Abstract: For building electrical systems, an electrical rough-in box holding tool is magnetically attached to a rough-in box and then, with the box attached to it, the tool is magnetically attached to a steel column at a proper location to hold the box in place while the box is being secured by screws driven into the column. Steel conduit is placed against the column and magnetically held in place by a conduit holder magnetically attached to the conduit and to the column to hold the conduit in the proper location while it is clamped to the column by a clamp fastened to the column by a screw. Armored cable is passed over a smoothly contoured guide magnetically secured to bar joists, both at the cable feeding end and at a cable pulling or drop end to facilitate the pulling of the cable from a supply coil on the floor up and over one bar joist and down around another bar joist spaced from it, for connection to the conduit which is connected to the rough-in box.