What Are KO Fillers?

KO fillers, also called knockout fillers or knockout seals, are fittings used to close unused factory knockouts in electrical boxes, pull boxes, cabinets, wireways and similar enclosures. Most are designed to snap into or fasten into a standard knockout opening after a raceway entry has been removed or a layout has changed. In practice, they are used to restore a neat, closed opening rather than leaving a hole exposed. Depending on the product style, KO fillers may be simple snap-in metal discs, locking styles, or fittings intended for more demanding enclosure conditions.

Where Are KO Fillers Used?

KO fillers are used anywhere an electrical enclosure has an unused knockout. Common examples include junction boxes, device boxes, pull boxes, panelboard cabinets, troughs, disconnect enclosures, control panels and retrofit work where conduit routing changed after rough-in. They are especially common in commercial tenant improvements, industrial maintenance, service upgrades and renovation work, where original knockout plans often change. They are also useful in stockroom and facility maintenance settings because they let crews correct open holes quickly without replacing the enclosure.

How To Choose KO Fillers

Start with the knockout trade size. A filler that does not match the actual knockout opening will either not seat properly or may loosen over time. Next, consider the enclosure type and location. A standard indoor steel box in a dry mechanical room may only need a basic metal snap-in filler. A cabinet in a washdown, outdoor or corrosive area may need a filler with more suitable material or sealing characteristics. Also check whether the enclosure listing or environmental rating needs to be maintained. On some jobs, the right answer is not just any filler that fits, but one specifically identified for the enclosure condition. For maintenance buyers, it is often worth stocking the most common trade sizes used in boxes and cabinets to avoid field delays.

Trade Rules Of Thumb

A practical rule of thumb is to treat every open knockout as something that should be closed as soon as the final raceway layout is known. Another useful field rule is to keep KO fillers on hand for the knockout sizes your crews open most often, especially on service and retrofit work. For indoor dry locations, simple snap-in metal fillers are often adequate for routine box and cabinet work. For outdoor, washdown or corrosive locations, do not rely on a basic filler without checking suitability. As a buying rule, if the enclosure rating, contamination control or appearance matters, spend a little more time confirming the exact filler style instead of using whatever is in the truck. These are practical guidelines only and not a substitute for the Canadian Electrical Code, equipment listing requirements or site-specific engineering.

Sizing Guidelines

KO fillers are generally selected by the trade size of the knockout, not by measuring the visible hole casually with a tape. On the job, the safest approach is to identify the original knockout size from the box or enclosure documentation, or compare against known fittings used in that opening. If the knockout was made for a standard conduit or connector trade size, choose the filler intended for that same opening. For purchasing, the most useful stock sizes are usually the common knockout sizes found in junction boxes, cabinets and panel accessories, but actual demand depends on the type of work your crews do. If exact fit is uncertain, verify before ordering in volume because a small mismatch can make a filler unusable.

Common Installation Practices

Typical installation starts by checking that the knockout edge is not bent, burred or distorted from removal. The opening should be clean enough for the filler to seat fully. Snap-in styles are usually pressed into place until fully engaged, while locking or fastening styles are installed according to the product design. Installers generally confirm that the filler sits flat, does not rattle loose and does not interfere with nearby conductors, fittings or deadfront components. In panel and cabinet work, crews should also confirm that the chosen filler does not create clearance issues. Always follow the product instructions and applicable code requirements, especially where enclosure integrity or equipment listing is important.

Common Mistakes

One common mistake is using the wrong trade size and forcing the filler into the opening. Another is assuming any metal disc is acceptable in any enclosure, including wet or corrosive locations. Installers also sometimes leave open knockouts temporarily and then forget to return, which creates a poor finish and can expose the enclosure to debris or accidental contact. In retrofit work, another mistake is ignoring damage around the knockout edge after a slug was removed roughly. A filler may not hold properly if the opening is distorted. Buyers can also create problems by stocking only one style when their crews work across dry indoor, outdoor and industrial environments with different requirements.

Brand Comparisons

Bridgeport is widely recognized in fittings and is a common choice when crews want familiar trade sizing and dependable fit across routine commercial and industrial work. Hubbell is also a well-known name, often preferred where buyers already standardize around Hubbell-related product lines or want consistency with other enclosure and wiring device purchases. Ortech can be a practical value option for standard applications where the main requirement is closing an unused knockout correctly without overbuying. In the broader market, T&B and Iberville are also commonly encountered, especially where existing installations already use those brands or where contractor preference is based on local supply patterns. In many cases, matching the installed brand is not mandatory for a simple filler, but matching style, fit and application suitability usually matters more than the logo on the part.

Related Products

KO fillers are often purchased alongside electrical boxes, pull boxes, device boxes, locknuts, bushings, conduit connectors, strain relief fittings, reducing washers and enclosure accessories. On service and retrofit jobs, they are also commonly added to orders with panel accessories, wireway fittings and general box hardware. If your team frequently opens knockouts during rough-in and changes layout later, it can make sense to stock KO fillers together with common connectors and closure accessories so the crew can finish the enclosure properly without a return trip.

Frequently Asked Questions

What does KO mean in KO fillers?

KO stands for knockout. A KO filler is used to close an unused knockout opening in an electrical box, cabinet or enclosure.

Are KO fillers only for metal boxes?

They are most commonly used with metal boxes and enclosures that have standard knockouts, but suitability depends on the specific enclosure and filler design. Always confirm the part matches the application.

Can I use any KO filler outdoors?

No. Outdoor or wet-location use depends on the filler material, design and the enclosure requirements. If weather resistance or enclosure rating matters, verify the product is appropriate for that use.

How do I know what size KO filler I need?

Select the filler by the knockout trade size, not by guesswork. If you are unsure, check the enclosure documentation or compare with the fitting size intended for that opening.

Do KO fillers help with code compliance?

They are commonly used to close unused openings and leave the enclosure properly finished, but compliance depends on the full installation, the equipment involved and applicable code and listing requirements.

Should maintenance teams keep KO fillers in stock?

Yes. They are inexpensive, frequently needed on retrofit and service work, and useful for correcting open knockouts without replacing the enclosure.