Short answer: for a sealed box bolted in full sun, pick bright metal over dark plastic. Under 1044 W/m² of peak sun at 35 °C ambient, a bright stainless box lands about 10 K cooler inside than a dark plastic one of the same size and load. The metal also gives your board a ground path that plastic cannot. Finish does most of that work: a dark skin absorbs about 0.9 of the incident sun, bright bare metal about 0.3. Plastic still wins when weight, cost or radio transparency drive the job and the load is a few watts in shade. The worked example below uses a 16 x 12 x 8 in box, so you can drop in your own size and load.
What the sun does to a sealed box
A sealed box sheds heat only through its walls. Its interior therefore settles at ambient plus two rises: what your electronics dump in, and what the sun puts through the skin. The sun's share is set by absorptivity, the fraction of incident radiation the surface absorbs. One enclosure solar-load calculation puts grey paint, absorptivity 0.50, at a 7.7 K rise under 1044 W/m² of sun, close to the clear-noon peak. Rise tracks absorbed fraction, so any other finish reads off that anchor: divide 7.7 K by 0.50, then multiply by your surface's absorptivity.
Internal heat is a different story, because the wall material barely moves it. The wall conduction values used here are 3.7 W/m²K for stainless, 5.5 W/m²K for painted steel and 3.5 W/m²K for plastic. On load alone, stainless and plastic finish within half a degree of each other. Colour separates the two, not the polymer or the alloy. Roof studies reach the same ordering: light colours hold an outer surface measurably cooler than high-absorptivity ones. Metal adds one quiet advantage, since it carries the absorbed sun sideways over the whole skin, while plastic leaves a hotter spot on the lit face. The orientation-by-month version of this arithmetic is in the solar heat load and temperature rise article.
Metal vs plastic, side by side
Same box, same 10 W load, same 35 °C afternoon in peak sun; the skin swaps, everything else holds.
| What you compare | Bright stainless | Dark plastic |
|---|---|---|
| Surface absorptivity (standard surface data) | about 0.3 | about 0.9 |
| Solar rise at 1044 W/m² peak sun | 4.6 K | 13.9 K |
| Wall conduction | 3.7 W/m²K | 3.5 W/m²K |
| Rise from a 10 W load, 16 x 12 x 8 in box | 5.0 K | 5.3 K |
| Interior at 35 °C ambient, peak sun, 10 W | 44.6 °C | 54.2 °C |
| Ground path for the board | standoffs and mounting panel | a wire you fit |
| Antenna inside the box | needs a feedthrough or an outside mount | RF passes through |
| Corrosion and UV | none; pick 304 or 316L by site | no corrosion; UV embrittles unprotected skins |
| Lid screws | stainless galls; use anti-seize | no galling |
| Weight and cost on a pole | higher | lower |
The rises come from the 7.7 K grey-paint anchor at 1044 W/m² and the wall values above.
Where metal wins
The sun margin. A bright skin plus lateral conduction is worth about 10 K in the example below, and 10 K is often the gap between a board that idles and one that throttles. Mill-finish aluminium behaves like brushed stainless here, which is why so many sealed light-alloy boxes live on poles in full sun. Installers who have run sealed, unventilated metal boxes for years report winters past -18 °C and summer afternoons past 38 °C without heat or cold failures.
The ground path. A metal box, metal standoffs and a metal mounting panel bond the board's ground to the box with no extra parts. Static is why that matters: discharge from storms and dry blowing snow is what kills ungrounded Ethernet ports in the field. Tie the box into the installation's grounding system with the circuit off and absence of voltage proven. The grounding and bonding guide covers what else on the installation has to be tied together.
Skin that outlives the electronics. Stainless shrugs off the UV that embrittles unprotected plastic within a few seasons outdoors. One habit comes with the metal: put a smear of anti-seize on the lid screws at assembly, because stainless threads gall and a seized screw can shear. Installers who skip it describe the lids as welded shut after a few seasons, and clean wax holds as a field substitute when no anti-seize is on the truck.
Where plastic wins
Weight, cost and radio waves. A plastic box weighs a fraction of its stainless twin and costs less at the same size, which is why it owns the pole-mounted radio job. RF passes straight through it, so the antenna can live inside the box. Metal forces the antenna outside or into a deliberate feedthrough plan, which the radio signal through a metal enclosure design options work through.
No corrosion, no galling. Plastic ignores salt spray and never seizes a screw thread.
The colour rule binds plastic too. A light grey, UV-stabilized skin absorbs about 0.5 of incident sun, the same as grey paint, so it takes the same 7.7 K solar rise. It is the dark skins that cook boards. Water tightness comes from the gasket and the glands, not the polymer, as the plastic junction box watertightness question makes clear. We cut polycarbonate and fiberglass to your layout, so the non-metallic path can still take your full cutout drawing.
A 16 x 12 x 8 in sealed box at 35 °C ambient in peak sun
The inputs:
- Box: 16 x 12 x 8 in (H x W x D), sealed and unventilated; this is the example's assumption, so substitute your own size.
- Ambient: 35 °C, a hot design afternoon.
- Peak solar irradiance: 1044 W/m², the figure the enclosure calculation uses, near the clear-noon maximum.
- Solar anchor: grey paint at absorptivity 0.50 gives a 7.7 K rise at that irradiance.
- Finish absorptivity: 0.3 for bright bare metal and 0.9 for a dark skin, standard surface data.
