Electrical Receptacle Box Installation in an Existing Wall Using Madison Bars

Dear Mr. Electrician: Using Madison Bars, I want to install an electrical outlet on a dedicated circuit for my window air conditioner.  I’ve got it figured out how to run the 12/2 NM-B cable from the circuit breaker electrical panel in the basement up through the wall under the living room window.  How do I install an electrical receptacle box using Madison Bars in the wall that will contain the wire and support the receptacle?

Answer: The hole above was made using a drywall saw. The metal outlet box was held against the wall and a pencil line was traced onto the surface.  Note the notches on the upper right and lower left corners.  They are to allow for the protruding screws on each side that all metal receptacle boxes of this type have.  NOTE: Text links below go to products on Amazon.

Installing an Electrical Receptacle Outlet Box using Madison Bars

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.
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.

It is important to have the hole cut exactly right.  If the hole is too big the ears on the electrical box will not hold against the wall as the Madison Bars pull the box inward.

The longer end of the Madison Bar steel switch box support is inserted first along side of the metal box.
The longer end of the Madison Bar steel switch box support is inserted first along side of the metal box.

Bring the cable into the electrical box and tighten the clamp enough so that the cable cannot be easily pulled out.  Do not over-tighten the clamp as it could cause an internal short circuit on the cable.  Wrap the ground wire around the 10/32 ground screw clockwise and tighten the screw so that the metal electrical box will be grounded.

With one Madison Bar steel switch box support pinched tightly, the other side of the metal electrical box protrudes slightly from the wall.
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.

With the cable clamped you can push the electrical box into the wall and insert one Madison Bar along side of the box.  Slide up the long side so that the short side of the Madison Bar can go into the wall.  With the opposite side of the electrical box protruding about 1/4″ – 3/8″ push the side of the box with the Madison Bar into the wall hard using a big flat head screwdriver.  While holding the one edge of the box in with the large screwdriver, bend the metal tabs of the Madison Bar inward towards the inside of the box.  Squeeze the tabs tightly with long nose pliers.

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.
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.

Insert another Madison Bar on the other side of the electrical box.  Push the box in with the large flat head screwdriver and bend the metal tabs over.  Pinch the tabs tightly with long nose pliers.

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 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.

Once the box is secure you can adjust it for straightness by lightly tapping it with a screwdriver and your pliers.

The wires are folded and the GFCI receptacle is pushed into the box.
The wires are folded and the GFCI receptacle is pushed into the box.

The Madison Bars tend to work best with drywall that is at least 1/2″ thick or thicker.  I sometimes had a problem using Madison Bars on 3/8″ drywall.  That is why it is always best to mount a new box next to a wall stud.  That gives you something to screw on to in case the Madison Bars don’t hold well.  Many times I will drive one #8 sheet metal screw through the side of the electrical box box anyway just to keep it secure.

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.
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.