At a 120 °F [49 °C] site, a sealed NEMA 4X PLC box needs refrigerated cooling, not vents or fans. Size about 1,000 W [3,400 Btu/h] of enclosure air conditioner at that ambient, and add 0.5 in [13 mm] of foam behind the walls. Keep the winter heat strip on its own thermostat so the two units stay out of each other's way. The arithmetic behind the number is short. Drives and most industrial electronics — PLC processors, I/O and 24 V supplies included — carry a 40 °C [104 °F] ambient rating unless their datasheet says more, and air that hot already sits 9 K past it. The worked example below assumes a 500 W panel in a 40 x 32 x 16 in box with afternoon sun on one wall. Swap in your own load, box size and wall color, and the same three steps hand you your number.
What that air temperature does to a sealed 4X box
A sealed box gives heat one way out: through the walls. With no air exchange, an internal fan only stirs the inside air; it evens hot spots and moves no heat outdoors. A stainless panel wall passes roughly 3.7 W/m²K overall, surface films included, so wall area sets the passive budget. The example box has 3.1 m² of wall, and 500 W against it lifts the interior 44 K above the outside air. That is 93 °C [199 °F] beside a PLC rated for 40 °C [104 °F].
The sun pushes the same walls the other way. A wall in direct afternoon sun runs above air temperature, and that extra push drives heat inward whether or not the panel is working. If your box has ever read hotter than the afternoon shade, the heat-gain walkthrough traces the extra degrees to the wall, the sun and the load. Start with the walkthrough for an outdoor enclosure running hotter than ambient.
Two levers exist: keep heat out of the walls, and pump heat out through them. The next sections size both.
The rating to design around
Start from the most sensitive datasheet in the panel. Drives and most industrial electronics carry 40 °C [104 °F] ambient unless a named datasheet allows more; some PLC lines extend to 55 °C [131 °F] or 60 °C [140 °F]. Drive manuals show how hard the limits bite: one maker's IP66 (Type 4X) drive line lists -10 to 40 °C and derates 2 % per °C above that. Read the PLC's ambient table and the 24 V supply's before you buy anything cold.
The enclosure rating follows the site, not the electronics. Type 4X is Type 4 plus corrosion resistance under NEMA 250, so it keeps the hose-directed water protection while standing up to brine, sanitizer or salt air. On most washdown sites 304 stainless is the grade; chlorides and coastal spray want 316L. Our 304 and 316L wall-mount boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed. The IP66 half adds the IEC 60529 dust-tight test and the powerful-jet test with a 12.5 mm nozzle at 100 L/min. On US jobs, NEC 110.28 lets the inspector hold the installation to the enclosure Type, so write NEMA 4X on the spec and treat IP66 as the export copy. The full tables sit in the NEMA enclosure types guide, and the salt-spray difference between the two Types is in the Type 4 vs Type 4X comparison.
Worked example: sizing the air conditioner
These inputs are example values; replace them with yours and rerun the three steps.
| Input | Example value |
|---|---|
| Panel heat (PLC, I/O, 24 V supply, HMI) | 500 W |
| Box outside | 40 x 32 x 16 in [1,020 x 810 x 410 mm] |
| Wall area | 3.1 m² |
| Wall U, bare stainless | 3.7 W/m²K |
| Design air ambient | 120 °F [49 °C] |
| Sun lift, west wall | +15 K (assumed) |
| Inside target | 95 °F [35 °C] |
Step 1 — prove passive cooling cannot hold. The wall equation is ΔT = Q / (U x A). With the full panel load against 3.7 W/m²K and 3.1 m², ΔT = 500 / 11.5 = 44 K, so the interior settles near 93 °C [199 °F]. Against a 40 °C [104 °F] rating, that is a 53 K miss.
Step 2 — add up what the air conditioner must pump. Hold the interior at 95 °F [35 °C], five kelvin under the rating. The sun-lifted west wall sits at 49 + 15 = 64 °C, so its 1.55 m² pass 3.7 x 1.55 x 29 = 166 W inward. The shaded 1.55 m² pass 3.7 x 1.55 x 14 = 80 W. Walls total about 250 W, and the cold side of the AC sees 500 + 250 = 750 W.
Step 3 — round up to the catalog, at your ambient. Order about 1,000 W [3,400 Btu/h] of cooling, and take the capacity figure quoted at the 120 °F [49 °C] column, not the 95 °F [35 °C] one. The same unit looks bigger in a mild-climate datasheet than it performs on your wall. The cabinet air conditioner sizing walkthrough applies these steps to a measured overheating panel.
Foam, color and shade before you upsize
Half an inch of closed-cell foam behind the walls is the cheapest frame size you will ever buy down. Foam at about 0.025 W/mK turns the composite wall into U = 1 / (1/3.7 + 0.013/0.025) = 1.3 W/m²K. Rerun the walls: sunlit 1.3 x 1.55 x 29 = 59 W, shaded 1.3 x 1.55 x 14 = 28 W, so the outside-driven share falls from 250 W to about 90 W — roughly two-thirds gone. The AC load lands near 590 W, and a 750 W [2,500 Btu/h] unit covers it with room to spare.
