Short answer: in a sealed box, the wall material barely moves the electronics temperature — the air films on each face do. At 60 W inside a 20 x 16 x 8 in (508 x 406 x 203 mm) enclosure, 304 stainless settles about 1 C cooler than polycarbonate: a 21 K rise against a 22 K rise. Painted steel beats both by about 7 C, because its surface radiates strongly and the metal spreads heat over the whole shell. So you pick metal for heat spreading, bonding and shielding, and plastic for RF transparency, isolation and low weight. Past roughly 64 W in that box at a 25 C ambient, no wall material saves the electronics and you plan active cooling instead. The tables below give you the wall numbers, the worked example and the material call for each environment.
The wall numbers that decide it
Enclosure heat math wraps the whole wall into one coefficient: conduction through the sheet plus convection and radiation off the faces, all bundled. Our enclosure heat calculation guide uses 3.7 W/m²K for stainless, 5.5 W/m²K for painted steel and 3.5 W/m²K for plastic, with fiberglass sitting alongside the plastics. The air films set nearly all of it: a 0.06 in (1.5 mm) stainless sheet adds under 0.1% of the roughly 0.27 m²K/W total resistance behind the 3.7 figure, and even a 0.16 in (4 mm) polycarbonate wall adds about 7%. The metal-versus-plastic intuition from sheet conductivity — 16 W/mK against 0.2 W/mK in the handbook tables — mostly vanishes once the wall is thin and the heat leaves into air.
| Property | 304 stainless (bare #4) | Painted carbon steel | Polycarbonate |
|---|---|---|---|
| Wall heat transfer, assembled | 3.7 W/m²K | 5.5 W/m²K | 3.5 W/m²K |
| Sheet conductivity | 16 W/mK | 50 W/mK | 0.2 W/mK |
| Surface emissivity | about 0.3 brushed | about 0.9 painted | about 0.9 |
| Continuous temperature limit | none for the metal | coating-limited, about 100 C | about 100 C (typical datasheets) |
| Corrosion | pits in chlorides; cracks above 60 C | rusts where scratched | immune |
| Equipment grounding | bonds into the fault path | same | separate conductor per device |
| RF shielding | tens of dB | tens of dB | transparent |
Paint helps bare metal through emissivity: handbook values run about 0.3 for brushed stainless against about 0.9 for paint, and plastics sit near 0.9 as well. The rest of the painted figure's advantage is spreading — metal carries heat sideways, so the entire shell warms and sheds, while a plastic box sheds mostly from the hot patch around its sources. Keep that in mind: spreading is the one place sheet conductivity pays, and it returns in a moment.
Worked example: 60 W in a 20 x 16 x 8 in box
A wall-mount box 20 x 16 x 8 in (508 x 406 x 203 mm) presents 2 x (20 x 16 + 20 x 8 + 16 x 8) = 0.78 m² of external wall. The rise over ambient is the load divided by (coefficient x area), so the same 60 W of electronics gives:
| Wall | Arithmetic | Rise over ambient | Interior at 25 C ambient | Interior at 35 C ambient |
|---|---|---|---|---|
| 304 stainless | 60 / (3.7 x 0.78) | 21 K | 46 C | 56 C |
| Painted steel | 60 / (5.5 x 0.78) | 14 K | 39 C | 49 C |
| Polycarbonate | 60 / (3.5 x 0.78) | 22 K | 47 C | 57 C |
Read that against the 40 C ambient rating most drives and electronics carry. At a 25 C ambient, only the painted steel box holds its average interior at or under 40 C; at 35 C ambient, every material misses. The load each wall holds to a 40 C interior is the 15 K budget times its coefficient-area product:
| Wall | Holds 40 C at 25 C ambient | Holds 40 C at 35 C ambient |
|---|---|---|
| 304 stainless | 15 x (3.7 x 0.78) = 43 W | 14 W |
| Painted steel | 15 x (5.5 x 0.78) = 64 W | 21 W |
| Polycarbonate | 15 x (3.5 x 0.78) = 41 W | 14 W |
Two levers move these numbers before material does. Every 10 K of hotter ambient takes about a third off the budget, turning the stainless 43 W into 14 W. An extra inch of depth in the same footprint adds about 12% of wall: a 20 x 16 x 10 in box presents 0.88 m² against 0.78 m² and holds 12% more load.
