Popular Mechanics recommends that the usual wiring be a 14 gauge, two wire cable with separate ground. However state and local building and electrical codes may vary and must always be consulted prior to beginning any home electrical wiring project.
A copy of the National Electric Code produced by the National Fire Protection Association contains attic wiring specifications and can be downloaded at EBooksData.org. While NEC codes can be superseded by states and municipalities, they are generally a good guideline. Wiring is accomplished by tapping into an existing attic circuit as long as 4 or 5 amps are available. If not, fish a new wire from an open 15 or 20 amp breaker in the service panel.