A home’s electrical panel — what electricians call a service panel but what most people know as their breaker box”— serves as the switchboard for the entire house's electrical system. It receives power from the utility company and distributes it to the individual circuits that supply all of the fixtures, outlets and other devices in the home. Learning the basics of electrical panel wiring is a good first step to understanding your home’s electrical system and is essential to performing safe repairs and upgrades.
Types of Panels
It’s important to note that not all electrical panel wiring uses breakers, although that’s been the standard for over 50 years. The precursor to the breaker-type panel is the fuse panel, which uses disposable fuses (instead of breakers) to protect the circuits from shorts and overloads. To shut off the power to an individual circuit, you have to remove the fuse.
While most homes have a single service panel, or "main" panel, it’s not uncommon to have a secondary panel, called a subpanel, that is supplied by the main panel. A subpanel essentially is a small version of a main panel and may be installed when a main panelhas no room left for adding circuits or to provide power (and easy panel access) to an addition or new garage.
Behind your service panel door are the columns of circuit breaker switches, with the ON/OFF levers that you click over to the ON side whenever a breaker trips. The inner panel surrounding the switches conceals the guts of the electrical panel wiring. Here’s what’sback there:
Three heavy-gauge service cables enter the panel from the electric meter; one of these is the neutral line, and it connects to the neutral bus bar — a metal strip with numerous screw terminals — inside the service panel. The two other lines are the hot lines. Theseconnect to the two hot bus bars in the panel via a large "main" circuit breaker. Each hot line carries 120 volts of power.
A panel must also have a heavy grounding wire (usually bare copper) that connects to the neutral bus bar on one end and to a grounding rod driven into the ground on the other end. In houses with metal water pipes, there may also be a ground wire between the panel and a nearby cold-water pipe.
All of the remaining cables going into the panel are the individual circuit cables from the house (and subpanel, if you have one). The hot wire (usually black or red) from each cable connects to a circuit breaker, while the neutral wire (usually white) connects to the neutral bus bar.
Circuit breakers are the primary safety devices in your electrical system and they are integral to electrical panel wiring. A breaker automatically "trips" and shuts off power to a circuit if it senses an overload, which can be caused by plugging in too many appliances on one circuit, or by a short. A short can result from things like loose wires, damaged insulation or a curious kidsticking a coat hanger into an outlet (ouch).
Breakers are rated for the power draw of each circuit. Standard breakers carry 120 volts and either 15 or 20 amps (amperes). High-voltage breakers carry 240 volts and 30 or more amps. The amp rating is the number stamped on the end of each breaker switch lever. All panels should have an index label on the panel door listing the main devices on each circuit (e.g. "kitchen plugs," "dryer," "living room lights").
Now that you know a little about electrical panel wiring, the most important rule to remember is: Always shut off the power to the area where you’ll be working. Before inspecting or removing a switch or other household device, for example, switch off the breaker serving the device’s circuit. And before doing anything with the panel — including removing the inner cover — shut off the main breaker. This cuts the power to all of the household circuits at once. However, the service cables and the points where theyconnect to the panel are always powered, so don’t go near them!
Philip Schmidt is a home improvement author and editor based in Colorado. He enjoyshoney-do lists and boring his family with random facts about houses.