Short answer: put any panel that carries 120 V AC in a real metal enclosure rated by its manufacturer, and pick the rating from the room — Type 12 for dry indoor dust, Type 4 in 304 stainless for hose-down washdown, Type 4X in 316L where salt or chemicals ride on the water. Size the backplate with 20-25% spare area, check the heat through the wall, and fix every cutout before you order. A plywood or open-frame cabinet fails the same job three ways at once: it gives fault current no metal path, it keeps out nothing, and it carries no rating you can write on a drawing.
Why plywood and open frames fail at 120 V AC
A rated enclosure does three jobs, and a timber box does none of them. It keeps fingers and dropped tools away from live parts. It gives the installation a bonded metal surface. It carries a type rating you can put on a drawing. Wood burns, splinters and soaks up washdown water, and an open frame does the opposite of enclosing.
Bond the box into the ground path. NEC 250.96(A) requires metal parts intended to serve as equipment grounding conductors to be bonded together so they can safely conduct any fault current imposed on them. Raceways, cables, equipment and enclosures all fall under that sentence. A ground lug on a steel wall takes ten seconds to fit; plywood gives you nothing to bolt it to. The grounding and bonding guide covers the rest of the path back to the source.
A homemade shell of any material — printed, plywood or folded sheet — has no tested rating behind it. The logic that sends a 3D printed outdoor enclosure to a rated box applies here too: once the panel leaves a dry bench, it belongs in a manufacturer-rated metal box.
The rating each environment needs
NEMA 250 defines each enclosure type and the tests behind it: Type 12 for indoor dust and drips, Type 13 for oil and coolant spray, Type 4 for hose-directed water, Type 4X for Type 4 plus corrosion resistance, Type 3R for outdoor rain and sleet, Type 6P for prolonged submersion. Read the table by the wettest thing that ever hits the box, not by the average day. One hose-down a week makes your panel a Type 4.
| Where the panel lives | What reaches it | Specify | Material that fits |
|---|---|---|---|
| Dry closet or control room | Dust and touch | Type 1 | Painted steel or 304 stainless |
| Plant floor, dusty | Dust and drips | Type 12 | 304 stainless or painted steel |
| Machine base | Oil and coolant spray | Type 13 | 304 stainless |
| Outdoor wall, no hose | Rain, sleet, sun | Type 3R or Type 4 | 304 stainless |
| Food or beverage washdown | Hose-directed water | Type 4 | 304 stainless |
| Coastal or chemical washdown | Hose plus salt or chemicals | Type 4X | 316L stainless |
| Pit, vault or flood line | Immersion | Type 6P | 316L stainless |
Two US notes round out the table. In US installations, NEC 110.28 states that IP ratings are not a substitute for enclosure type ratings. Its Table 110.28 lists which type suits which location, and carrying both a NEMA type and an IP code satisfies US and international specifications at once. Every rating in this article is a manufacturer rating for the specified design: our stainless wall-mount boxes are rated NEMA 4X-type / IP66 — not UL listed. A UL listed enclosure has been evaluated by UL to UL 50E, which is a separate question from the rating itself. The UL 50E vs NEMA 250 guide explains the difference, and the NEMA enclosure types guide carries the full type list.
The X is tested, not a marketing letter: under NEMA 250's Table 5-1D, Type 4X enclosures get 200 hours of salt spray alongside an AISI 304 stainless test specimen. The NEMA 4 vs 4X vs IP66 guide works through those tests.
304 or 316L stainless
The grade follows the chemistry of the water. 304 in a #4 brushed finish covers indoor washdown, food and beverage lines and most outdoor sites. 316L is the call for salt air, brine, chlorides, pharma and chemical areas, because its molybdenum raises resistance to pitting and crevice corrosion. The 304 vs 316L guide carries the full comparison.
Two boundaries decide the hard cases. About 60 C (140 F) is the chloride stress-corrosion-cracking threshold for both grades, so hot brine is 316L's problem too. And 304 can pit at room temperature where salty deposits sit in a gasket crevice, so an indoor location alone does not make it safe wherever brine dries on the shell.
Coastal air is its own call, and the closer the salt the stronger the case for 316L: the article on 304 stainless in coastal air walks that line for outdoor boxes. Wherever salt lands and dries, wet-dry cycling does the rusting — the wet-dry cycling article shows why rinse water and drainage matter as much as the grade.
How big the box should be
Start from your sketch, not from a catalog page. Add the backplate footprint of every mounted item, then hold back 20-25% spare area for the additions you have not drawn yet. Keep components 2 in off every wall so wiring duct can run around them, and let the door gasket take its own clearance. The enclosure sizing guide adds wire bending space and conductor fill on top of this arithmetic.
Worked example, for a 120 V AC control panel with a 24 V DC supply, relays and I/O terminals: say your layout totals 120 in² of component footprint. Spare at 25% sets the target: 120 × 1.25 = 150 in² of usable backplate. A 20 x 16 x 8 in (H x W x D) outside box leaves a mounting panel of about 19 x 15 in once walls and door lip take their half inch. With a 2 in duct band on all four sides, the usable area is (19 - 4) × (15 - 4) = 15 × 11 = 165 in², which clears the target by 10%. The 8 in of depth carries the duct frame, the devices on it and the door gasket. If your own sum lands over the usable area, step up a size or move I/O to a second box; the out-of-space article compares adding space against redesigning.
