The flow of electricity through a wire is often compared to the flow of water through a pipe. Although the analogy is hardly precise, it's useful in describing a force that many people find difficult to understand.
Electricity is basically the flow of subatomic particles called electrons through a conductor, a metal such as copper wire whose resistance to electrical flow - measured in units called ohms - is low. Materials such as rubber or plastic that offer high resistance to electrical flow are called insulators.The rate of electrical flow, or current, is measured in amperes (amps). The electrical pressure that causes current to flow through a conductor is measured in volts. Light fixtures and small appliances operate on 120 volts; electric ranges and other heavy-duty appliances require 240 volts.
The amount of power delivered by a current under pressure to a lamp or appliance is measured in watts. To determine watts, multiply amps times volts. A thousand watts equals a kilowatt. A kilowatt used for an hour is a kilowatt-hour, which is the unit your utility company uses to measure and bill for electricity.
To do its job, electricity must flow in a closed loop, or circuit, from a power source to a power user, such as a light bulb, and back to the source. If the circuit is interrupted, say by an open switch, the flow stops.
The electricity generated by a power plant and delivered to your home by your local utility is called alternating current. It flows alternately, first in one direction, then in the opposite, completing 60 cycles every second. Batteries supply direct current, which flows in one direction only.
At the business end of an electrical system are the wall outlets, technically called receptacles and switches that make things happen. Inside the hollow walls and ceilings of your house, a maze of wires branches out from the main service panel, located near where the power line enters your house. The main panel is typically on a wall in your basement, utility room or attached garage.
The main electrical power lines, the service, usually come in overhead from a nearby utility pole. The electrical wires on the pole connect to an electrical grid that connects the entire neighborhood to powerful electrical generators.
The electrical wires at the very top of the pole may carry about 10,000 volts, much more than household wiring can stand. So transformers, the large cylinders you see hanging on utility poles, reduce the voltage to 120 volts, our household standard.
Electrical service is routed from the utility pole to your home and then to the service entrance head located high on one side of the house. Here the utility's wires are spliced to the service entrance wires. These are encased in metal or plastic tubing called conduit.
From the service head, the wires pass through a meter into the main service pan-el where current is split up and distributed through branch circuits throughout the house.
Branch circuits are designed to carry only a certain amperage - typically 15 or 20 amps. If a circuit draws more amperage than it can handle, the wires can overheat and cause a fire. To prevent this, each circuit is protected at the service panel by an overload protection device.