Your home's electrical system begins with main service wires that enter your home from an overhead utility line or underground feeder wires and connect to the main service panel, usually located in a utility space. Up to this point, the system hardware belongs to a power utility company. But from the main service panel, the current is divided into individual branch circuits, each of which is controlled by a separate circuit breaker.
Circuit Breakers Start Each Branch Circuit
The main service panel is controlled by the main circuit breaker that serves as the primary disconnect for the power supply to the main service panel. This is normally a 100- to 200-amp two-pole circuit breaker providing current at 240-volts and feeds it to two 120-volt hot bus bars running down vertically through the panel.
Below the main circuit breaker, there are two rows of smaller circuit breakers, and it is these that form the beginning of the individual branch circuits that run to all areas of your home to provide power. These individual breakers will be either 120-volt breakers, tapping into only one of the hot bus bars in the panel; or they will be 240-volt breakers that connect to two of the 120-volt bus bars. Thus, your branch circuits will be either 120-volt circuits—which feed all the standard outlets and lighting circuits; or they will be 240-volt circuits—which feed circuits that feed major appliances, such as an electric clothes dryer, an electric range, and central air conditioning units.
Branch Circuit Amperage
Both 120-volt and 240-volt branch circuits can vary in the amount of power they deliver—a quantity measured by amperage. Branch circuits for 120-volt circuits are usually 15-amp or 20-amp circuits, although occasionally they will be larger than that. For 249-volt circuits, the amperage is more often 30-, 40-, or 50-amps. The amperage of each branch circuit can be read by the printing on the lever of each circuit breaker. The wires attached to that circuit must also be sufficient to handle the load of the branch circuit; attaching wires that are too small for the circuit amperage poses a definite danger of fire. The ratings of individual wire gauges are as follows:
- 15-amps: 14-gauge copper wire
- 20-amps: 12-gauge copper wire
- 30-amps: 10-gauge copper wire
- 45-amps: 8-gauge copper wire
- 60-amps: 6-gauge copper wire
- 80-amps: 4-gauge copper wire
- 100-amps: 2-gauge copper wire
Normally this is not an issue, as the original circuits in your home are likely wired correctly. However, anytime a circuit is being extended, it's critical that the new wiring is the appropriate gauge for the circuit amperage. It is a common DIY mistake to wire with incorrect gauge size.
There are several different types of branch circuits in your home.
- Dedicated appliance circuits. These serve only one appliance and are often required by Code. They can be 120- or 240-volt circuits, and serve appliances such as electric ranges, dishwashers, refrigerators, garbage disposers, air-conditioners, and clothes dryers. Normally, any appliance that has a motor will require a dedicated circuit.
- Lighting circuits. These are what they sound like—circuits that serve general lighting needs in rooms. Normally, a lighting circuit will serve several rooms, and most homes will have several. One advantage of separating the lighting circuits from the outlet circuits is that each room will be left with some means of lighting them if one circuit is shut off. While working on the lighting circuit, for example, a plug-in lamp can be used to illuminate the space.
- Outlet circuits. These are circuits that serve only general-purpose plug-in outlets. They can be specific to a room or a group of rooms. A second-story in a small house, for example, may have one or two outlet circuits that serve multiple rooms.
- Room circuits. Depending on how the home has been wired, sometimes the circuit layout has all lights and outlets in a room served by individual circuits.