Electrical boxes are the workhorses of electrical repairs and remodels, serving as end or junctiion points for electrical cables. Outlets, light switches, ceiling lights, ceiling fans, and transition wires are just a few of the items that are installed in electrical boxes.
While there is an abundance of sizes, shapes, and attachment variations with electrical boxes, all of them fall into two categories: plastic boxes or metal boxes.
In many cases, your choice of a metal or plastic electrical box is one of personal preference, economy, and ease. In a few cases, though, the choice is clear-cut and obvious, mainly with respect to grounding. Most do-it-yourselfers prefer plastic electrical boxes, while electricians use both metal and plastic boxes.
Metal Electrical Boxes
More expensive than plastic
Heavier than plastic boxes
Knock-outs sometimes difficult to remove
Sharp edges can chafe hands
Requires separate clamps; not built-in
Metal electrical boxes were used long before plastic (PVC) electrical boxes were introduced to the market. Ultra-strong, fireproof, and incapable of being melted, metal boxes offer the greatest level of security for all electrical wiring applications.
Even when slightly stressed, plastic boxes can twist. With enough stress, plastic boxes will distend. Even hammering the plastic box to the stud the wrong way can twist the box out of shape. Metal electrical boxes have no such problem: It is virtually impossible to bend or crush metal electrical or junction boxes.
For do-it-yourselfers, metal electrical boxes can be slightly more difficult to work with. Metal boxes have rear and side knock-outs that require supplementary clamps. In addition, metal boxes' sharp edges can take a toll on hands. For this reason, it is recommended that you wear gloves when working with metal boxes.
When Metal Boxes Are Highly Recommended or Required
Use a metal electrical box when metal-sheathed cable (also called armored BX cable) or metal conduit is running in or out of the box. Metal cable and conduit depend on the contact from its metal sheathing to the metal box to complete grounding.
Also, use metal boxes with interior exposed applications. Typically, electrical boxes are recessed in walls. But areas such as unfinished basements and mudrooms may not have a complete wall system that allows for the box to be enclosed by drywall. A typical installation in these cases involves attaching the exposed box directly to a masonry wall. Because the wires are also exposed, the metal conduit is required.
Metal boxes' strength comes in handy when mounting especially heavy items like ceiling fans.
Metal boxes can also be used with Romex or NM wiring if desired, though special precautions must be taken to ensure proper grounding. Contact between an attached device (such as a light switch or outlet) and the metal box completes the grounding contact. Even if the device does not complete the ground, Romex or NM wiring can always be used with metal electrical boxes by attaching the bare or green grounding wire to the box by a screw.
Other Metal Box Considerations
Metal boxes are strong, plus they provide a strong attachment to 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.
Metal boxes will not warp. With their high-stress material, metal boxes are often stronger than the work material built around them. Some metal boxes have pre-attached clamps for gripping the electrical cable. Other boxes 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 than plastic boxes.
Plastic Electrical Boxes
Holes (doors) easy to open or remove
No sharp edges
Less expensive than metal boxes
Can twist out of shape
Doors don't always hold cables firmly
Cannot provide grounding
Can melt in fire
Plastic (made off polyvinyl chloride, or PVC) is fast becoming the standard material for electrical boxes, especially for do-it-yourself work.
Plastic boxes are lightweight, cheap, and simple to work with. Holes are easy to punch out in the back or sides. PVC can melt when subjected to sufficiently high temperatures but it does not conduct electricity. Many plastic boxes come with built-in clamps for the wires, further lowering overall project cost.
When Plastic Boxes Are Recommended
It is recommended but not required that you use a plastic electrical box when you have Romex (or NM) cables leading in or out of the box. Electrical code does not require that you use NM cable with plastic boxes.
The overriding concern is that the application should be grounded. Metal-sheathed wiring depends on bonding with the metal electrical box for grounding. Using metal-sheathed wiring with plastic electrical boxes, without taking other grounding measures, severs that ground and is highly dangerous.
Other Plastic Box Considerations
Plastic electrical boxes are easy for do-it-yourself home remodelers to work with. The least expensive plastic boxes come with pre-attached nails for nailing into studs. Pre-sets on the box indicate the thickness of 1/2-inch drywall to help prevent the installer from nailing the box face flush with the stud.
Old-work or remodel boxes have wings so they can be attached directly to drywall. These are used when there is no free access to a stud.
Another type of plastic box has metal brackets that attach to the studs but are adjustable forward and backward with a Phillips screwdriver. These are the most expensive type of plastic box but offer the most flexibility when the eventual thickness of the wall is still undetermined.
Many do-it-yourself electricians gravitate toward plastic electrical boxes when using Romex or NM wiring. For one, home improvement centers tend to favor plastic boxes. For another, plastic boxes are lighter, their edges are softer, 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, eliminating the need for additional clamps.
On the downside, plastic boxes have a tendency to become misshapen when stressed. When plastic boxes receive a sharp blow, they can crack. The pre-attached door clamps, while convenient, often stubbornly hold the cable in place even when you want to remove the cable. Remodel (old work) boxes attached to drywall can pull away from the drywall and their attachment wings often break.
Explore the 2020 NEC: 314 Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures. National Fire Protection Association.