How a Home's Electrical System Works
The typical American home has a primary circuit coming in from the electrical supplier with two 120 volt "hot" wires and one neutral wire. These wires connect to the main electrical panel and then possibly from there to one or more sub panels. The main panel or sub panel contain circuit breakers (or fuses for older installations). One breaker is used for each of the home's "branch" circuits. Occasionally a breaker may be "double tapped" to control more than one branch circuit, although this may be dangerous and likely violates your local electrical codes. Most branch circuits have one 120 volt hot wire, one neutral wire and one ground wire. A few specialized branch circuits have two hot wires instead of one. Each of the two hot wires carry 120 volts which results in 240 volts being supplied on the circuit. Such circuits are commonly used for air conditioners, electric ranges and dryers.
You may have noticed that electrical devices have ratings listed such as 120v, 115v or 110v. These are all designed to run on a standard circuit in your home. Your electrical supplier delivers 120 volt current to your home. However as electricity travels through the wire, it dissipates some of its energy and as a result the voltage drops. By the time it gets to the farthest outlet in your home, the voltage may have dropped from 120v down to 108v or so. Voltage drops over a distance are normal and to be expected.
Electrical current is commonly measured in watts, amps and volts. The voltage is analogous to water pressure; it measures the "pressure" of the electricity being "pushed" through the wire. Wattage is the quantity of electricity being pushed through the wire. Amperage is the result of wattage divided by voltage. So if you see a device rated for "15 amps", that means when supplied with 120 volt current, it is designed to handle up to 1800 watts
(15 amps * 120 volts = 1800 watts).
Circuit breakers and fuses are important safety devices. They work effectively as a switch and are designed to stop the flow of electricity. While a circuit breaker is very much like a switch, fuses have a one time usage and must be replaced each time they protect a circuit. When too much load is placed on a circuit, the breaker "trips" and interrupts the flow of electricity. An excessive load may result from too many devices simultaneously in use on a circuit, a faulty device or a short circuit, amongst other possibilities.
Without a circuit breaker, an overloaded device could be damaged , a fire could result from the heat generated by the device or wiring or from electrical sparking. Circuit breakers however are not very effective at protecting people from electrocution. The problem is that they do not trip quickly enough to protect you from harm. That is why most electrical codes now call for GFCI outlets in wet locations such as kitchens and bathrooms. GFCI outlets trip very quickly reducing your exposure to dangerous electrical current.