Plastic electrical boxes have their pluses and minuses. Because they are plastic, there is no need to attach a ground wire to it. Since it is made of a non-conductive material, switches and outlets cannot short out if they touch the side of the box.
Plastic boxes usually come with tapped screw holes for easy attachment of switches and outlets. These boxes come in a single-gang, double-gang and even multiple-gang configurations.
You can choose between plastic boxes with nail-on brackets, complete with nails, or a cut-in version that has tabs that spin out when tightened to hold the box securely to the wall.
The disadvantages of plastic boxes are their brittleness and wire support brackets. If you look at the box, you'll notice that the device screw holes are also plastic. If the screw is lined up when installing the device, you're fine. But get the screw cross-threaded and you have a problem. Suddenly the screw won't tighten down and the device is flopping in the wind. If the metal hole in a metal box strips out, you can just tap it out and it's as good as new. Not so easy with a plastic box.
The same holds true for the nail-on mounting brackets. If you hammer the nails carefully and don't accidentally hit the plastic bracket, you're fine. But the slightest miscue of the hammer head on the plastic bracket or body and the box is toast. Metal boxes can take a pounding and still function.
The plastic boxes sometimes have plastic push-in tabs to hold the nonmetallic sheathed (NM) wire. These tabs serve their purpose, but we’d much rather have a wire connector or a metal clamp with a screw and tighten the tension to hold the wires in place. In this case however, this box has knockouts to remove with no fastener for the wires.
Plastic octagon boxes are often used for supporting ceiling fixtures and making electrical connections, but they do have their weight limits. These boxes are usually found on walls and on ceiling for connections and use with light fixtures. The wiring must comply with the National Electrical Code guidelines to ensure safety. Installing light fixtures isn't all that hard with these boxes, but ceiling fans should have a bracketed box capable of holding the additional weight they have.
Junction boxes come in many shapes and sizes. Depending on what sized wire, how many wires are in a given box and the number of wire splices in that box, you may need a little more room than a standard box can give you. You may have to opt for a deeper or larger box to accomplish the splicing you have planned. Then again, you may need both a physically larger and deeper box for the electrical job that you are working on.
Each electrical project has its own specific needs of electrical boxes. Some may require square, octagon, metal or plastic boxes. Your project may be a new installation or a remodel job. There are specific boxes for each of the choices and they both have options to accommodate the space you need to place the right amount of wiring within them.
If you have a choice between a plastic or metal box, definitely choose the metal box every time. The plastic boxes are cheaper and easier to install, but it’s not a bad idea to choose quality over price. A metal box will survive a fire, but will your plastic box?