Short answer: a field-punched knockout keeps the panel's UL listing when you close it with a listed conduit fitting rated for the enclosure type and install it per the fitting's instructions. An open hole, or a fitting rated below the enclosure's type, takes the panel outside its listing. How many holes the wall can take is a steel question, not a counting question: keep one full hole diameter of flat metal between opening edges. For trade-size 1 conduit that means a 1.375 in [35 mm] knockout with 1.375 in of steel beside it, centers 2.75 in [70 mm] apart. The same cut also asks whether the box keeps its NEMA rating, a separate question whether drilling affects an enclosure's rating answers; this page is about the listing of the finished assembly.
Does punching a new knockout void the panel's UL listing?
No, when the finished entry is a listed fitting of matching type, seated on a clean hole and installed per its instructions; that completed entry is what the listing covers, and plenty of panels are built for exactly this.
Enclosures arrive both ways: some with a full pattern of factory knockouts, some with a bare wall, so every conduit entry on the bare-wall kind is cut in the field. If a field cut by itself broke a listing, those products could never be installed as listed. What the listing covers is the completed assembly, and UL 508A's supplement on field-installed components says it plainly: a component goes in per its instructions and works within its marked ratings. A listed conduit hub, sealing fitting or connector marked for your enclosure type sits squarely inside that.
So your two touchpoints are the enclosure construction you started with and the fitting you add. Keep the hole to the fitting's stated size, keep it in a location the panel maker allows, and the listing follows the panel. For what a third-party listing is worth next to a manufacturer rating, read UL 50E vs NEMA 250: listed or manufacturer-rated.
When does a field-cut hole void the listing?
In four ways: a hole left open, a fitting rated below the enclosure type, a hole the fitting cannot seat on, and a location the panel's instructions keep off limits.
| What you find | What it breaks | The fix |
|---|---|---|
| Open knockout, no conduit yet | The enclosure boundary and contact protection | Listed closure plug of the right trade size |
| Indoor zinc fitting on a 4X wall | The enclosure type at its weakest link | Stainless or coated fitting rated 4X |
| Oversize or oval hole | Locknut or thread engagement | New hole at the fitting's stated diameter |
| Cut across a seam, fold or gasket face | Fitting seating and wall stiffness | Relocate to flat metal and patch nothing |
| Cut in a marked no-entry area | The construction the panel was listed with | Move to a zone the instructions show |
UL 50's Table 71-1A (13th edition) standardizes the knockout diameters behind conduit fittings, so punch to the diameter the fitting's instructions print — for trade-size 1 conduit, 1.375 in [35 mm]. Carry the enclosure type through the fitting, too: Type 4X enclosures earn the X with an extra 200 h of salt-spray exposure against a 304 stainless specimen in the NEMA 250 / UL 50E test tables, and a plated-steel fitting on a stainless wall rusts years before the wall does. Outdoors, conduit hubs vs sealing locknuts on top entries settles what to thread in; how to cut a clean hole in a stainless wall settles the cutting itself.
How many knockouts can one panel wall take?
The wall is full when you can no longer keep one hole diameter of flat steel between opening edges; the working limit is remaining metal, not a number of holes.
Work your side wall with real numbers. Say the top entries are full and the side offers 6 in [150 mm] of flat steel between the door seam and the first existing entry, and your new run is trade-size 1 conduit. Two entries need 1.375 in [35 mm] per hole plus 1.375 in of steel between them: 1.375 + 1.375 + 1.375 = 4.125 in [105 mm] edge to edge, which fits your 6 in with room to spare. A third entry adds another gap-and-hole pair, 2.75 in more, for 4.125 + 2.75 = 6.875 in [175 mm] — past what that wall holds. The wall takes two; the third goes to the opposite side or into a larger enclosure.
Counting breakers is not a sizing method, either. A 60-circuit panel ships with a wall its maker sized for that conduit load, and matching one knockout to every breaker multiplies holes far past any need. Every opening removes stiffness, and a gasketed door needs its flange flat to seal, so when entries crowd, spread them across walls or step up a box size — both cheaper than a wall that flexes when you lean on the door.
Does the panel's short-circuit rating allow extra holes?
No — a short-circuit current rating such as 22 kA states the fault current the assembly safely withstands until the upstream device clears, and it says nothing about conduit entries.
UL's own explainer on SCCR versus arc flash keeps the two apart, and they measure different failures. The SCCR is proven with a fault applied and the protective device clearing it; it is a withstand number for the assembly, and your new 1 in [25 mm] entry leaves it untouched. Arc-flash performance is the other animal: ordinary panelboards and control panels are not tested to contain an internal arcing fault, so no hole count changes an arc-flash study, and the incident energy at the panel — say 8 cal/cm² — sizes PPE for live work and nothing else. The interrupting rating on the main breaker belongs to the device, not the box.
So use the door numbers for what they say. With 10 kA available at the service and a 22 kA assembly, you have fault-withstand margin, and the cutting decision still comes down to the steel-spacing rule and the maker's entry zones.
