The first step in understanding your home's circuits is to know how basic electricity works. Electricity moves through any metal conductor, such as a wire or the metal contacts and other components inside a switch or receptacle. The electrical current must move in a loop, or circuit. If the circuit is broken at any point, the flow of power stops.
Power comes to your home through the power company's transmission and distribution lines, passes through a meter, and enters the service panel.
In the service panel, the power energizes two strips of metal called hot bus bars. Circuit breakers or fuses attach to the hot bus bars. Power must pass through a breaker or fuse before it leaves the service panel and goes into the house through a branch circuit.
Each branch circuit supplies power to a number of outlets. An outlet is any place where power leaves the wires to provide service. Devices (receptacles and switches), ceiling lights and fans, and appliances (such as a water heater or a dishwasher) are outlets.
Power leaves the service panel via a hot (energized) wire—one with insulation that is black, red, or a color other than green or white—and returns to the panel through a neutral wire—one with white insulation. Another wire, bare or with green insulation, provides the ground.
The neutral and ground wires connect to the separate neutral bus bar in the service panel. That bus bar is connected to the neutral line from the power line.
The thicker the wire, the more electrical current it can safely carry. If too much current passes through a wire, the wire overheats, the insulation can fail, and a fire or shock could result. The service panel (often called the fuse box or breaker box) provides protection against this possibility.
A breaker or fuse is a safety device. When it senses that its circuit is drawing too much power, it automatically shuts off. A breaker "trips" and can be reset; a fuse "blows" and must be replaced. Both devices keep wires from overheating.
Volts are the amount of force exerted by the power source. Household wiring carries 120 volts. (Actual voltage varies constantly but stays within an acceptable range, from 115 to 125 volts.)
Most outlets supply 120 volts, which is provided by one hot wire bringing the power to the outlet and one neutral wire carrying it back to the service panel. Some heavy-duty appliances, such as large air-conditioners, electric ranges, and electric water heaters, use 240 volts, supplied by two 120-volt hot wires with one neutral wire.
Though the force pushing current through all wires is the same—120 volts—fixtures and appliances use different amounts of power. Amperes (amps) measure the amount of electrical flow. Wattage (watts), the amount of power an electrical device consumes, is calculated by multiplying the voltage times the amperage.
Circuits are designed to carry a specific maximum electrical flow, usually 15 or 20 amps for 120-volt household circuits. Wires for a 20-amp circuit are thicker than those for a 15-amp.
When a circuit breaker trips or a fuse blows, it is a sign that the circuit is overloaded. The only solution is to remove some electrical load from the circuit. If you have a toaster, waffle iron, electric skillet, and other appliances running on the same circuit, plug some into another circuit.