Electrical boxes serve as end or transition points for electrical wires. Outlets, light switches, lights, and ceiling fans are but a few of the devices that are installed in electrical boxes. While there is an endless variety of sizes and shapes of electrical boxes, all can be divided two camps: plastic or metal electrical boxes. To confuse matters, your choice of plastic vs. metal is abundantly clear-cut in only a two instances, with other instances being at your discretion.
When to Use a Plastic Electrical Box
Plastic (stiff polyvinyl chloride, or PVC) is fast becoming the standard material for electrical boxes. This material is lightweight, cheap, and simple to work with. Holes are easy to punch out in the back or sides. PVC can melt when subjected to high-enough temperatures but it does not conduct electricity. Many plastic boxes come with pre-attached clamps for the wires, lowering overall project cost.
Highly Recommended or Required
Metal-sheathed wiring depends on bonding with the metal electrical box for grounding. Using metal-sheathed wiring with plastic electrical boxes severs that ground and is highly dangerous.
Plastic electrical boxes are easy for do-it-yourself home renovators to work with. The least expensive plastic boxes come with pre-attached nails for nailing into studs. Other boxes, called old-work or remodel boxes, have wings so they can be attached directly to drywall. Another set of boxes have metal brackets that attach to the studs but are adjustable in/out with a Phillips screwdriver. These are the most expensive type but offer the most flexibility.
Many do-it-yourself electricians tend to find it easier to work with the plastic electrical box in conjunction with Romex or NM wiring. Plastic boxes are lighter and their holes are easier to knock out. Also, many plastic boxes have doors that act as clamps to hold the electrical cable to the box.
On the downside, plastic boxes have a tendency to become misshapen when stressed. When they receive a sharp blow, plastic boxes can crack. Remodel (old work) boxes attached to drywall can pull away from the drywall.
When to Use a Metal Electrical Box
Metal boxes were used long before PVC came onto the scene. Ultra-strong, fireproof, and incapable of being melted, metal boxes offer the greatest level of security for the do-it-yourselfer. They are somewhat more difficult to work with, and their sharp edges can take a toll on your palms after a while.
Highly Recommended or Required
Use a metal box when you have metal-sheathed or BX wiring leading in/out of the box. However, metal boxes can also be used with Romex or NM wiring, if desired.
The metal electrical box is well suited for metal-sheathed wiring. This type of wiring depends on the contact from its metal sheathing to the metal box to complete grounding.
However, NM or Romex wiring does not use its sheathing for grounding. It uses a bare wire contained within the sheathing. Contact between the device (light switch, outlet, etc.) and the metal box completes the ground. As such, Romex or NM wiring can be used either metal electrical boxes or plastic electrical boxes.
Some metal boxes have pre-attached clamps for gripping the electrical cable. Others require you to purchase separate clamps, driving up the total cost of the box.
Generally, do-it-yourself electricians may find metal boxes slightly more cumbersome to work with.
Metal boxes are strong, plus they provide a strong attachment with the stud. When attachment to the stud is a major worry, choose a metal box. Metal boxes mean you can drive strong screws into the studs, ensuring a maximum amount of holding power.
It is also a good idea (but not a requirement) to use a metal box when the box will be exposed beyond your finish surface, as in exterior applications.
Metal boxes will not distend, nor will they pull away from the stud when properly attached. With their high-stress points, metal boxes are often stronger than the work material built around them.