Inside Your Main Electrical Service Panel
When you open the panel door, you gain access to the circuit breaker switches, but that's all. And that's as far as most homeowners need to go. However, to get inside the panel to install or replace a circuit breaker, you have to remove the protective cover around the breaker switches, known as the dead front cover. The dead front cover is typically held in place with a screw in each corner. Removing the cover provides access to all components of the panel. Some panels have a separate door and cover; others have a door and cover as parts of the same unit.
Main Circuit Breaker
The main circuit breaker is a large breaker usually located at the top of the panel but sometimes near the bottom or along one side. It controls all the power of the branch circuit breakers (the breakers controlling individual circuits) in the panel.
Power comes from the utility service lines, flows through the electrical meter on the outside of your house, and continues into the service panel. However, some systems include a separate disconnect switch between the meter and the panel. The main breaker is used to turn power to all the branch circuits ON or OFF at the same time.
Hot Bus Bars
The two thick, black service wires feeding the main circuit breaker each carry 120 volts from the electric meter and feed the two "hot" bus bars in the panel. Circuit breakers snap into place onto one or both of the bus bars to provide power to the circuits. Single-pole circuit breakers provide 120 volts and connect to just one hot bus bar. Double-pole circuit breaker provide 240 volts to a circuit and snap into both hot bus bars. The electrical current leaves the service panel through the hot wires that are connected to the circuit breakers. Single-pole breakers have one hot wire (usually black), while double-pole breakers have two hot wires, which may be black, red, white, or another color.
Neutral Bus Bar
Once the power leaves the electrical service panel through the hot wire(s) of a circuit and does its work through the electrical devices (light bulbs, outlets, etc.), the electrical current returns back to the service panel through the neutral (usually white) circuit wire, which is connected to the neutral bus bar. The bar connects to the main service neutral and returns the current back to the electric utility grid.
Ground Bus Bar
Some service panels have a separate bus bar for ground-wire connections, instead of a neutral/ground bus. In this case, the ground bus is electrically connected to the neutral bus in main service panels only; in subpanels, the ground bus and neutral bus are not connected to each other.
The circuit breaker is the weak link in each electrical circuit. But that's a good thing, as it's designed to fail safely. When a circuit draws more current than it is designed to handle, the wiring gets hot and becomes a fire hazard. Excessive current in a circuit is prevented by the use of overcurrent safety devices, such as circuit breakers (or, in older systems, fuses). Circuit breakers connect to the hot bus bars and come in a variety of types and capacities.