Electrical Receptacle Box Installation in an Existing Wall Using Madison Bars The hole cut-out ready for a metal receptacle box. I held the face of the electrical box against the wall and penciled an outline to follow with my Drywall Saw. This photo shows the 12/2 NM-B cable brought into the metal box and clamped. The ground wire is attached to the box using a separate 10-32 ground screw.Consult Article 314 in the National Electrical Code for the correct size electrical box needed for the amount of wires and devices that will be contained within. The longer end of the Madison Bar steel switch box support is inserted first along side of the metal box. With one Madison Bar steel switch box support pinched tightly using needle nose pliers, the other side of the metal electrical box protrudes slightly from the wall. After inserting the second Madison Bar the protruding edge of the metal electrical box needs to be pushed hard against the wall as the Madison Bar is pinched over. You can use a large flat head screwdriver for this, the bigger the easier. This ensures that the box will be held in place tightly. Use needle nose pliers to crimp the Madison Bars tightly against the metal electrical box. The box is held firmly in place by the Madison Bars. It should not be loose. Note how the upper and lower screw hole tabs on the box are set back into the wall slightly. Inside the rear of the box is the bare copper ground wire attached to the metal box using a 10/32 screw. The wires are folded and the GFCI receptacle is pushed into the box. The finished GFCI receptacle installation using Madison Bars to support the box. I usually prefer to install the box next to a wall stud so that a wood or sheet metal screw can be screwed through a hole in the box directly into the wood stud in addition to the Madison Bars. That makes the box extremely secure. However in this case the customer wanted the receptacle installed in this exact location. Click here for my post about installing an electrical receptacle.