People depend on electricity constantly, and when the power goes out in a storm or there's a tripped breaker or another problem in an electrical circuit, understanding the basic components of an electrical system can help you get things running again. It's also important to know who is responsible for what portion of your electrical service. The utility company handles the line portion of your service, which includes everything up to the attachment point on your house. From there, it's called the load side, and everything on the load side is your responsibility.
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Electrical Service Connection and Meter
Your home's electricity starts with the power service and electric meter. The utility company's service cables (whether overhead or underground) extend to your house and connect to the utility's electric meter. The meter measures the amount of electricity your home uses and is the basis for the charges on your electric bill. The meter runs only when electricity is used in the house.
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Many home electrical systems include a dedicated disconnect switch that is mounted on an outside wall of the home near the electric meter. In the event of a fire or flash flood, or if work needs to be done on the system, a disconnect switch allows you to shut off the power from outside the home so you don't have to enter the home to turn off the power. If an electrical system does not include a separate disconnect switch, the main circuit breaker in the home's main service panel (breaker box) serves as the system disconnect.
03 of 09
Main Service Panel
After passing through the meter, your electrical service feeds into your home's main service panel, commonly known as the breaker box. Two large "hot" wires connect to big screw terminals, called lugs, inside the service panel, providing all the power to the panel. A third service cable, the neutral, connects to the neutral bus bar inside the panel. In simple terms, electricity is supplied to the house on the hot wires. After it flows through the household system, it is fed back to the utility on the neutral wire, completing the electrical circuit.
04 of 09
Main Circuit Breaker
The service panel contains a large main breaker that is the switch controlling the power to the rest of the circuit breakers inside the panel. It is sized according to your home's service capacity. A standard panel today provides 200-amp (ampere) service. Older panels were sized for 150, 100, or fewer amps (amperes).
A main breaker of 200 amps will allow a maximum of 200 amps to flow through it without tripping. In a tripped state, no current will flow to the panel. In systems without an external disconnect switch, the main breaker serves as the household disconnect.
Turning off the main breaker stops the flow of power to all of the branch circuit breakers in the panel, and therefore to all of the circuits in the house. However, power is always flowing into the panel and to the service lugs even when the main breaker is shut off unless the power is shut off at a separate disconnect switch. Power is always present in the utility service lines and the electric meter unless it is shut off by the utility.Continue to 5 of 9 below.
05 of 09
Branch Circuit Breakers
The breakers for the branch circuits fill the panel (usually below) the main breaker. Each of these breakers is a switch that controls the flow of electricity to a branch circuit in the house. Turning off a breaker shuts off the power to all of the devices and appliances on that circuit. If a circuit has a problem, such as an overload or a fault, the breaker automatically trips itself off.
The most common cause of a tripped breaker is a circuit overload. If you're running a high-demand appliance, such as a vacuum, toaster, or heater, and the power goes out, you've probably overloaded the circuit. Move the appliance to a different circuit and reset the breaker by switching it to the ON position. If the breaker trips again—without the appliance plugged in—you must call an electrician. There may be a dangerous fault situation in the circuit.
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Devices are all the things in the house that use electricity, including switches, receptacles (outlets), light fixtures, and appliances. Devices are connected to the individual branch circuits that start at the breakers in the main service panel.
A single circuit may contain multiple switches, receptacles, fixtures, and other devices, or it may serve only a single appliance or receptacle. The latter is called a dedicated circuit. These are used for critical-use appliances, such as refrigerators, furnaces, and water heaters. Other appliances, such as dishwashers and microwaves, usually are on dedicated circuits, too, so that they can be shut off at the service panel without interrupting service to other devices. This also reduces the incidence of overloaded circuits.
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Switches are the devices that turn on and off lights and fans in your home. They come in many different styles and colors to suit your design needs. There are single-pole, three-way, four-way, and dimmer switches. When you flip a switch off, it "opens" the circuit, meaning the circuit is broken or not complete and the power is interrupted. When the switch is on, the circuit is "closed," and power flows beyond the switch to the light or another device it is controlling.
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Electrical outlets, technically called receptacles, provide power to plug-in devices and appliances. Televisions, lights, computers, freezers, vacuums, and toasters are all good examples of devices that can be plugged into an outlet. Standard outlets in a home are either 15-amp or 20-amp; 20-amp outlets can provide more electricity without tripping a breaker. Special outlets for high-demand appliances, such as electric ranges and clothes dryers, may provide 30 to 50 or more amps of power.
In potentially wet areas of a home, such as bathrooms, kitchens, and laundry rooms, some or all of the outlets must have GFCI (ground-fault circuit-interrupter) protection, provided by GFCI outlets or a GFCI breaker.Continue to 9 of 9 below.
09 of 09
Your home’s wiring consists of a few different types of wiring, including non-metallic cable (commonly called Romex), Bx cable, and wiring concealed in conduit. NM cable is the most common type of circuit wiring. It is suitable for use in dry, protected areas (inside stud walls, on the sides of joists, etc.) that are not subject to mechanical damage or excessive heat.
Bx cable, also known as armored cable, consists of wires running inside a flexible aluminum or steel sheath that is somewhat resistant to damage. It is commonly used where wiring for appliances, such as dishwashers and garbage disposers, is exposed.
Conduit is a rigid metal or plastic tubing that protects individual insulated wires. It is used in garages, sheds, and outdoor applications where the wiring must be protected from exposure.
Wires running inside NM cable, Bx cable, or conduit are sized according to each circuit's amperage. Wire size is given in its gauge number. The lower the gauge, the larger the wire, and the more current it can handle. For example, wiring for 20-amp circuits is 12-gauge, which is heavier than the 14-gauge wiring used for 15-amp circuits.