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HOW TO


POWER DISTRIBUTION
How Electricity Is Provided
Utility companies generate electricity in a variety of ways. One of the most common methods uses the energy of running water to power a generator. Electrical power created in this way is termed hydroelectricity. To harness the energy of flowing water on a scale this enormous, a dam may be built across a narrow gorge in a river or at the head of a man-made lake. Water backed up behind this dam, in what is called the forebay, is then allowed to flow through a submerged passage, or penstock, in a controlled release. The massive force of this elevated water spins the generator's giant turbines as it falls, producing electricity. Electrical power produced in this way is called AC power, or alternating current.
Transmission
Once a utility company produces electricity, it must then transmit it through a distribution system for use by its customers. For ease of transmission, the electrical power is raised to many thousands of volts and conducted over high-voltage transmission lines to the utility company's regional switching stations, where it is then stepped down to a lower voltage for transmission to local substations. A typical transmission starts at 230,000 volts, is stepped down to 69,000 volts at a switching station, then is stepped down further at a substation to 13,800 volts for direct distribution to a local area. Once at your home, this is again reduced, to 240 volts. All homes are wired for 240 volts which can be stepped down to 120 volts.
Point of Use

To be stepped down, the electricity that arrives at your home must first pass through a utility transformer. It then leaves this transformer via three terminals, mounted on its side, which are connected to three wires. These wires constitute the service drop that leads to your house service entrance. They include two insulated hot wires, or legs, and a grounded neutral. The two hot wires can each provide 120 volts or supply 240 volts of power between the two hot legs. The neutral conductor is usually bare on overhead and insulated in underground service laterals.
A glass-domed meter is connected to the two hot wires leading from the utility transformer. This meter, generally mounted on the outside of your house, is provided by your utility company to measure the amount of electrical energy in kilowatt hours consumed by your household. This is the rate of energy consumption in kilowatts multiplied by usage in hours. Directly from the utility meter, the two hot wires and the grounded wire continue on to a service-entrance panel (SEP), which distributes power throughout your house. The service panel also contains circuit breakers or fuses that will open if a short circuit or overload occurs in the system.

Service-Entrance Panel
it is the service-entrance, or main, panel that controls the flow of power to individual circuits within your home. These circuits may be 120volt, 240-volt, or both (120/240-volt). All 240volt devices pull current from both of the hot insulated legs. At any given moment, electricity is exiting from one terminal on the utility trans-former and returning by the other. Current flows from one terminal, travels through the service drop to the house, and then dowel the service-entrance conduit or cable into the meter base. From here it flows through the meter into the main panel and is then distributed to each of the circuits within your home, flowing through the main panel via one (or two) insulated hot leg(s), or wire(s), and returning to the panel via another insulated wire-directly through the utility meter and back to the transformer. The final result is that you never actually "consume" electricity-you just borrow it (although you transform much of its energy, which is what you pay for).
All 120-volt devices draw from one of the two hot insulated wires going to the device and use
the grounded white wire as the return. The grounded conductor is connected to ground (via a ground electrode) at both the transformer and at the main service-entrance panel. All 120/240volt appliances draw from both of the hot insulated wires as well as using the grounded conductor as the return. An electric clothes dryer, for example, uses 240 volts to heat the element but also uses 120 volts for the timer, motor, and alarm circuits. Such circuits carry current on all three wires at the same time. A 120/240-volt appliance, like a clothes dryer, needs two insulated hot wires, one insulated grounded wire, and one grounding conductor. A 120-volt duplex outlet needs an insulated hot wire, an insulated grounded wire, and a bare or green grounding wire. A 240-volt-only appliance needs just two insulated hot wires and a grounding wire service-entrance panel (SEP) by means of two insulated hot conductors and a bare grounded neutral wire. Power that enters your home must first flow through the utility company's electric meter to be measured. The electricity then goes to your service panel, where it is distributed to the various electrical circuits in your home. Though power may be generated by other means, it is delivered through this same system.

At a hydroelectric plant, the massive kinetic force of elevated water that drops down through a dam penstock in a controlled release turns giant turbines that generate electricity. Electrical current travels over high-voltage power lines to a step-down utility transformer near your house. The current then leaves the transformer, enters your home as available voltage for use (120-volt and 240-volt), and then returns to the transformer. It is transmitted to your main service-entrance panel (SEP) by means of two insulated hot conductors and a bare grounded neutral wire. Power that enters your home must first flow through the utility company's electric meter to be measured. The electricity then goes to your service panel, where it is distributed to the various electrical circuits in your home. Though power may be generated by other means, it is delivered through this same system.


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