The correct hole saw size for 3/4 EMT is typically 7/8 inch. This size is necessary because standard electrical box knockouts are designed to accommodate the trade size of the conduit, and 7/8 inch closely matches the internal dimensions needed for 3/4 inch Electrical Metallic Tubing (EMT) to pass through securely, especially when using locknuts or fittings.
Choosing the Right Tool for EMT Knockouts
When you work with electrical wiring, making clean holes in metal boxes or enclosures is a big part of the job. This is especially true when you are drilling 3/4 EMT conduit into a junction box or panel. Choosing the right tool ensures a tight fit and safe installation. If the hole is too small, you damage the conduit or the box trying to force it in. If the hole is too big, the connection is loose, which is a safety hazard.
Hole Cutter vs. Hole Saw: What is the Difference?
People often mix up these tools. A hole saw spins and cuts a full circle out of the material. A hole cutter size for conduit can sometimes refer to a slug-bitting knockout tool, which pushes the slug out rather than cutting it away slowly.
- Hole Saw: Best for making many holes in softer materials like wood or thin plastic. They can work on thin metal if they are specifically rated for metal cutting.
- Knockout Punch: This tool uses hydraulic or manual force to punch a clean, perfect circle instantly. For standard conduit sizes, this is often faster and leaves a cleaner edge than a hole saw.
When dealing with metal electrical enclosures, many pros prefer a knockout punch set if they do the job often.
Deciphering Electrical Box Knockout Sizes
Electrical boxes and panels have pre-stamped indentations called knockouts. These are not always the exact inner diameter of the conduit. They are standardized sizes meant to ensure the conduit fitting seats correctly.
Standard Conduit Trade Sizes and Knockout Matches
The “trade size” of the conduit (like 3/4 inch) does not always mean the hole should be exactly 3/4 inch wide. Here is a quick look at common matches for electrical box knockout sizes:
| Conduit Trade Size | Typical Knockout Size (Inches) | Recommended Hole Saw Size (Inches) | Primary Use |
|---|---|---|---|
| 1/2 inch EMT | 7/8 inch | 7/8 inch | Small junction boxes |
| 3/4 inch EMT | 1 inch | 7/8 inch or 1 inch | Standard conduit runs |
| 1 inch EMT | 1 3/8 inch | 1 1/4 inch | Larger panels |
| 2 inch EMT | 2 5/8 inch | 2 1/2 inch | Large services |
Note: The 7/8 inch hole saw is often preferred for 3/4 EMT because it allows the conduit to pass through and lets the locknut grip the box wall securely. Some manufacturers list the knockout as 1 inch for 3/4 EMT.
Why the 7/8 Inch Preference?
When you are installing conduit connectors hole size matters for the locknut. A standard 3/4 inch EMT connector has a body designed to fit into a certain size opening. The 7/8 inch hole is often the industry standard specified for 3/4 EMT locknuts on thin metal boxes. Using a 1-inch hole saw might leave too much gap, leading to a weak connection.
Selecting the Right Hole Cutter Size for Conduit
The goal when drilling 3/4 EMT conduit or drilling into a box for it is to achieve a precise fit. This dictates your hole saw selection for electrical conduit.
Hole Saw Requirements for Metal
You cannot use a standard wood or bi-metal hole saw on thick metal panels. You need a specialized tool.
- Material: Choose a carbide-tipped or cobalt hole saw. These resist the high heat generated when cutting steel or thick galvanized metal.
- Arbor and Shank: Make sure the hole saw arbor fits your drill chuck securely. A wobbly arbor ruins the cut.
- Speed: Always drill metal slowly. High speed creates excessive heat, dulls the teeth quickly, and can cause the saw to bind or break.
If you are cutting a hole in the actual EMT pipe itself (not the box), the size for drilling through metal conduit is determined by what you need to run through it (if you are making a junction point) or if you are using a specific type of coupling that requires a certain hole size. Usually, you only drill holes in the enclosure, not the conduit itself, unless you are making a transition point.
Using a Knockout Punch Set
For electricians doing this work daily, a hydraulic or manual knockout punch set is usually the fastest and best option.
Knockout Punch Set Advantages:
- Creates perfectly round holes.
- Edges are smooth and clean.
- Much faster than drilling and sawing metal.
- Sets are readily available for standard knockout punch size for 3/4 EMT (which aligns with the 1-inch die).
When you buy a knockout set, it will be labeled by the trade size it punches for, matching the knockout standards mentioned earlier.
Step-by-Step Guide to Drilling a Metal Box for 3/4 EMT
If you do not have a knockout punch, following these steps ensures you use the right size hole saw safely.
Step 1: Determine the Correct Size
Confirm your box manufacturer’s recommendation. If in doubt, default to the standard 7/8 inch hole saw size for 3/4 EMT if your knockout mark is small, or use the 1-inch size if the knockout indentation is larger. Measure the required size for your specific connector if you have it in hand.
Step 2: Mark the Center Point
Use a sharp center punch to create a small divot where the center of the hole will be. This stops the drill bit, and later the hole saw pilot bit, from “walking” across the smooth metal surface.
Step 3: Drill the Pilot Hole
Use a high-quality metal drill bit (HSS or Cobalt) slightly smaller than the hole saw’s pilot bit. Drill slowly until the pilot bit pokes through the back of the box. This stabilizes the larger hole saw.
