The most basic electrical circuit in house wiring is the 110-volt, 15-amp outlet. Most electrical codes allow a minimum of 14-gauge wire, but this should really only be used if the circuit only includes one outlet, and that outlet is close to the breaker box. 12-gauge wire is a better choice, since it allows less voltage drop for longer runs. Also, the typical circuit will consist of a 20-amp breaker and 12-gauge wire, to which two or more 15-amp outlets are connected.
Use 12/2 wire with ground for most circuits. This consists of a black hot wire, and white neutral wire, and a bare wire for the ground connection. The black wire connects to the circuit breaker and to the smaller of the two slots in the receptacle. The screw will be brass-colored. The white wire connects to the neutral bus in the junction box and to the larger slot of the outlet with a silver-colored screw. The bare wire connects to the ground bus and to the green ground screw in the outlet.
Go up one size for long wire runs, wires enclosed in conduit, or bundles of several wires. This means 12-gauge for a single 15-amp outlet, or 10-gauge if the circuit will include more than one outlet. Conduit and bundles of wires increase the temperature of the wires, and long runs of 100 feet or more increase voltage drop, which lowers the efficiency of any appliance used on that circuit.
Choose the correct wire type. In most cases this will be NM plastic-sheathed wire, which is generically known as ROMEX®. If the wire is to run underground or through a potentially wet location, UF is the preferred type of wire. Wires which are run outdoors will need to either be UF run inside a conduit, or BX or AC cable. BX and AC are designations for a flexible cable with an aluminum or steel sheath.
Ground Fault Circuit Interrupters
Use a Ground Fault Circuit Interrupter (GFCI) for any outlet in a garage, kitchen, bathroom, crawl space or basement, as well as any outdoor outlet. These safety devices sense current flowing to ground and immediately stop the current flow. This can save your life if the path to ground happens to be your body. GFCIs are required by the National Electrical Code in all of the above listed locations.
Most lighting circuits use 14-gauge wire; specifically 14/2 with ground ROMEX®. Because a lighting circuit is not likely to have large inductive loads placed upon it, this is acceptable. The number of lighting fixtures connected to a single circuit should not add up to more than 1500 watts, even if all fixtures use 100-watt bulbs and all are switched on at once. Large outdoor lighting circuits should use 12-gauge wire or larger.