't Alter Prongs on Plugs Never - clip or file down the prongs on a grounded or polarized plug. Go to the heart of the problem: Test and upgrade your circuit and receptacle.

Limitations of Older Systems

An older home may have r\electrical service that is inadequate or even unsafe. It can be confusing, as well. If you are unsure about your home’s wiring, have a professional check it out. Some older systems have only two wires—one hot (rather than two) and a neutral—entering the house. This means you will not be able to have any 240-volt circuits for large appliances.

Modern electrical service provides at least 100 amps of power, which is enough to power a medium-size house with an average number of appliances. A house built in the 1950s or before may only have 30-amp service (the circuit box will have only two fuses) or 60-amp service (four fuses). With so few circuits, the number of fixtures and appliances you can run will be limited.

Limited service - This 60-amp service panel has four 15-amp fuses and a switch lever for shutting off all power. If you have this or a 30-amp (two-fuse) box, it’s a good idea to have a professional upgrade you to a 100-amp service panel with breakers.

Sources of shorts and faults - Loose taped wires, old wire damaged because it’s exposed, and multiple wires slipping off a single terminal screw may seem like minor problems, but are not. Drastic consequences, such as fire and electrocution, are the reasons codes are strict about good wiring practices.

Ungrounded, unpolarized circuits - If your outlets have two slots that are the same size, then they are neither polarized nor grounded. This leaves you with no protection against shocks from defective fixtures or appliances using that outlet. At the very least you need to install polarized outlets.

Knob-and-tube wiring - This type of wiring was common in houses built before World War II. The individual wires are wrapped in a rubberized cloth and have no additional protection. There is no ground wire. These wires should be replaced with modern cable, particularly in areas where they are exposed, such as attics and basements.

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