Electrical projects always begin at the service panel, which is either a breaker box or a fuse box. When a short or an overload shuts down power to a circuit, this is where you go to restore the flow. It's also where you cut off power to a circuit before starting a project.

Breaker box diagram

Power arrives from the meter through two main power wires, each of which carry 120 volts of electricity into the house. Usually, these are black and/or red. In addition, there is a white main neutral wire, which carries electricity back to the utility. The main hot wires are connected to a main power shutoff. When you turn this off, you don't de-energize the hot wires, but you cut power to everything else in the box.

Emerging from a breaker box's main shutoff are two hot bus bars. The 120-volt breakers are each attached to one of these bars. (This means that if one of the main hot wires gets damaged outside your house, you will lose power to about half of the circuits in your house.) Each 240-volt breaker is attached to both bus bars, giving them twice the power. When a circuit is overloaded or a short occurs, the breaker trips and shuts off power before the wires heat up and become a danger.

The main neutral wire is connected to the neutral bus bar. This bar is connected to a system ground wire, which leads to a grounding rod. White wires for every circuit, and possibly bare or green ground wires, also connect to the neutral bus bar. As a result, each 120-volt circuit has a black or colored wire leading from a circuit breaker, a white wire leading to the neutral bus bar, and possibly a bare copper or green-covered ground wire also connected to the neutral bar. Each 240-volt circuit has two wires leading to a circuit breaker. In addition, the 240-volt circuit has a neutral and, possibly, a ground wire, connected to the neutral bus bar. Systems with conduit or armored cable do not need separate ground wires--the conduit or metal sheathing act as ground conductors.

For how to troubleshoot the several types of circuit breakers and how to check for the cause of shorts, see Troubleshooting Circuit Breakers, Related Projects.

Fuse box diagram

If you have an older home that has not been rewired in the last 25 or 30 years, chances are that its electrical heart is a fuse box rather than a breaker box.

Fuse boxes are wired and work the same way as breaker boxes, but instead of tripping as a breaker does, a fuse "blows" when there's too much current in its circuit. When this happens, you must eliminate the short or the overload, remove the blown fuse, and screw in or plug in a new one.

As with a breaker box, power comes in through two main power wires. (In a house with no 240-volt equipment, there may be only one of these.) Current flows through a main disconnect, in this case, a pullout block that holds a pair of cartridge fuses.

Next in line are a series of plug fuses that protect the black hot wires of the individual circuits, often called branch circuits. Unscrewing a fuse disconnects its circuit. A neutral bus bar receives the main neutral wire as well as all the neutral wires for the branch circuits. A system ground wire leads from the neutral bus bar to a grounding rod outside the house.

For tips on this, see Troubleshooting Fuses, Related Projects.

A plug fuse

A plug fuse is threaded and screws into the fuse box. Handle only the rim. Do not touch the threads while removing or replacing the fuse. For information on identifying and replacing a blown fuse, see Troubleshooting Fuses, Related Projects.

Larger 240-volt circuits, as well as main shutoff fuses, often are protected by pullout blocks that contain cartridge fuses. If you need to pull out a cartridge fuse that is not in a pullout block, do not use your fingers. Instead, get a fuse puller.