18ga is plenty big enough
i recently bought (arrived past monday) a set of (4) LED under cabinet strip lights, each light 11.8" 4w each, runs on 12v, came with 12v wall wort, and wireless remote, for off/on dimming etc.. even 'strobing'
yesterday i mounted (2) of them above the dining table and tapped into the power for little halogen spot/puck light, pulled the light and tapped into the wiring, and use the on/off rocker switch that used to control the puck light
i bought the white light aka daylight version, really nice
i have (2) strips left, they will eventuality be over the sofa, under the cabinet, again replacing the the 30w puck/reading spots
the wire that came with mine must be 22ga,
i had to cut down and strip the end of one of the coupling wires (that goes between strips)
of course each light is only around 350 mill-amps, maybe less
all four together is rated at 16w aka 1.25 amps at 12v