Step 4: Attach and Use the Hole Saw
Attach the correct EMT fitting hole saw (7/8 inch or 1 inch) to the arbor. Ensure the pilot bit is engaged in the hole you just made.
- Apply cutting fluid or wax liberally to the saw teeth. This cools the tool and extends its life.
- Use medium pressure. Let the teeth do the cutting; do not force the saw.
- Maintain a very slow, steady drill speed.
Step 5: Clearing the Slug
Once the saw cuts through, the metal slug might remain stuck inside the saw. Turn off the drill. Use a screwdriver or a designated punch tool to push the slug out of the back of the saw cup.
Step 6: Finishing the Hole
Inspect the edges. If there are sharp burs, use a half-round file or a dedicated reamer tool to smooth them down. Sharp edges can quickly damage wire insulation, which is a serious code violation.
Fathoming Conduit Connector Needs
The final hole size must accommodate the installing conduit connectors hole size mechanism—usually a locknut threaded onto the connector body.
The locknut needs enough surface area on the outside of the box wall to seat firmly and grip the metal without slipping.
- Hole too small: You might crush the connector body or scrape the paint off the box, leading to rust.
- Hole too large: The locknut might spin freely or only engage a few threads, creating a poor mechanical and electrical bond.
For 3/4 inch EMT, the difference between a 7/8 inch and a 1-inch hole is minor but significant in professional installation standards. When in doubt on a new installation, check the packaging of the specific connector you plan to use, as manufacturers often specify the required knockout size.
Maintenance for Your Hole Cutting Tools
Proper care ensures your tools last longer, especially when cutting metal like EMT conduits or enclosures.
Caring for Metal Hole Saws
- Cooling is Key: Always use cutting oil when working with metal. If you don’t have oil, candle wax or even dish soap mixed with water can help reduce friction heat.
- Storage: Store hole saws in their original case or a dry spot. Rust is the enemy of fine teeth.
- Sharpening: If the saw starts smoking or requires extreme force, the teeth are dull. Cobalt and carbide saws can often be professionally sharpened, saving you the cost of replacement.
Maintaining Knockout Punches
Knockout punches require less upkeep but need care to keep the dies and pins aligned.
- Keep the cutting edges of the dies clean from metal shavings.
- Lubricate the screw or hydraulic cylinder periodically as recommended by the manufacturer. A seized punch is useless.
Comparison: Drilling Metal Conduit vs. Cutting Knockouts
While the topic is mainly about the enclosure, sometimes a worker needs to make an access point or coupling point directly on a piece of 3/4 EMT conduit.
| Action | Tool Used | Ideal Hole Size | Primary Concern |
|---|---|---|---|
| Attaching Connector to Box | Hole Saw / Knockout Punch | 7/8 inch or 1 inch | Secure fit of the locknut |
| Drilling Through Conduit | Hole Saw (Cobalt recommended) | Determined by internal fitting size | Avoiding sharp burs inside the pipe |
When drilling 3/4 EMT conduit itself, you must be precise because the conduit wall is thin. If you use a hole saw that is too large, the conduit might deform, making the coupling connection weak. Always select the smallest size that allows the internal fitting to pass cleanly.
Finalizing the Hole Size Decision
For the vast majority of electrical work involving 3/4 inch EMT connectors and standard metal enclosures, the required hole size hovers around 7/8 inch to 1 inch.
If you are using a standard UL-listed connector designed for 3/4 inch conduit, the physical dimensions usually mandate an opening slightly larger than 3/4 inch diameter but smaller than 1 1/16 inch. This places the 3/4 inch conduit hole size squarely at 7/8 inch for the tightest, most secure fit using a hole saw. If you are using a large knockout punch set, the corresponding die will likely be 1 inch.
Always prioritize the connector specification over general rules if you have the part in hand. A tight, clean hole ensures your installation meets electrical codes for mechanical integrity and grounding continuity.
Frequently Asked Questions (FAQ)
What size hole saw do I use for 3/4 inch EMT?
The most commonly accepted hole saw size for 3/4 EMT is 7/8 inch. This size provides the best fit for standard locknuts used to secure 3/4 inch EMT connectors to electrical boxes.
Can I use a standard bi-metal hole saw on an electrical box?
You can use a standard bi-metal hole saw on thinner steel or aluminum boxes, but it will dull very quickly and generate a lot of heat. For reliable results and longevity, use a cobalt or carbide-tipped hole saw specifically designed for metal, especially when working with thicker panels.
Why is the knockout size sometimes listed as 1 inch for 3/4 conduit?
The 1-inch size corresponds to the standard die size used in knockout punch size for 3/4 EMT. This size allows the locknut threads to fully seat against the metal surface of the box, ensuring a secure connection according to industry standards, even though the actual conduit inner diameter is smaller.
What is the best way to make a clean hole in a metal panel?
The fastest and cleanest method is using a hydraulic or manual knockout punch size for 3/4 EMT. If a knockout punch is unavailable, a sharp pilot hole followed by a slow-speed, lubricated metal hole saw is the next best option. Always file down any sharp edges afterward.
Does the hole size change if I use different types of conduit fittings?
Yes, the required installing conduit connectors hole size changes based on the fitting type. Set-screw connectors often need a slightly smaller hole than compression or threaded fittings. Always check the documentation provided by the fitting manufacturer to determine the precise EMT fitting hole saw diameter they recommend.