You don't need to be an electrician to understand the basics about how your home is wired. A general understanding of how cables and wires work is part of being a responsible homeowner, and our informational guide will walk you through everything you need to know. You'll learn what each wire color means, how to differentiate types of conduit, and much more.
Cable and Wire Basics
Most house wires—the wires that run from the service panel through walls and to electrical boxes—are solid-core, meaning they are made of a single, solid strand. Light fixtures and some switches have leads—wires made of many strands of thin wire, which are more flexible. The thicker a wire, the lower its number; for instance 12-gauge wire is thicker than 14-gauge.
Cable refers to two or more wires encased in a protective sheathing. Cable packaging indicates the gauge and number of wires. For example "12/2 WG" means two (black and white) 12-gauge wires plus a ground wire.
Nonmetallic (NM) cable, sometimes called Romex, has two or three insulated wires plus a bare ground wire all wrapped together in plastic sheathing. Many local codes permit NM cable inside walls or ceilings, and some codes allow it to be exposed in basements and garages. Underground feed (UF) cable has wires wrapped in solid plastic for watertight protection. Use it for outdoor projects.
Armored cable encases insulated wires in metal sheathing for added protection. BX (also called AC) has no ground wire, only a thin aluminum bonding wire unsuitable as a ground; the metal sheathing provides the path for grounding. Metal-clad (MC) has a green-insulated ground wire. Some local codes require armored cable or conduit wherever wiring is exposed.
Conduit—pipe that wires run through—offers the best protection against damage to wires. It also makes it easy to change or install new wires: Pull the wires through the conduit rather than cut into walls to run new cable.
Metal conduit once was used as a path for grounding; recent codes require a green-insulated ground wire. PVC (plastic) conduit is cheaper than metal but not quite as strong. Metal Greenfield and plastic EMT tubing are flexible types of conduit. They are useful when working in tight spots.
Wire Colors and Sizes
The thicker a conductor, the more amperage (amps) it can carry without overheating. A 14-gauge wire can carry up to 15 amps; a 12-gauge wire, up to 20 amps; and a 10-gauge wire, up to 30 amps. Never overload a wire—for instance, never wire a 20-amp circuit with 14-gauge wire.
Wires coated with insulation that is black, red, or another color are hot wires, carrying power from the service panel to the electrical device. White wires are neutral, meaning they carry power back to the service panel. Green or bare wires are ground wires. Beware: If wiring has been done incorrectly, the color of the wires in your house might not indicate which are hot.