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Changing Single-Gang Electrical Box To Double-Gang


I would like to install 2 or 3 halogen under cabinet lights and wire them into an existing wall outlet, changing the outlet to a double box so I can control my new lights with a dimmer switch. Can you describe how to do this so I can tap into that power source? 

Don C.



Changing a single-gang box to a double-gang box isn't all that easy. It could be done, I guess, but most electricians would try another approach... just add another single-gang "old work" box a few inches away and connect it to the existing box.

This can only be done if there is not already a big tangle of wires in that existing box. If there is only one or two cables leading into that box, you should be okay. If there are 3 cables, you may not be allowed to add any more. The National Electrical Code has restrictions on how many individual wires are allowed in a box of a given size. Their intent is to prevent over-heating of wires inside the box. I know this rule is violated a lot by do-it-yourselfers, but that doesn't make it right or safe.

If I had to change to a double-gang box I would remove the old box by cutting the mounting nails with a metal cutting blade in a reciprocating saw. This is done by shoving the blade between the box and the stud. A little bit of drywall might need to be cut away to find the stud. Of course, the circuit breaker needs to be turned off and the outlet needs to be removed so the ends of the wire are just dangling. Caution needs to be the rule here... it is possible to cut the insulation of a wire nearby. I would shine a bright flashlight in behind so I could see the location of any cables around the box. A hand-powered hack-saw is safer, I guess, because you can tell if you are hitting a cable. Once the nails are cut I would pull the box out, with the wires just sticking out of the hole in the wall.

Next I would get a two-gang old-work box, but only the type that has the two little rotating clamp tabs. I'm not sure this can be communicated in an e-mail. I think there are pictures of an old-work box somewhere on The only type I use are the blue Carlon brand with the two rotating clampy things. These are available in single and double width, maybe wider. But... you need to be very careful when cutting the opening in the drywall. If you cut too big it's all over but the cryin!

With the proper hole for the 2-gang old work box (I always test-fit the box) I would put the wires in the box and mount the thing. Then I would restore the wiring the way I wanted it.

This is a project for someone who has a decent level of electrical experience.

Note that code absolutely FORBIDS using a dimmer to supply power to an outlet, even if the intended use of the outlet is for a dim-able light fixture. There is always the chance that some motorized appliance can be plugged in, and motors WILL NOT work properly if a dimmer is used. In fact, that is a good way to destroy a motor, and possibly overheat a motor and cause a little fire.

Some under-cabinet lights are meant to be hard-wired, that is where the cable is connected inside the fixture. The package usually says "Direct Wire" when this is the case. These are the only type of lights that are approved for dimming.

Bruce W. Maki, Editor.





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Copyright 2002

Compiled February 21, 2002