Electrical cable is available in a number of gauges and configurations. Electrical resistance in wire decreases as the wire becomes thicker, and this allows the wire to safely carry higher levels of current. The American wire gauge (AWG) method is commonly used to specify the thickness of wire, and the lower-numbered gauges represent thicker wire. The most common type of household wiring, type NM (ROMEX®) is referenced by gauge and number of conductors. Wiring listed as 14/2 is 14 gauge wire with two insulated conductors, plus a third uninsulated ground wire. There are specific wire gauge recommendations for lights and outlets.
National Electric Code
The National Fire Prevention Association maintains the National Electric Code (NEC). Local jurisdictions may adopt the NEC in its entirely, or amend the NEC in response to local conditions. Check with your local building inspector or planning office.
Ampacity is the term used in the NEC to define the maximum safe sustained current level in a wire. Wire ampacity is matched to the capacity of the circuit breaker which supplies the circuit with current. The value of the circuit breaker, expressed in amps, defines the minimum thickness wire that must be used. The NEC allows for thicker (lower gauge number) wire to be substituted.
Type NM Ratings
The 2005 NEC, table 310.17 lists the allowable ampacities. These numbers are for conductors in free air, with a maximum temperature of 83 degrees. This is a common household condition. Determine the value of the circuit breaker that controls the circuit you are wiring. If the circuit breaker is 15 amps, 14 gauge wire can be used. For a 20 amp circuit breaker, 12 gauge wire is specified. A 30 amp circuit breaker requires 10 gauge wire. While you can always substitute a lower gauge wire, they are more expensive and difficult to work with.
In practice, most lighting circuits are controlled by a 15 amp circuit breaker, and therefore 14 gauge wire is acceptable. Circuits with power outlets are often controlled by 20 amp breakers and should be wired with 12 gauge wire. Always match the gauge of wire to the capacity of the circuit breaker.