How to Reduce an Oversized Knockout Without Compromising the Bonding Path
Match the existing knockout to the desired fitting trade size, then verify quantity, orientation, listing, conduit compatibility and bonding.
An electrical reducing washer adapts a larger knockout in a metal box or enclosure to a smaller conduit-fitting trade size. Select it by matching the existing knockout size to the desired fitting size—but treat that as only the mechanical decision. Before use, verify the exact washer’s listing, conduit compatibility, installation instructions, enclosure and knockout condition, and any separate grounding or bonding requirements, especially at service equipment.
For pump-system maintainers, this mismatch may arise at a metal control enclosure or disconnect. The same rule applies: a washer that closes the mechanical gap does not, by itself, settle the electrical bonding question.
What a reducing washer does—and what the size means
This guide concerns electrical reducing washers, not plumbing reducers or similarly named general-hardware washers. The electrical fitting spans an oversized box or enclosure knockout and provides a smaller opening for the selected conduit fitting.
The notation reads:
Existing knockout trade size × desired fitting trade size
A 1 × 3/4-inch reducing washer, for example, adapts a 1-inch trade-size knockout for a 3/4-inch trade-size fitting. A 3/4 × 1/2-inch washer adapts a 3/4-inch knockout for a 1/2-inch fitting.
These are nominal trade sizes. Do not place a ruler across the opening and assume its measured diameter is the conduit trade size. Identify the knockout from the enclosure documentation or markings, or use dimensional information for the exact fitting system.
Size-selection table: match the opening to the fitting
Start with two values:
- The trade size of the knockout already open.
- The trade size of the fitting that must enter it.
Then choose the corresponding reduction. The part numbers below are LH Dottie examples, not universal identifiers; they are documented in the manufacturer’s reducing-washer catalog.
| Existing knockout | Desired fitting | Washer notation | Sourced example |
|---|---|---|---|
| 3/4 in. | 1/2 in. | 3/4 × 1/2 in. | LH Dottie RW21 |
| 1 in. | 1/2 in. | 1 × 1/2 in. | LH Dottie RW31 |
| 1 in. | 3/4 in. | 1 × 3/4 in. | LH Dottie RW32 |
| 1-1/4 in. | 3/4 in. | 1-1/4 × 3/4 in. | LH Dottie RW42 |
| 1-1/4 in. | 1 in. | 1-1/4 × 1 in. | LH Dottie RW43 |
| 1-1/2 in. | 1-1/4 in. | 1-1/2 × 1-1/4 in. | LH Dottie RW54 |
| 2 in. | 1-1/2 in. | 2 × 1-1/2 in. | LH Dottie RW65 |
Seattle City Light’s project standard includes these seven reductions among its approved configurations. That standard is evidence that the sizes are established products, not a complete list of everything available commercially.
Part numbering remains manufacturer-specific. A matching size—or a similar catalog number—does not establish identical construction, certification, dimensions, conduit compatibility, or installation requirements. Check the exact:
- Manufacturer and catalog number
- Material and finish
- Product certification and markings
- Permitted conduit or connector type
- Quantity required at each knockout
- Installation instructions
Commercial catalogs contain many additional sizes and models. A partial manufacturer or project list should not be treated as the complete market.
Basic installation arrangement: verify the product before assembly
Installation quantity depends on the product or specification. American Fittings says its galvanized-steel RW Series requires two washers per knockout, covers nominal sizes from 1/2 through 4 inches, and is intended for rigid or IMC steel conduit. The manufacturer also identifies UL 514B and NEMA FB1 among the standards associated with that series (American Fittings RW Series).
Seattle City Light likewise specifies one washer inside and one outside for the products covered by its fittings and reducing-washer standard. Neither specification proves that every product sold as a reducing washer always requires two pieces.
For a product that calls for a pair, the conceptual relationship is:
Smaller fitting or conduit
│
▼
[outer reducing washer]
[enclosure wall and oversized opening]
[inner reducing washer]
[hardware required by the fitting design]
This is not a universal assembly sequence. Connector shoulders, locknuts, bushings, and other hardware vary by fitting design. Use the instructions for the exact washer-and-fitting combination to determine:
- Inside and outside placement
- Raised-side orientation
- Tightening method
- Permitted enclosure-wall thickness
- Surface preparation
- Required locknut, bushing, or other hardware
Trade-forum participants commonly report placing the raised portions in the opening to help center the washers. Some also report reversing washers when thick sheet metal prevents tightening. Those field reports are not model-specific installation authority and do not establish that reversal is permitted by a product listing. Do not reverse washers, add unsupported stacks, or substitute hardware merely to make the assembly clamp down.
