Electrical cable for lighting circuits can be run in several sizes. There is, however, a minimum size for a lighting circuit defined in the 2008 National Electrical Code (NEC).
Wiring for a residential general lighting circuit is required to have a minimum rating of 15 amps which calls for No. 14 American wire gauge (AWG) per NEC 310.16. Many contractors install the wires for lighting circuits so they are dedicated to lights only.
There is an alternative method of wiring the circuits where the electricians can mix the lights with outlets on the same circuit. Receptacle circuits are required to be 20 amp circuits according the NEC and require No. 12 AWG conductors using the chart in NEC 310.16. If mixing lights and receptacles on the same circuit, you must use No. 12 wire to feed the lights.
There are larger lights rated for higher amperage, such as street lights and large yard lights. The amperage the light is designed and rated for determines the size the wire needs to be. The NEC 310.16 chart for wire gauge and amperage is used to figure out what size wire is needed according to its amp rating.