What do you do when an electrical junction box is set deeply into a wall and you plan to remodel a wall, as we did, by adding wainscot, paneling, or trim? This requires the installation of a junction box extension.
Any exposure to flammable material by spliced wiring is not allowed by NEC. It needs to be fully protected by the J-Box – with no gaps allowed. For this, you need to extend the junction box with an electrical box extension.
With an existing J-box than cannot be moved without demo of the wall, you can add a J-box extender. There are several different types, and I’ve used three of them
Carlon box extender for most residential wiring
Metal box extender for condo wiring or exposed conduit wiring
Arlington BE 1-5 below is carried by Home Depot
For new construction, or remodel where you can install a new J-box, I really like the Carlon adjustable box. I used these for all planned wainscot installation locations.
Regardless of which one you use, always choose the best product that will do the job, not the least expensive. Carlon sells tons Zip Box Blue, thin wall plastic J-boxes that meet NEC, if installed correctly. Problem is, they flex, and for DIY it is all too easy to push the box out of shape. Pass over the Zip Box Blue thin wall boxes, and pay the extra money for the Carlon SuperBlue Hard Shell Boxes. They are easier to install, and you get more capacity, and no worries about fire.
Again, consult the NEC and your local building code.