A small polycarbonate miniature case with molded knockouts and a small stainless enclosure with factory-cut gland holes solve the same problem two different ways. The polycarbonate case is fast: snap out an M12 to M50 knockout, screw on a cord grip, mount it on feet, done. The stainless box is durable: it survives washdown, sun and chemicals, and every cable entry is a clean, factory-made hole sized to the gland. Pick polycarbonate for protected indoor junction duty where speed and cost drive the job. Pick stainless for washdown, outdoor, or corrosive sites, or anywhere you must state an ingress rating and stand behind it.
The short answer
Use a molded-knockout polycarbonate case when the box lives indoors, stays dry, carries a few small cables, and nobody hoses it down. Use a small stainless enclosure when the location sees water, cleaning chemicals, sunlight, or physical abuse. The knockout itself is the deciding detail: a molded knockout is a pre-weakened spot in the wall, and the moment you remove it, the seal you get is the fitting's seal, not the case's. A factory-cut gland hole in stainless is a machined opening that stays under your control. For a deeper look at plastic boxes outdoors, read is a plastic junction box watertight outdoors.
Side by side
| Polycarbonate miniature case (molded knockouts) | Small stainless enclosure (factory gland holes) | |
|---|---|---|
| Typical size range | Roughly 4 x 4 in up to about 12 x 10 in | 6 x 4 x 4 in to 12 x 10 x 6 in in our catalog, larger on request |
| Cable entry | Molded metric knockouts, M12 to M50, snapped out on site | Holes cut to your gland sizes at the factory |
| Rating after entry | Depends on the fitting; the open knockout breaks the seal | Rated NEMA 4X / IP66 by the manufacturer; not UL listed, with matched glands |
| UV behavior | Degrades over years unless UV-stabilized; chalks and embrittles | No degradation; 304 holds outdoors away from heavy salt spray |
| Chemical resistance | Good against many acids and salts; attacked by some solvents and alkaline cleaners | 304 suits most food and cleaning chemicals; 316L for heavy chloride exposure |
| Hinge and closure | Plastic strap hinge, tamper screws or screw cover | Metal hinged gasketed door with latch |
| Impact | Thick-wall versions take a hit; thin walls crack in cold | Steel wall shrugs off ladders and tools |
| Grounding | No grounding path; needs internal bonding of metal raceways | Full metallic equipment grounding path |
| Installation speed | Fastest: snap knockout, fit gland, mount on feet | Fastest when cutouts are factory-made; no field drilling |
| Best duty | Clean indoor junctions, signal wiring, small terminals | Washdown, outdoor, corrosive, or anything with a stated rating |
The general trade between plastic and metal boxes is covered in GRP vs stainless steel junction box selection; this article stays on the miniature end, where the knockout decides most jobs.
Where the polycarbonate case wins
Speed is the real product. Molded knockouts from M12 to M50 mean no drill, no hole saw, no deburring, and no swarf in the box. A through-base mounting pod outside the gasket seal lets you screw the case to the wall without piercing the sealed volume. Tool-free strap hinges and tamper-resistant screws cut installation minutes further.
Nonconductive walls suit low-voltage signal work. A plastic box needs no bonding jumper to the wall, weighs a fraction of steel, and never corrodes at a scratched screw hole. Thick-wall miniature cases with reinforced corners absorb the kind of knocks a junction box under a desk or inside a machine frame actually receives.
Cost favors plastic at small sizes, and the molded DIN-rail mounting pods let you land a terminal strip or a small relay board without a backpanel. For a controls cabinet interior, a junction room, or a sheltered machine bay, that package is hard to beat. If your question is whether a plastic box can go outside at all, the rating facts are in the nonmetallic enclosures guide.
Where the stainless box wins
Washdown ends the argument. Hot, pressurized water finds every pre-weakened knockout edge, every plastic hinge, and every screw cover gasket. A hinged gasketed stainless door, rated NEMA 4X / IP66 by the manufacturer, is built for hose-down duty. The details that matter in washdown zones, including water temperature, are covered in washdown water temperature and your enclosure.
Sunlight is the second. Ultraviolet exposure slowly embrittles polycarbonate; a stainless wall does not care. For permanent outdoor mounts, 304 stainless is the default, and the limits of 304 near salt water are covered in can 304 stainless be used outdoors in coastal air.
Seal integrity is the third, and it is the one engineers miss. Removing a knockout leaves a molded edge whose flatness and roundness you did not machine. A factory-cut hole in stainless is cut to the gland maker's dimension, deburred, and sealed by a cord grip torqued against a flat wall. The rating you keep after mounting devices is covered in what NEMA rating a panel keeps after you cut it and mount devices. Finally, a metal box gives you a real equipment grounding path, which a nonmetallic box cannot; the grounding rules are in the grounding and bonding guide.
The knockout question, in detail
A molded knockout is a membrane, not a fitting. While it is in place, the wall is continuous. The instant you snap it out, three things happen:
- The opening edge is as-molded, with draft and possible flash, so the gland's sealing surface is less predictable than a machined face.
- The knockout waste can fall inside the box if you are not careful, and a sliver of plastic across a terminal is a fault looking for a place to happen.
- Any knockout you remove and do not fill is an open hole, and a half-snapped knockout left proud of the wall can catch a wipe or a glove.
