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When to Use a Metal Electrical Box

- Mar 31, 2018 -

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.

Other Considerations

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.