knockout plugs for electrical boxes
The following is a detailed description of **knockout plugs** used for electrical boxes, covering their functions,
types, and key installation points:
I. Definition and Functions
**Knockout plugs** are accessories used to seal the **unused knockouts** of electrical boxes (such as embedded concrete boxes,
metal/plastic junction boxes). Their main functions are as follows:
- **Sealing and Protection**: Prevent water, dust, and insects from entering the box, maintaining an **IP66/IP67 protection rating**.
- **Safety and Insulation**: Avoid direct contact between wires and the box body to prevent short circuits or electric leakage.
- **Compliance with Regulations**: Meet the requirements of **NEC 314.17(B)** (knockouts must be sealed) and the **UL 514B** standard.
II. Core Types and Materials
1. Classification by Structure
| Type | Characteristics | Applicable Scenarios |
| Standard Plugs | Circular/square, directly embedded into the knockout | Indoor dry environments, non-waterproof boxes |
| Waterproof Plugs | Equipped with silicone sealing rings or rubber pads | Outdoor, wet or buried electrical boxes |
| Reusable Plugs | Screw-type or snap-type design, can be disassembled multiple times | Scenarios where the circuit needs to be frequently modified |
2. Classification by Material
| Material | Advantages | Disadvantages | Applicable Box Bodies |
| Plastic (PVC/Nylon) | Good insulation, lightweight, low cost | Poor temperature resistance (<80℃) | PVC electrical boxes, indoor environments |
| Rubber | High elasticity, strong waterproofness | Prone to aging (requires UV-resistant materials) | Outdoor waterproof boxes |
| Metal (Galvanized Steel/Brass) | Resistant to high pressure, can be grounded | Requires insulation treatment to prevent conductivity | Metal electrical boxes, industrial environments |
III. Key Specifications and Adaptability
1. **Size Matching**:
- Common sizes of knockouts: **1/2 inch, 3/4 inch, 1 inch** (corresponding to the diameter of the conduit).
- It is necessary to strictly match the **type of knockout** (circular, square, oval) of the electrical box.
2. **Installation Methods**:
- **Press-in Type**: Directly knock into the hole (suitable for plastic plugs).
- **Screw Fixing Type**: Fasten with a central screw (suitable for metal plugs, and an insulating washer is required).
IV. Installation Steps
1. **Cleaning the Knockout**:
- Use a **file** to remove the burrs on the edge of the knockout to ensure it is flat.
2. **Selecting the Appropriate Plug**:
- In outdoor or wet environments, a **waterproof plug + silicone sealant** must be selected.
3. **Installing the Plug**:
- **Press-in Type**: Align with the hole position and push in firmly to ensure that the edge is completely embedded.
- **Screw Type**: Put the plug into the hole, tighten the screw from inside the box, and add a **sealing washer**.
4. **Testing the Seal**:
- Spray water for a 5-minute test to check if water enters the box.
V. Precautions
1. **Protection Rating Requirements**:
- Plugs for buried or concrete boxes need to meet **IP67** (short-term immersion). It is recommended to use **double sealing**
(plug + box cover sealing ring).
2. **Grounding Continuity**:
- Metal plugs need to be electrically connected to the box body through **grounding screws** (only applicable to metal
electrical boxes).
3. **Prohibited Behaviors**:
- Do not directly block the knockout with tape or cement (does not comply with regulations).
- Do not forcefully use a small-sized plug to fill a large hole (a matching plug needs to be replaced).
4. **Labeling Management**:
- Use a **label pen** to mark the purpose of the corresponding conduit (such as "lighting", "socket") on the surface of the plug for
convenient subsequent maintenance.
VI. Special Accessories and Extensions
1. **Waterproof Knockout Assembly**:
- Consists of a **rubber plug + metal pressure ring + screw**, suitable for high-pressure water spray environments (such as
electrical boxes near swimming pools).
2. **Adjustable Plugs**:
- Equipped with a **threaded sleeve**, which can be adapted to box bodies of different thicknesses (such as differences in the
wall thickness of concrete boxes).
3. **Fireproof Plugs**:
- Made of high-temperature-resistant materials (such as ceramic fiber), used at the penetration points of electrical boxes in fire
prevention zones.
VII. Common Questions
- **Q: How to remedy an unused knockout that has been knocked off?**
A: Use an **oversized plug** (such as 1.5 times the hole diameter), or install a **metal patch panel + sealant**.
- **Q: What should be done if a plastic plug ages and cracks?**
A: Replace it with a **UV-resistant rubber plug** and apply **silicone sealant** on the surface to extend its service life.
- **Q: Can multiple plugs be installed in the same knockout?**
A: No. Only one plug can be installed in each hole, and the redundant holes need to be covered with a **special blocking plate**.
VIII. Examples of Application Scenarios
- **Concrete Streetlight Bases**: Use **stainless steel screws + rubber plugs** to seal the redundant knockouts and prevent
rainwater from seeping in.
- **Power Distribution Boxes in Underground Garages**: Use **fireproof plugs** to block the spare holes to meet fire prevention
regulations.
- **Outdoor Junction Boxes**: Install a waterproof plug in the **3/4-inch knockout** and match it with a conduit locknut.
If you need specific model recommendations (such as brands like Panduit and Thomas & Betts) or installation diagrams, you can
provide the type of electrical box for further clarification.
