Installing an outlet box in concrete block can be challenging. The easiest way to provide electricity in areas where the walls are made of concrete blocks is to surface mount all conduit using pressure treated wood that has been glued to the wall. However, if you must install the box in the wall in order to have the outlet flush-mounted, your task is more difficult. The most feasible method is to fish the wires from above through cable placed in the hollow cores of the blocks. You may encounter difficulties if there are obstructions in the cavities, such as excess concrete from construction.
Plan your cable run through the hollow spaces where the blocks overlap. When concrete blocks are laid, the hollow sections line up and provide a path for your cable.
Create a hole for your outlet box. Trace a line to give a guide for breaking the block. A rotary hammer drill fitted with a carbide tipped drill makes an excellent tool, but if this isn't available, a star drill will work. Drill holes at the corners of your outline and then use a ball peen hammer to break away the rest of the concrete.
Drill a hole through the mudsill above the concrete wall for your cable. Make sure this hole is in line with the cavities you are fishing through.
Use a chain to work around obstructions in the cavities. You may encounter obstructions, typically excess mortar that has been deposited in the cavities by the mason during construction. A heavy chain makes a good tool for fishing through the gaps. By jiggling it around from above, you should be able to find a clear path for the cable to run through.
Attach the end of your cable to the chain and then pull the cable up through the cavities so that it can be connected to a circuit in the house. Make sure the connection is strong to prevent the cable coming loose while you are pulling it.
Cut the end of the cable coming out of the hole where the outlet is placed. Make sure to leave at least six inches of cable beyond the surface of the wall. Using a cable connector, land the cable in the box.
Slide the box into the hole in the concrete. In order to fasten the box to the concrete, use screws approved for anchoring in concrete. Pilot holes for the screws can be drilled with a rotary hammer. Fill in the gaps around the box with expanding foam.