An improved system for lighting a floor area in a building. A plurality of lighting fixtures mounted in a ceiling structure for lighting the floor area are connected in one or more branch circuits. Each branch circuit is defined by a plurality of plural conductor branch circuit cables which distribute power to the fixtures. Each cable connects in series or parallel to both the next cable and the next fixture in the branch circuit. Circuit connections are easily modified for changing power distribution and switching needs by selective insertion of adapters between two series connected cables or between a cable and a connected fixture. Adapters provide for switching circuits and for converting circuit connections.