what size screws in electrical box

In an electrical box, the size and type of screws depend on the material of the box (metal or plastic) and its purpose 

(fixing the cover plate, installing the box, grounding, etc.). The following are common **American screw specifications**

 and their application scenarios:

 I. Metal Electrical Boxes (Metal Boxes)

Metal boxes (such as steel boxes and aluminum boxes) usually use **machine screws (Machine Screws)**, and the common 

specifications are as follows:

1. Cover Plate Screws (Cover Plate Screws)

- **Specifications**:

  - **6-32 UNC** (Diameter ≈ 0.138 inches, Pitch ≈ 0.05 inches)

  - **8-32 UNC** (Diameter ≈ 0.164 inches, Pitch ≈ 0.05 inches)

- **Length**: Usually **1/2 inch** or **5/8 inch**, specifically depending on the thickness of the box.

- **Application**: Fix the metal cover plate or devices (such as sockets and switches) to the metal box.

2. Mounting Screws (Mounting Screws)

- **Specifications**:

  - **10-24 UNC** (Diameter ≈ 0.190 inches, Pitch ≈ 0.0417 inches)

  - Or **#10 wood screws** (used for fixing to wooden structures).

- **Length**: From **3/4 inch** to **1.5 inches**, depending on the mounting surface (such as wooden beams and metal frames).

- **Application**: Fix the metal box to the building structure.

3. Grounding Screws (Grounding Screws)

- **Specifications**:

  - **6-32 UNC** or **8-32 UNC**, with a **green coating** (indicating grounding).

- **Application**: Connect the grounding wire to the grounding terminal of the metal box.

II. Plastic Electrical Boxes (Plastic Boxes)

Plastic boxes (such as PVC boxes) usually use **self-tapping screws (Self-Tapping Screws)**, and the common specifications 

are as follows:

1. Cover Plate Screws

- **Specifications**:

  - **#6 self-tapping screws** (Diameter ≈ 0.138 inches)

  - **#8 self-tapping screws** (Diameter ≈ 0.164 inches)

- **Length**: **1/2 inch** or **5/8 inch**, which needs to match the depth of the threaded hole of the plastic box.

- **Application**: Fix the plastic cover plate or devices to the plastic box.

2. Mounting Screws

- **Specifications**:

  - **#8 or #10 self-tapping screws**, with a length from **3/4 inch** to **1.5 inches**.

- **Application**: Fix the plastic box directly to wood or gypsum board (some plastic boxes come with built-in mounting ears).

III. Key Precautions

1. **Material Matching**:

   - For metal boxes, **galvanized or stainless steel screws** should be used to prevent rust; ordinary steel screws can be

 used for plastic boxes.

2. **Anti-loosening Requirements**:

   - In a vibrating environment (such as an industrial site), **locking screws** or **spring washers** should be used.

3. **Electrical Codes**:

   - According to the regulations of the National Electrical Code (NEC) in the United States: Metal boxes must be connected 

to the grounding wire through **grounding screws**, and plastic boxes do not need to be grounded (but insulation must be

 ensured).

4. **Refer to the Manufacturer's Instructions**:

   - The recommended screw specifications are usually marked on the packaging of the box or the cover plate (such as "Use 

#6-32 screws").

 IV. Common Questions and Answers

V. Summary

- **For metal boxes**: Use **6-32 or 8-32 machine screws** for the cover plate, and **10-24 screws** or wood screws for installation.

- **For plastic boxes**: Use **#6 or #8 self-tapping screws** for the cover plate, and **#8 self-tapping screws** for installation.

- **For grounding**: Metal boxes must use **green 6-32/8-32 grounding screws**.

Select appropriate screws according to specific scenarios to ensure firmness and compliance with electrical safety standards.