Where should the new side entry go?
In the flat middle of the wall, at least one hole diameter from any seam, fold, gasket face or existing opening, with the fitting body clear of everything behind the wall.
One safety note first: lock out the feed, test for zero voltage at the terminals you will work near, and punch only with that compartment dead — a steel slug dropping across 480 V bus is the way this job goes wrong.
Then check, outside to inside:
- Outside the box: flat metal for the full fitting and its locknut; a conduit route that supports rigid conduit within 3 ft [900 mm] of the enclosure so the wall carries no lever load.
- Both sides of the wall: connector body, locknut and conductors clear of bus, terminals, insulators, barriers and door hardware before the punch, not after.
- At the terminal: bending space for the conductors — NEC 312.6 sets it by wire size, 2.5 in [63.5 mm] at a terminal for one 2 AWG conductor.
- In the panel's plan: land entries in the wiring compartment the instructions show, and keep conductor fill within what NEC 312.8 allows in cabinets holding switches or overcurrent devices.
What will the inspector ask to see?
The finished entry: a listed fitting whose marking matches the enclosure type, every hole closed, and the installation instructions that show your location is an allowed one.
The code path is short. In US installations, NEC 110.28 ties equipment to its location through Table 110.28, and enclosures covered there carry a type number from that table — a wet-location panel marked 4X with a 4X-rated fitting tells the whole story at a glance. On an industrial control panel, NEC 409.110 puts the enclosure type number on the panel, and UL 508A is the standard for listing one. In workplaces under OSHA, 29 CFR 1910.303(a) calls for approved equipment, and 1910.399 defines approved as accepted, certified, listed or labeled by a nationally recognized testing laboratory — which is why the fitting's listing mark carries so much weight.
If the panel's instructions are silent on side entries, ask the panel maker while the wall is still intact. A maker can confirm a proposed location on paper quickly; a ragged, unapproved hole is far harder to walk back. Keep the fitting instructions in the panel file with the one-line drawing.
When should the openings come cut from the factory?
With three or more entries, a repeat job, or a gasketed 4X door: order the enclosure with every opening cut, deburred and sized to its fitting before it ships.
A factory opening arrives at the exact diameter the fitting expects, with no burr under the gasket and no swarf in the wiring compartment, laid out by someone working from the flat pattern. For one or two entries on an existing panel, field punching with a listed fitting is normal shop practice. For repeat work, factory vs shop machining of enclosure cutouts covers the break-even, and choosing factory cutouts for repeat work shows how to lock the pattern once. The knockout diameter and gland sizing guide carries the dimensional table behind those standard diameters.
Decide it this way:
- One or two entries, existing listed panel, permitted zone available: field punch, listed fitting of matching type, close anything you open.
- Three or more, crowded walls, or repeat builds: a made-to-order box with factory openings; in our custom cutout designer you set the material, width, height, depth and every cutout in 304 or 316L stainless, delivered in 10 business days from your approval of the drawing.
- Instructions that fence off the whole side: settle it with the panel maker before cutting, or re-specify the enclosure.
Our enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed. The fitting and type logic above is the same either way, but a spec that demands a listed box needs a listed box.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 14 × 8 in | 304 stainless | 16 x 14 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can you add a knockout to the side of a listed panel?
Yes. Punch the hole, close it with a listed conduit fitting rated for the enclosure type, and install it per the fitting's instructions; the panel stays within its listing. Many enclosures ship with no knockouts at all, so field-cut entries are expected practice. Keep about one hole diameter of flat steel between openings so the wall keeps its stiffness.
What closes a knockout you end up not using?
A listed closure made for the enclosure type: a metal gasketed plug on a NEMA 4X wall, snap-in plastic caps only on dry indoor Type 1 boxes. Closures come in the standard trade sizes from 0.5 in to 2 in [12 to 50 mm]. Tape and unlisted plastic discs leave the hole as the weak point for water and fingers.
Is a panel's SCCR the same as an arc-flash rating?
No. An SCCR of, say, 22 kA is the fault current the assembly withstands until the upstream device clears it. Ordinary panelboards are not tested to contain an internal arcing fault, so hole count changes nothing there; incident energy in cal/cm2 sizes PPE for live work. Use the steel-spacing rule for cutting decisions, and the SCCR for fault withstand.
How far apart should knockouts be on a panel wall?
Keep one full hole diameter of flat metal between opening edges, and the same to any seam, fold or gasket face. For trade-size 1 conduit that is a 1.375 in [35 mm] hole with 1.375 in of steel on each side, putting centers about 2.75 in [70 mm] apart. When you cannot keep that spacing, the wall is full.
Do factory-cut openings hold a rating better than field punches?
They remove the field risks: the hole is cut to the fitting's exact diameter, with no burr under the gasket and no swarf inside. One or two entries on an existing panel is normal field work; for repeat jobs or sealed 4X doors, ordering the box with its openings cut is the cleaner, more repeatable path.
