Understanding the Basics
A plastic electrical box—sometimes called a PVC junction box—comes with small, thin discs called knockouts. These are designed to be removed so wires can pass through.
The trick is removing them cleanly without breaking the box or leaving sharp edges. Many DIYers damage the box by forcing it too hard or using the wrong tool. Once cracked, it’s nearly impossible to fix securely.
Knowing how to knock out holes safely keeps your electrical work neat, code-compliant, and professional-looking.
The Right Way To Knock Out Holes
Step 1: Identify the Knockout Spot
Look closely at the box. You’ll see round, slightly raised circles molded into the plastic. These are the knockout tabs. Choose the one nearest to where your cable or conduit will enter.
Avoid knocking out a hole too close to another wire entry—you’ll weaken the box.
Step 2: Support the Box
Hold the box firmly or place it on a solid surface. Plastic boxes flex easily, and that’s what causes cracks.
Tip: If the box is already mounted, brace it gently with your other hand while working.
Step 3: Use the Right Tool
You don’t need brute force. The best tools include:
- Screwdriver and pliers
- Utility knife
- Knockout punch tool (for clean, round holes)
- Needle-nose pliers (for delicate twists)
| Tool Type | Best For | Notes |
|---|---|---|
| Flat-head screwdriver | Standard knockouts | Easy to control, minimal damage |
| Utility knife | Thin tabs | Ideal for precise cutting |
| Knockout punch | Perfect circles | Great for conduit fittings |
| Pliers | Twisting tabs free | Prevents cracking |
Step 4: Pop the Knockout
Insert the tip of a screwdriver into the edge of the knockout circle. Tap it lightly with your hand or hammer until it bends inward.
Grab the loosened disc with pliers and twist gently until it snaps off.
Never yank it—twisting is safer and cleaner.
Step 5: Clean the Edges
After removal, inspect the hole. Trim any rough plastic edges with a utility knife.
A smooth opening prevents wire insulation damage and helps grommets fit securely.
Alternative Methods For Tight Spaces
Sometimes the knockout is hard to reach—especially in old-work boxes already mounted in a wall.
Try these alternatives:
- Drill method: Use a step bit or spade bit to carefully bore through the knockout center.
- Pry-and-twist method: Insert a small screwdriver, pry one side, then rotate to snap the ring free.
- Heated blade method: For tough plastic, heat the tip of a knife (use caution) and melt through the thin tab cleanly.
Always wear gloves and safety glasses—plastic fragments can fly off.
Why It Matters
A properly removed knockout ensures:
- Tight wire fittings
- Reduced strain on cables
- Protection against moisture and dust
- Longer lifespan of your electrical box
Cracking or over-enlarging the hole leads to loose connections, which can cause electrical hazards or code violations.
Common Mistakes To Avoid
| Mistake | Result | Prevention |
|---|---|---|
| Hitting too hard | Cracked box | Use light, controlled taps |
| Using a large screwdriver | Oversized hole | Match tool size to knockout |
| Ignoring burrs | Cable damage | Trim smooth after removal |
| Twisting with force | Broken plastic | Gently wiggle, not yank |
Think of it like opening a soda can—you only need to pop the seal, not crush the can.
Key Benefits of Doing It Right
- Keeps electrical boxes code-compliant
- Saves time during installation
- Prevents replacement costs
- Makes wiring neater and safer
The best electricians aren’t fast—they’re precise. Taking a few extra seconds to pop a knockout correctly makes all the difference.
Conclusion
Knocking out a hole in a plastic electrical box is simple—but it rewards patience and finesse. With a steady hand, light pressure, and the right tool, you’ll have a clean, perfect opening ready for your wire or conduit.
Remember: slow and steady saves your box.
Key Takeaways
- Identify the correct knockout before removing.
- Support the box to avoid cracking.
- Use proper tools like pliers, screwdriver, or knockout punch.
- Twist gently, never force.
- Smooth edges after removal for a safe, clean fit.
Frequently Asked Questions (FAQ)
1. How do I remove a knockout without breaking the box?
Use a flat screwdriver and gentle taps to loosen the knockout. Support the box to prevent flexing, then twist the tab off with pliers.
2. What tools work best for small knockouts?
A utility knife or needle-nose pliers gives more control for smaller, delicate tabs. Avoid large tools that can crack the box.
3. Can I drill a new hole instead of using a knockout?
Yes, but use a step bit and go slowly. Drilling too fast can melt or deform the plastic.
4. What should I do if I crack the box?
If the crack reaches the mounting screw or wire entry, replace the box. Damaged boxes can’t safely hold wiring or fittings.
5. Why are knockouts included in electrical boxes?
They allow custom wire routing without compromising the box’s integrity. You only open what you need.
6. Can I reuse a knockout once it’s removed?
No. Once popped out, it cannot be reattached securely. Use a knockout plug if you need to close it again.
7. What size hole do I need for standard conduit fittings?
Most ½-inch or ¾-inch conduit connectors fit standard knockouts. Always check the fitting label for exact dimensions.
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