Push the same box to 200 W sealed and the material argument ends: 200 / (3.7 x 0.78) = 69 K in stainless, 73 K in polycarbonate, 46 K in painted steel. Interiors sit at 94, 98 and 71 C at a 25 C ambient — past every electronics rating in the first two, and near polycarbonate's roughly 100 C continuous datasheet limit. That is the regime where you buy cooling rather than a different wall: a heat exchanger, a cooling unit or through-wall conduction. The why a sealed driver box runs hot article shows how a concentrated load outruns the wall average, and the sealed PoE switch sizing example runs the same arithmetic end to end.
Where metal wins
Metal's real thermal trick is lateral spreading, and it has a usable number. A fin-spreading estimate at a 5 W/m²K surface film carries heat about 70 mm across a 0.06 in (1.5 mm) stainless panel before the air takes over, and only about 13 mm across a 0.16 in (4 mm) polycarbonate wall. You bolt drivers, power supplies and cold plates to a metal back plate, and the whole panel becomes the heatsink; on plastic, each hot device needs its own finned sink or a bonded spreader plate. The through-wall conductive cooling article measures that path on a sealed box.
Grounding is structural in metal. NEC 250.96(A) requires metal parts serving as equipment grounding conductors — raceways, equipment and enclosures included — to be bonded together so they safely conduct any fault current likely to be imposed on them. The box is that bonded part. Plastic hands you the same job as wire: a green conductor to every device and a bus for them to meet on. Before reaching into any panel, isolate it and verify absence of voltage.
An unbroken metal cabinet also shields radiated noise by tens of dB, which keeps drive switching off nearby radios and sensors; plastic passes the band untouched. Mechanically, metal takes gland threads, hinge loads and hose impact without inserts, and a grounded wall sheds static so dust does not cling in a dry shop.
Where plastic wins
An antenna inside polycarbonate just works: a 0.12 in (3 mm) wall costs well under 1 dB at 2.4 GHz, while an unbroken metal panel blocks the band by tens of dB. With metal you route the antenna out through a rated gland or specify a window, and every one of those penetrations is a gasketed cutout that has to hold the seal. When radio gear is the payload, plastic is the honest choice.
The wall is an insulator, which you want around exposed terminals, mixed-voltage sections and bench or training hardware: there is no insulating sheet to fit behind devices and no bonded surface for a slipped probe to find. FRP adds chemical resistance around sanitizers and brine, where metals need careful grade selection.
Weight and corrosion carry the rest. Wall for wall, polycarbonate is 1.2 g/cm³ against 8.0 for stainless — about one-sixth the load on the hanger — and it cannot rust or pit. The chloride limit cuts the other way too: where salt or sanitizer deposits sit on a wall hotter than about 60 C (140 F), 304 and 316L both risk stress-corrosion cracking, and the molybdenum in 316L buys pitting resistance without moving that threshold. A plastic wall has no such temperature cliff.
Plastic pays for this in spreading and temperature: 0.2 W/mK in the sheet against 16, so hot spots stay hot, and typical datasheets cap polycarbonate near 100 C of continuous service and ABS near 70 C. Outdoors the tilt changes with solar gain and UV, which the outdoor metal-versus-plastic article covers. If a plastic box is fixed and the load is not, calculating internal temperature rise in sealed plastic enclosures applies the same film arithmetic to molded sizes.