This guide assumes the equipment has been de-energized and absence of voltage has been verified under the applicable safety procedure. It does not provide an energized-work procedure. If the equipment, conductor status, or safe-work requirements are uncertain, stop and use a qualified electrician.
Mechanical fit is not the same as grounding and bonding
A secure mechanical fit establishes that the fitting is held in the opening. It does not by itself establish an acceptable fault-current path.
The result depends on the complete installation, including:
- Exact washer listing and markings
- Enclosure type and listing
- Service or non-service status
- Voltage to ground
- Condition of the opening and remaining rings
- Required conductive contact
- Manufacturer instructions
- Locally adopted code
- Utility or project requirements
- Authority having jurisdiction
UL 514B and NEMA FB1 recur in the supplied manufacturer and utility specifications. That does not mean every reducing washer has identical certification, limitations, or conduit compatibility. Verify the current certification information and markings for the exact product rather than relying on a retailer category, package appearance, or forum quotation.
A tight locknut or a basic continuity indication should not be treated as application approval. Whether a metal raceway may participate in the equipment-grounding path is a listing-and-installation question, not merely a fit test.
Bonding decision guide: service versus non-service installations
This table is a screening tool, not a compliance determination. Current adopted-code provisions, current product certification information, equipment instructions, and the AHJ control the actual installation.
| Question | Screening result | What must be verified |
|---|---|---|
| Are service conductors or service equipment involved? | Do not rely on reducing washers alone. | UL explains that service-bonding provisions treat reducing washers as an impaired connection requiring an appropriate bonding method around the connection (UL grounding and bonding guidance). |
| Is this a non-service installation? | Conditional suitability may exist for a particular listed metal washer. | Verify the current listing, instructions, enclosure, rings, surfaces, voltage, adopted code, and AHJ requirements. |
| Do concentric or eccentric rings remain? | Do not assume the rings provide bonding continuity. | Determine whether the specific enclosure’s knockouts are listed for bonding or whether another approved bonding method is required. |
| Has knockout integrity been compromised? | Do not rely on the connection without further evaluation. | Check the enclosure instructions and required bonding provision; obtain AHJ review where applicable. |
| Is a conductor-connection point required at the fitting? | A listed grounding or bonding bushing may be applicable. | Match the bushing, fitting, conductor, enclosure, and installation to current requirements. |
For non-service installations, historical UL category language reproduced in trade sources described conditional grounding suitability for listed metal reducing washers. Because the available material is historical, the following are verification points, not current universal rules:
- A qualifying metal enclosure
- A reported minimum enclosure thickness of 0.053 inch
- Non-service conductors
- Removal or permitted treatment of concentric and eccentric rings
- Knockouts listed for bonding when rings remain
- Required conductive contact
- Installation under the applicable electrical code
A 2015 UL Question Corner article, drawing on the 2014 UL White Book, addressed panelboards and safety switches with remaining concentric rings. It called for a listed bonding bushing unless the knockout arrangement qualified for the stated bonding exception (IAEI explanation). That article is useful historical guidance, but it must be checked against current certification records and the locally adopted code.
A listed grounding or bonding bushing provides a means of connecting a grounding or bonding conductor. Whether one is required depends on the actual enclosure, knockout arrangement, service status, voltage, fitting, and governing requirements. There is no single arrangement that can be prescribed for every reducing-washer installation.
Concentric knockouts, coatings, and damaged openings
Concentric and eccentric knockout rings introduce additional joints into the connection. Their presence does not automatically establish bonding continuity.
UL distinguishes qualifying metallic outlet boxes evaluated for bonding through concentric or eccentric knockouts from other enclosure categories. Do not automatically extend an outlet-box evaluation to:
- Panelboards
- Safety switches
- Cabinets
- Cutout boxes
- Junction boxes
- Pull boxes
Check the specific enclosure’s listing and markings. Appearance alone does not establish how its knockouts were evaluated.
