A modular lighting system for use with shelving and a variety of other applications. A power track is adapted to be disposed within an upright support of a shelving unit. A power plug with vertically-aligned prongs inserts through existing openings in the upright support to reach conductors within the power track. The power plug has connecting structure to connect to a cable, and frangible portions that allow the cable to exit in a number of positions. Light bars carry the same connecting structures, can connect to the cable and snap together to extend continuously across a shelf. A second power track and second plug may provide power to the power track within the upright. The second plug carries the same connecting structure as the other components.