Then check the heat, because a sealed stainless box has one way out for watts: through the wall. Use k = 3.7 W/m²K for stainless walls, the value our sizing guides use, and the 20 x 16 x 8 in box presents about 0.78 m² of surface. Say the losses run 40 W. A 24 V DC supply delivering 240 W at 90% efficiency loses about 27 W on its own (240 × [1/0.9 − 1]); relays and a filter add the rest. Rise over ambient is 40 ÷ (3.7 × 0.78) ≈ 14 K, so at 25 C ambient the inside air sits near 39 C — just under the 40 C (104 F) that most control electronics carry unless a datasheet allows more. At 35 C ambient the inside reaches about 49 C and the panel cooks. Every 10 W you cut saves about 3.5 K, and a bigger box helps less than you expect, because area grows slowly.
Above that, cut the losses or cool actively. An air-to-air heat exchanger or a cooling unit sized to the load keeps the rating, while a fan-filter would drop the whole assembly to the fan unit's rating — a trade a washdown box cannot afford. Outdoors, add the sun to the load: the real heat gain article works through what a hot afternoon does to a box. Keep it sealed rather than vented, because an open vent breathes humid air in and takes the rating with it; the vent vs sealed article has the outdoor numbers.
Cutouts and cable entry before you order
Every hole is easiest at the factory. Specify the HMI window from the display module's outline, each operator device position, and every gland entry when you order; the cutouts, glands and windows guide covers placement. Enter from the bottom wall where you can — water landing on a top entry has one place to go, through the seal.
Match every gland to its cable and to the rating. The second digit of IP66 is the powerful-jet test, run with a 12.5 mm nozzle at 100 L/min, and a matched cord grip on a clean hole holds that level. An assembly is only as rated as its lowest part: an unrated vent, louver or snap-in plug sets the number the whole box carries.
If a hole appears late, mark it, pilot it, cut it, deburr it and seal it to the box's own level. The watertight sealing for custom cutouts page shows what a sealed hole needs. When cable entry starts to crowd one wall, a removable gland plate gathers it all in one panel — the gland plate article explains when that beats individual holes.
Mount it where you can work on it
Wall-mount boxes come with feet, and four stainless fasteners matched to the grade hold them. The bolts and anchoring article covers sizes and hardware.
Two heights set the mounting band. Keep the center of the highest breaker handle, at its highest position, at or below 2.0 m (6 ft 7 in) above the floor under NEC 240.24(A). In front, NEC 110.26(A) asks for at least 3 ft (900 mm) of clear working depth for equipment at 0-150 V to ground in Condition 1, at least 30 in wide or the equipment width, and room for the door to open 90 degrees.
Keep the door flange and gasket clean, and set the latches so the gasket compresses evenly along the whole flange. An even squeeze is what the hose test sees.
What to put on the order
| Line | Example entry |
|---|---|
| Material | 304 stainless, #4 brushed finish; 316L for salt, brine or chemical areas |
| Outside size, H x W x D | 20 x 16 x 8 in (the worked example; your layout sets yours) |
| Rating | Manufacturer-rated NEMA 4X-type / IP66 — not UL listed |
| Door | Hinged gasketed door, removable mounting panel inside |
| Cutouts | HMI window and operator holes to your drawing; 4 x M20 gland entries, bottom wall |
| Hardware | Wall-mount feet; stainless bolts matched to the grade |
| Delivery | Delivered in 10 business days, anywhere in the world |
Custom runs from one piece, so the size in your sketch is the size you get. Delivery is 10 business days, anywhere in the world — counted from your order, or from your approval of the drawing for custom work. If you would rather filter than calculate, the enclosure selector narrows the choice by environment, listing needs, inside size and DIN module count, and the custom stainless enclosures page takes your drawing straight to a quote.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 20 × 16 × 8 in | 316L stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 10 × 6 in | 316L stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 316L stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 14 × 8 in | 316L stainless | 16 x 14 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does a dry indoor panel need stainless steel?
No. In a dry electrical closet under 30 C (86 F), a Type 1 painted-steel box protects against contact and a Type 12 box adds dust and drip protection. Stainless earns its place once water appears: hose-down makes it Type 4 in 304, and salt or chemicals in the water make it Type 4X in 316L. Material and rating are two separate lines of the same decision.
How much spare space should a new panel leave?
Plan 20-25% spare on top of the mounted footprint. Multiply your component area by 1.25 for the minimum usable backplate: the article's 120 in² example needs 150 in², and the 20 x 16 x 8 in box delivers 165 in² usable. That margin absorbs a second DIN rail and the next I/O card without moving a terminal.
Is IP66 enough for an outdoor enclosure?
For water, mostly: the second 6 is the powerful-jet test, a 12.5 mm nozzle at 100 L/min, which out-performs most rain. In the US, though, NEC 110.28 states IP ratings are not a substitute for enclosure type ratings, so an outdoor specification should name the NEMA type too — Type 3R or 4 for rain, 4X where corrosion counts. Sun and salt are material calls, not IP digits.
When does 304 stainless stop being enough?
When chlorides ride on the water: coastal salt air, brine, process chemicals, pharma washdown. Remember the two boundaries — about 60 C (140 F) is the chloride stress-corrosion threshold for both grades, and 304 can pit at room temperature under salt deposits held against the metal in gasket crevices. Away from direct salt spray, 304 outdoors is often fine; beachfront goes 316L.
Can I drill extra holes on site and keep the rating?
Yes, if every hole is sealed to the box's own level. Match each cord grip to its cable, deburr the hole, and keep entries off the door where the gasket sits. The assembly rating is the lowest-rated part: an open knockout, a louver or a snap-in plug with a lower IP becomes the number the whole box carries.