- Wall conduction: 3.7 W/m²K for stainless, 3.5 W/m²K for plastic.
- Load: 10 W, a small router-style board with margin; your datasheet decides.
The rule in words: interior temperature = ambient + solar rise + load rise, where the solar rise = 7.7 K × (absorptivity ÷ 0.50) and the load rise = load ÷ (wall k × total wall area).
The arithmetic:
- Total wall area: 2 × (16 × 12 + 16 × 8 + 12 × 8) = 832 in² = 0.537 m².
- Load rise, stainless: 10 ÷ (3.7 × 0.537) = 10 ÷ 1.99 = 5.0 K. Plastic: 10 ÷ (3.5 × 0.537) = 10 ÷ 1.88 = 5.3 K.
- Solar rise, dark plastic: 7.7 × (0.9 ÷ 0.50) = 13.9 K. Bright stainless: 7.7 × (0.3 ÷ 0.50) = 4.6 K.
- Interior, dark plastic: 35 + 13.9 + 5.3 = 54.2 °C. Bright stainless: 35 + 4.6 + 5.0 = 44.6 °C.
The result: 9.6 K apart, call it 10 K, and both sit above the 40 °C (104 °F) ambient most electronics are built for unless the datasheet allows more.
The decision: with the sun off the wall, both skins hold this 10 W load right at 40 °C — 40.0 °C in stainless, 40.3 °C in plastic — so weight, cost and RF make the call. In peak sun the finish makes the call instead. 44.6 °C in bright stainless is survivable for a board rated past it; 54.2 °C in dark plastic is not. Every 2 W of extra load adds about 1 K inside, since 1 ÷ 1.99 ≈ 0.5 K/W, so about 10 W is the ceiling for a 40 °C board in this shaded stainless box. Past that, cut the sun with a shield or move the box out of the midday sun.
The decision by site
- Full sun, hot climate, load over a few watts: bright stainless or mill-finish aluminium with a brushed or light finish. Above about 30 °C design ambient, add a shield that blocks the direct beam; the sun shield solar heat load article works that arithmetic. Already running a dark box? The dark outdoor enclosure cooling fixes recover most of the same gap.
- Salt air, brine or chemical washdown: 316L stainless, because 304 pits under chloride deposits; the 304 vs 316L grade guide carries the rest of that choice.
- Antenna inside, pole or wall mount, tight budget: light grey UV-stabilized plastic or FRP, load under about 5 W, mounted so midday sun misses it. A 5 W load in the example box adds 2.7 K in plastic, so a shaded 35 °C afternoon lands near 38 °C. Bond the board with a wire, because the box gives no path of its own.
- Lightning country, dust storms, dry snow: metal, with bonded standoffs and a ground lug on the wall; a plastic box has to carry the same bond as a wire run.
- Keep the box sealed either way. A sealed box with desiccant stays drier than a vented one, and internal fans only stir the air; the vented versus sealed enclosure comparison and the internal fans in a sealed enclosure article carry both sides of that trade.
What to put on the order
- Grade: 304 stainless, #4 brushed, for indoor washdown and most outdoor sites; 316L for salt air, brine and chlorides.
- Size: the stainless line runs from 6 x 4 x 4 in to 60 x 36 x 16 in (H x W x D); any other size is built to your drawing from one piece.
- Cutouts: put the gland entries and the antenna feedthrough positions on the drawing now, even for an antenna you will add later; the custom enclosure builder takes them one cutout at a time.
- Rating: order the box manufacturer-rated NEMA 4X / IP66 — not UL listed; the rating is the manufacturer's, and NEMA does not test or endorse enclosures.
- Selection: the stainless enclosure size finder filters the line by grade, outside size and the device it carries.
- Delivery: delivered in 10 business days, counted from your order; for custom work, from your approval of the drawing.
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 |
|---|---|---|---|
| 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 |
| 30 × 24 × 10 in | 304 stainless | 30 x 24 x 10 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
Is light grey plastic good enough in full sun?
It closes most of the solar gap: a light grey skin absorbs about 0.5 of incident sun like grey paint, so it adds the same 7.7 K at 1044 W/m², against about 14 K for a dark skin. In the 16 x 12 x 8 in example box, a 5 W load then lands near 45 °C at 35 °C ambient — above the 40 °C most electronics carry, so shade it or keep it to morning sun.
How do I ground electronics inside a plastic outdoor box?
Run a conductor, because the box offers no path of its own: bond the board's ground terminal to the installation ground with a short, direct lead. A 30 cm lead beats a 1 m coil, because a static discharge takes the lowest-inductance path. In a metal box the standoffs and mounting panel do this job without extra parts; ungrounded boards are the ones that lose Ethernet ports to storms and dry blowing snow.
Do internal fans cool a sealed box in the sun?
Only the hot spots. An internal fan evens the air and trims the board-to-wall difference by a few degrees, but every watt still leaves through the walls, and none of it touches the 7.7 to 13.9 K the sun adds. Keep the box sealed: moving air in and out needs a rated vent, and that trades heat relief for dust and water.
Which survives salt air better, stainless or plastic?
Plastic cannot corrode, but 316L stainless answers where plastic is weakest. 304 pits under chloride deposits even at room temperature, and about 60 °C is the chloride stress-corrosion cracking threshold for both grades, so coastal and brine-wash boxes belong in 316L and should be rinsed. Plastic ages instead: unprotected skins embrittle within a few seasons, so choose UV-stabilized grades.