The decision by environment
Start from the load and the hottest ambient you expect, then let the environment pick the surface:
| Environment | What to order | The number that decides |
|---|---|---|
| Dusty shop, sealed electronics up to 64 W | Painted carbon steel, Type 12 indoors | 64 W held to a 40 C interior at 25 C ambient, against 43 W in stainless |
| Food or beverage washdown | 304 stainless, Type 4X | hose-directed water and daily chemicals, with no coating to scratch through |
| Salt air, brine, coastal outdoor | 316L, Type 4X | chlorides crack both grades above about 60 C (140 F), so keep the walls under it |
| Radios, antennas or RF gear inside | Polycarbonate or FRP | under 1 dB loss at 2.4 GHz through 0.12 in of polycarbonate |
| Load past what the wall holds | Active cooling, not a different material | 69-73 K of rise at 200 W in the example box, whatever the wall |
| Bench or instrument build, 20 W | Either material; pick by RF, weight and durability | a 20 W load adds about 7 K in the example box |
Every type number above is a manufacturer rating for the specified design: our boxes are rated NEMA 4X / IP66 by the manufacturer and not UL listed, and NEMA does not test, certify or endorse enclosures.
When the load beats the wall, three escapes exist: a filtered fan package, a sealed heat exchanger or cooling unit, and through-wall conduction. The fan route spends rating to buy airflow — a louver or fan-filter holds only its own rating, so the assembly drops with it, as cooling louvers and what they cost a rating works through. For a box that must stay sealed, passive vents or a fan for electronics weighs the tradeoffs.
What to put on the order
- Material from the table above; when the only open call is grade, the 304 vs 316L guide settles it and the 304 or 316L at a glance page shortcuts the common cases.
- Size from the heat arithmetic, not the parts count: worst-case load, hottest ambient, wall figure for your material, then a common 25% margin so a hot day does not trip overtemperature protection.
- Depth is the cheapest capacity: one extra inch in the example footprint buys about 12% more wall and held load.
- Decide sealed versus vented before drawing cutouts, then rate every gland, vent and fan-filter to the target, because the weakest part sets the assembly rating.
- List each cutout with size and position; each one is gasketed in the drawing so the rating survives the field.
- Build route: 304 and 316L boxes are cut from one piece to your size and cutout layout, while polycarbonate and fiberglass come as molded sizes cut to your layout. Either way the box is delivered in 10 business days, counted from the order or from your approval of the drawing; the custom enclosure builder takes the dimensions directly.
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 | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 10 in | 304 stainless | 20 x 16 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Is aluminum better than stainless steel for cooling electronics in an enclosure?
Sealed and unpainted, aluminum can trail stainless: handbook emissivity for mill aluminum runs about 0.1 against about 0.3 for brushed stainless, so its assembled wall figure sits below the 3.7 W/m²K stainless value. Anodize or paint it and it reaches painted-steel territory near 5.5 W/m²K. Bulk conductivity is where it shines: common enclosure alloys conduct 140-170 W/mK against about 16 for 304, so clamped devices spread heat across an aluminum wall fastest of all.
Can I add vents or a fan to save a plastic enclosure that runs hot?
Yes — airflow beats the wall difference: every 10 CFM (4.7 L/s) of through-flow carries about 5.7 W per 1 C of rise, with air at 1.2 kg/m³ and about 1 kJ/kgK of specific heat. Two counts against it: the fan's own few watts land inside the box, and any vent or fan-filter holds only its own IP rating, so the sealed rating drops to the vent's. If the sealed rating matters, a heat exchanger moves the heat without openings.
Does 316L stainless cool any better than 304?
No: both grades conduct about 16 W/mK in the sheet and use the same 3.7 W/m²K assembled wall figure, so identical electronics run identically in either. Pick 316L for chlorides, salt spray and chemical splash, not for temperature: its molybdenum buys pitting and crevice resistance, and the chloride stress-corrosion threshold near 60 C (140 F) applies to both grades alike.
At what temperature does a plastic electrical enclosure fail?
Typical datasheets cap polycarbonate near 100 C of continuous service and ABS near 70 C, with heat deflection around 130 C for loaded polycarbonate. The worked example shows the limit is reachable: 200 W sealed in the 20 x 16 x 8 in box drives the polycarbonate interior to about 98 C at a 25 C ambient, and the wall beside a hot component runs hotter still. Above about 200 W sealed, the material itself becomes the constraint, so metal plus active cooling is the answer.