Metric knockouts in the M12 to M50 range match standard metric cord grips, so the fitting choice is easy. Match the gland's clamping range to your actual cable diameter; that matching job is covered in cable gland clamping range. If you expect to run cables with oversized molded connectors, read industrial cable gland solutions for large molded connectors before you pick the entry size, because a connector that will not pass through the gland changes the whole entry design.
Stainless enclosures take the opposite approach: you specify every hole before the box is built. That costs you a drawing cycle but buys a hole that is exactly the gland's dimension, in a wall thick enough for full thread engagement. Thin plastic walls are a different story, compared in rubber grommet vs cable gland for a thick panel.
Sizing and hardware differences
A miniature polycarbonate case is usually just a shell. Subpanels, DIN rails and inspection windows are accessories you add, and the usable depth shrinks with each one. A stainless enclosure of the same footprint gives you a removable mounting panel behind a hinged door, so terminals land on the panel and the door stays clear.
Working sizes from our catalog make the comparison concrete. A 6 x 4 x 4 in 304 enclosure holds a small terminal block set with gland holes in the base. An 8 x 6 x 4 in 304 enclosure fits a DIN rail strip and a breaker or two. Step to a 10 x 8 x 4 in 304 enclosure and you have room for a small power supply. A 12 x 10 x 6 in 304 enclosure keeps nearly the full footprint usable inside, and swallows a radio, a small controller and its wiring with room to spare. All are rated NEMA 4X / IP66 by the manufacturer; not UL listed, and all are delivered in 10 business days from your order, or from drawing approval for custom work.
The decision by environment
- Indoor, clean, signal-only: polycarbonate miniature case. Speed and cost win; nothing in the environment stresses it.
- Indoor with occasional wipe-down: either, if the plastic case's rating covers the fittings. Stainless if the wipe-down is a hose.
- Food, beverage, pharma washdown: stainless, hinged gasketed door, glands in the base, hinges out of the spray lane.
- Outdoor, sheltered: UV-stabilized plastic can serve; stainless is the set-and-forget choice.
- Outdoor, full weather or coastal: 304 stainless inland, 316L near salt water or heavy chlorides.
- Anywhere an inspector wants a type rating on the box: stainless with a stated NEMA type, or a plastic case with the equivalent rating, and glands that match it.
The stainless enclosure selector walks the environment and size questions when the stainless route is your next step, and 304 stainless enclosures shows the small-size range directly.
What to specify
For the stainless route, the order carries five decisions: grade (304 or 316L), outside height, width and depth, every gland hole with its diameter and location, the mounting-panel option, and door hardware. Put each cable entry on the drawing with its gland size, not "a few holes in the bottom." State the environment, because washdown and coastal sites change the grade and the gasket choice. Gasket material trade-offs are covered in foam or rubber gasket for an enclosure door seal and in the stainless steel enclosures guide.
For the polycarbonate route, specify the case size, which knockouts you will remove, the fittings for each, and the mounting method. Confirm with the case's documentation that the rating survives knockout removal, and buy sealed plugs for every knockout you snap out and do not use. Treat the plastic case as a component with a service life in sun, and as a nonconductive box that still needs its metal raceways bonded inside.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 6 × 4 × 4 in | 304 stainless | 6 x 4 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 8 × 6 × 4 in | 304 stainless | 8 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 10 × 8 × 4 in | 304 stainless | 10 x 8 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can I tap a stainless enclosure for M20 glands instead of ordering cutouts?
Yes, with the right tooling, but a field-drilled hole is the weakest link in the sealing chain: any burr, wrong hole saw speed or missing gasket seating can drop the box below its rated performance. Ordering the enclosure with factory-cut gland holes keeps the cut clean, the wall flat and the seal surface correct. It also saves hours on site and removes the risk of swarf left inside the box. If you must drill in the field, follow the gland maker's hole size exactly and deburr both sides.
Do molded knockouts hold a NEMA 4X rating?
A molded knockout holds its rated ingress protection only while the membrane is intact. Once you snap out a knockout, the rating depends entirely on the fitting you install: a properly torqued cord grip or sealed plug maintains a watertight entry, while an open or partially removed knockout breaks the seal completely. Check whether the case's rating was ever evaluated with knockouts removed. For stainless enclosures rated NEMA 4X / IP66 by the manufacturer, only listed or matched glands preserve that performance after cutting.
Which is better for a small outdoor junction box, polycarbonate or stainless?
For permanent outdoor duty, stainless is usually the safer choice. Sun degrades most plastics over years, so a UV-stabilized polycarbonate case works but has a finite weathering life, while 304 stainless does not embrittle or chalk. Stainless also shrugs off impacts from ladders, tools and vandalism that would crack a plastic wall. Use polycarbonate outdoors only for sheltered, low-risk locations, and paint-free stainless everywhere the box must survive a decade of weather.
Is a miniature polycarbonate case good for washdown areas?
Only if it carries an appropriate rating with the knockouts removed and fittings installed, which many miniature cases do not. Food and beverage washdown adds hot, high-pressure water and chemicals that stress plastic hinges, screw covers and gaskets. A stainless enclosure with a hinged gasketed door, rated NEMA 4X / IP66 by the manufacturer, is built for exactly that duty. Keep the hinges and latch on the side away from the spray lane.
