A hanger bar assembly for supporting an outlet box between spaced apart support elements including a first channel member being longitudinally slidingly engagable with a second channel member forming a longitudinally adjustable support bar. The first channel member has a plurality of longitudinally extending grooves and the second channel member having a plurality of longitudinally extending rails slidingly received in the grooves such that lateral movement between the first and second channel members is resisted. A clamping device is provided for urging the first and second channel members into respective forced engagement thereby restricting movement between the first and second channel members. A securement device attached to the support bar for securing the support bar to the spaced support elements is further provided. The first and second channel members have apertures formed therein to accommodate fasteners for securing end plates to the support.