If knockout integrity has been compromised during installation, UL says a bonding jumper should be provided to maintain the required fault-current path (UL guidance for compromised knockouts). The final method still has to agree with the enclosure instructions, current adopted code, and any required AHJ determination.
Do not make assumptions about paint, lacquer, enamel, powder coating, oxidation, or other material at contact surfaces. Follow the exact fitting and enclosure instructions and the applicable current code requirements. Likewise, do not scrape, grind, drill, or add a blank plate unless the alteration is supported by the enclosure documentation and applicable approval.
An improvised patch may solve the visible opening while leaving the listing and bonding questions unanswered. If a factory knockout is damaged or unsuitable, obtain an approved repair or redesign rather than treating a washer stack as a universal fix.
Pre-installation selection checklist
Before buying or using a reducing washer, record and verify:
- [ ] Existing knockout trade size
- [ ] Desired conduit or fitting trade size
- [ ] Washer notation, with the larger opening first
- [ ] Exact manufacturer and catalog number
- [ ] Metal or nonmetal construction
- [ ] Finish and corrosion-protection requirements
- [ ] Compatible conduit or connector type
- [ ] Current product listing and visible markings
- [ ] Required number of washers per knockout
- [ ] Required orientation
- [ ] Required locknut, bushing, or other hardware
- [ ] Enclosure material and permitted wall thickness
- [ ] Condition of the opening and remaining knockout rings
- [ ] Whether the enclosure’s knockouts are listed for bonding
- [ ] Service or non-service status
- [ ] Voltage to ground
- [ ] Surface and conductive-contact requirements
- [ ] Separate bonding jumper, bushing, or other provision
- [ ] Locally adopted electrical code
- [ ] Project, owner, or utility specification
- [ ] AHJ acceptance where required
Catalogs include steel and galvanized-steel washers with zinc-plated, galvanized, and ball-burnished finishes. These attributes vary by model; finish alone does not establish certification or bonding suitability.
American Fittings documents its RW Series for rigid or IMC steel conduit. That documentation does not establish compatibility with EMT, flexible conduit, PVC fittings, or cable connectors. Other models must be evaluated from their own certification and manufacturer documentation.
Project-approved lists can also be narrower than commercial availability. Seattle City Light’s standard specifies selected sizes, named manufacturers, steel construction, specified finishes, and paired installation for the stock items it covers. A commercially available part is not automatically approved under that project standard.
If the product listing, fitting compatibility, knockout integrity, or bonding method cannot be verified, stop. Do not treat the reducing washers as the complete solution without review by a qualified electrician or the AHJ.
Can a reducing washer be used at service equipment?
It may provide the mechanical size reduction, but it must not be treated as the complete service-bonding method. UL’s service guidance identifies reducing washers as an impaired connection around which an appropriate bonding method is required. Confirm that method against the current adopted code, equipment instructions, utility requirements, and AHJ.
Do reducing washers come in pairs?
Some products or specifications require two washers per knockout. American Fittings specifies two for its RW Series, while Seattle City Light specifies one inside and one outside for the products covered by its standard. Packaging quantity is not the same as installation quantity, so follow the exact product instructions.
Can I use a rigid/IMC reducing washer with EMT or flexible conduit?
Not from a rigid/IMC specification alone. Documentation for rigid or IMC steel conduit does not establish suitability for EMT, flexible conduit, PVC fittings, or cable connectors. Verify both the washer and fitting for the intended combination.
Which way should the raised side of a reducing washer face?
The supplied evidence does not establish one universal orientation for every model. Trade discussions say the raised portion commonly helps center the washer, but reports of reversing it to accommodate thick sheet metal do not establish listed use. Follow the manufacturer’s orientation and wall-thickness instructions. If those instructions are unavailable or the fitting will not tighten correctly, choose a documented alternative.
Select the reducing washer by the existing knockout and desired fitting trade sizes, then verify the exact product’s quantity, orientation, listing, and conduit compatibility. Before energizing, resolve the separate bonding question from the service status, enclosure and knockout listings, ring condition, current adopted code, and AHJ requirements. If any part remains uncertain, obtain qualified electrical review rather than assuming a tight metal connection is adequate.