When to Use a Metal Junction Box

There are two types of junction boxes in common use today: metal junction boxes and plastic junction boxes. Both have their own advantages, and both are useful for certain applications.

For many applications, the biggest advantage of a metal junction box is its durability. Metal junction boxes can withstand a huge variety of adverse environmental conditions, including crushing impacts, open flames, and extremely hot or cold temperatures. They also serve as natural grounding points for metal-sheathed wires and other components that require electrical grounding.

Plastic junction boxes are generally less expensive and may be easier to work with. However, some plastic junction boxes are less resilient than metal boxes, so they may not be the ideal choice if you need maximum protection and maximum durability from your junction box.

When to Use a Metal Electrical Box

You should use a metal electrical box if:

You’re using metal-sheathed BX cable. Metal-sheathed cable is designed to be used with a metal junction box to ground the wires.

It’s extremely important the electrical box is securely attached to a stud. The rigid and warp-resistant nature of metal electrical boxes means they’re compatible with the heavy-duty screws you’ll need for an extra-secure stud attachment.

Your project is in an exposed interior space. Some interior locations, such as unfinished basements, will have non-recessed electrical boxes directly attached to a surface (usually a masonry wall). Since these applications leave wires more exposed, you should use a metal junction box to protect them from damage.

You need a more durable type of electrical box. Some plastic electrical enclosures can be damaged if they’re placed under pressure. Metal electrical enclosures are generally more resilient.

These aren’t the only circumstances in which you should choose a metal junction box, but they’re some of the most important. If you’ve decided a metal junction box is a right call, see our guide to metal electronics enclosure options for some great choices.

When Not to Use a Metal Electrical Box

Conversely, there are also some reasons you might choose a plastic junction box for your project instead. A plastic electrical box might be a better choice for your application if:

You’re using non-metallic sheathed wire (often called Romex wire). Romex wire and plastic junction boxes are frequently used together, but metal enclosures may be acceptable as well (depending on the project). If you’re using metal-sheathed wire, you may be able to use plastic enclosures, but you’ll always need to take the appropriate steps to ground the wire first.

It’s safe to use either metal or plastic enclosures if you want to save money. Plastic electrical boxes are typically less expensive and require less additional equipment, such as clamps, than metal electrical boxes. (Remember, though, no amount of savings is worth trading away safety if the application requires metal enclosures.)

You want junction boxes easy to handle and work with. Some types of sheet metal enclosures have sharp edges capable of causing injury, although wearing gloves or choosing a finished sheet metal enclosure can usually eliminate these risks.