Plastic electrical boxes, sometimes called junction boxes, have plenty of pluses, including low cost, convenience, and ease of installation, but they're not the best choice for every application. When plastic doesn't make sense, the standard alternative is a metal box. Here's a look at the places where plastic is the right and the wrong choice.
Plastic boxes are nonconductive, which means they won't conduct electricity if they're touched by a live wire or a fixture or switch with a short circuit or fault. This also means you don't have to ground the box with a short wire (called a pigtail) connected to the circuit's ground wire. Metal boxes must be grounded.
Plastic junction boxes are easy to install in many situations. They're easiest to install during new construction when the wall studs or ceiling joists are exposed. You simply hold the box against the wood framing and drive the two nails that come pre-attached to the box. They even have markings on the side for setting the proper depth so that the box is flush with the face of the wall when the drywall goes up. Boxes for new construction are called "new work." There are also special boxes for remodeling work, called "old work." These have little ears that flip out to grab the backside of the drywall when the box is screwed in place. Metal boxes also come in old work types, but they aren't always as easy to use.
Plastic junction boxes have integrated cable clamps, little spring tabs that hold electrical cable snug once it's inserted into the box. By contrast, many metal boxes require a separate cable clamp that is secured to the cable and the box. There's nothing wrong with a separate clamp; it's just easier and a bit quicker to use the plastic tabs.
Plastic is cheaper than metal. Both are pretty cheap, but plastic wins in most cases. The fact that they don't need clamps also saves you a little bit more.
Durability and Strength
This is where plastic junction boxes loose some points. 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.
Also vulnerable are the two screw holes for installing a switch, outlet, or another device to the box. As with anything plastic, the threads inside the mounting holes 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. If 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. The screw must hold the device firmly in place.
When mounting light fixtures and ceiling fans, use metal boxes. Although there are some plastic boxes that are designed to support these fixtures, it’s hard to have faith in the plastic threads that support a fixture over your head. For heavy fixtures and all ceiling fans, be sure to use boxes with heavy-duty braces or mounting brackets designed for this purpose.
Plastic junction boxes aren't a good fit for the outdoors, and they can't be used with metal conduit, which is often required outdoors or when the wiring runs are not concealed inside wall or ceiling cavities. You can find "outdoor-rated" plastic boxes, but when you think about the effects of snow and ice and years of direct sunlight (the arch enemy of plastic), metal sounds like a much better bet.