Plastic junction boxes have their advantages and disadvantages. Although they are easy to install, they are not always the best choice for every application.
In electrical installations, electrical boxes are used to install electrical devices, make splices and install light fixtures. This leads to a choice of types of boxes, plastic or metal boxes. Plastic boxes are nonconductive and easy enough to install. They have two nails, both on one side of the box, that allow the box to be fastened to the studs in your home. Plastic boxes have markings on the side that show different depths that the box can be set out to allow flush alignment with the outer wall of drywall or wall surface. This allows the outlet, switch, or light fixture to sit flush against the box opening and the wall face for proper alignment and installation.
Plastic boxes have their disadvantages also. Along with the box being plastic, the nail brackets are also plastic. They are easily broken off when installing a box or when you have to take a box off and reinstall one. Once this bracket clip is damaged or broken off, your mounting options are all but gone.
One look at the face opening of the box and you’ll notice there are two screw holes that make it possible to install a switch, outlet, or other device to the box. As with anything plastic, the threads inside the mounting holes are threaded and they are easily stripped if the screws are not installed properly.
Quite often, you may be tempted to put any old screw into these holes, but the threads are usually set to a 6-32 thread. In the event the threads do become stripped, you may be able to still use the box by installing a short drywall screw into the hole, that is, if it isn’t cracked or damaged. We want the screw to firmly hold the device in place so that it doesn’t short out.
As for mounting light fixtures and ceiling fans, I recommend using metal boxes. Although there are some plastic boxes that are designed to support these fixtures, I personally have little faith in the plastic threads that support a fixture over my head. And while we are discussing that matter, keep in mind to use the specially designed light fixture and fan mounting boxes that come complete with mounting brackets. These setups are specifically designed to handle the weight that some of these lights and fans have.