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 below was made using a Compass 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 applicable products on Amazon.com
Installing an Electrical Receptacle Outlet Box using Madison Bars
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.
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 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.
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.
Once the box is secure you can adjust it for straightness by lightly tapping it with a screwdriver and your pliers.
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.
Click here for my post about installing an electrical receptacle.