Shade and a light wall attack the same term at its source. Keeping the sun off the west wall erases the +15 K lift and the 59 W behind it, leaving 56 W of pure ambient leak through the foam. The shading and color choices behind that wall temperature are worked through in the solar heat gain article on shading and color, and a sheet-metal sun shield cuts the solar load before it touches the box. Foam slows heat in both directions, so it cannot shrink the internal load; the AC carries that part in every case.
When a fan, an exchanger or plant air fits instead
A filtered fan is the right tool at milder sites, and it rewrites the rating. The filter becomes the weakest part of the assembly, so the box then carries the filter's rating, not its 4X. It also delivers whatever the outside air is, which works when shaded air stays at or below 100 °F [38 °C] and dust is the only enemy. Do not put a fan-filter on a box you chose for brine, washdown or salt air — the rating you bought was the seal.
An air-to-air heat exchanger keeps the box sealed and its rating, but it can only pull the interior toward ambient. Take a mid-frame unit rated 25 W/K as the example input: carrying the full panel load takes ΔT = 500 / 25 = 20 K, so the interior sits near 69 °C [156 °F]. At a gentler site — 95 °F [35 °C] air, a 40 °C limit, 5 K of headroom — that same exchanger moves 125 W, enough for a small I/O panel and not for this one.
Plant air is the third option. A vortex cooler mounts through the wall, keeps the box sealed and rated, and cools below ambient with no compressor of its own — see what it takes to cool below ambient in a sealed enclosure. Its air consumption and sizing numbers are in the FAQ below. The full trade-offs are lined up in the fan, air conditioner and heat exchanger comparison, and the shortlist for a sealed box in sustained heat is in the hot-ambient cooling options for sealed enclosures.
The heater and the thermostat split
Keep the heat strip. Winter condensation inside a sealed 4X box kills more PLCs than summer heat, and it is cheaper to prevent than to dry out. Give the two units a wide split: heater on below 41 °F [5 °C], air conditioner on above 95 °F [35 °C]. With 30 K between them, the strip adds zero summer load, because its thermostat holds it off long before the AC wakes. A small internal circulation fan spreads the strip's warmth across the backplate instead of letting it pool. The year-round settings are laid out in the heater and air conditioner condensation article.
Cut the wall once and keep the rating
The air conditioner mounts through the wall on its gasketed flange, and the flange is what holds Type 4X. Cut to the unit's template, and let the flange land fully on flat metal. Power and signal conduits get their own glanded cutouts, each sealed to the same rating. Order all of it in the drawing and it arrives cut clean on your custom stainless enclosure; custom work rides the standard build, delivered in 10 business days, counted from your approval of the drawing. What the panel keeps after you cut it and mount devices is walked through in the rating-after-cutouts article, and the dimensions worth fixing early are in the cutouts and glands guide. Two last details: route the AC's condensate the way its manual shows, and land the AC on a circuit the panel disconnect kills with everything else. The 120 V disconnect article covers when that switch belongs in the panel.
What to put on the order
Send the drawing with these lines filled in:
| Line | Spec |
|---|---|
| Material | 304 stainless; 316L for salt air, brine or chlorides |
| Rating | Manufacturer-rated NEMA 4X / IP66; not UL listed |
| Size | 40 x 32 x 16 in [1,020 x 810 x 410 mm] outside, or your layout |
| Cutouts | AC opening to the unit's template; gland plate for power and I/O |
| Insulation | 0.5 in [13 mm] closed-cell foam, walls and roof |
| Finish | Light color, or shade the west wall |
| Cooling | Enclosure AC, about 1,000 W [3,400 Btu/h] at the 49 °C design ambient; thermostat 95 °F [35 °C] |
| Heat | 100 W anti-condensation heater, thermostat 41 °F [5 °C] |
The decision, by site:
| Site and load | Build |
|---|---|
| Design air 49 °C, full example load, washdown or brine | 4X stainless, ~1,000 W AC sized at that ambient, 0.5 in foam, shaded or light wall, 100 W heater |
| Shaded site, air never past 100 °F [38 °C], dust only | Filtered fan; the assembly carries the filter's rating |
| Sealed box, 95 °F [35 °C] air, ~125 W load | 25 W/K-class air-to-air exchanger |
| Clean dry plant air at the wall | Vortex cooler sized to the load; FAQ has the numbers |
| PLC datasheet allows 55 °C [131 °F] | Target 122 °F [50 °C] inside; the AC drops one frame |
For a 120 °F southern summer, the first row is the order: a sealed 4X stainless box, the AC sized at your ambient, foam behind the walls, and a thermostat split that lets the heat strip wait for winter. If the stainless size is still open, the stainless size finder filters every 304 and 316L box by line, grade and size, and the enclosure selector sorts by environment, module count and inside space.
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 |
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can shading alone replace the air conditioner at 120 F?
Rarely. Shade brings the sun-lifted walls back down to air temperature, but air at 49 °C [120 °F] is already 9 K above the 40 °C [104 °F] rating most PLCs carry. Even an empty sealed box in full shade drifts to 49 °C inside. Foam and shade cut the example air conditioner from 1,000 W to about 750 W; they shrink the unit, they do not remove it.
Does the 0.5 in foam make the box run hotter from its own heat?
It slows heat in both directions, so yes, the internal load has a harder path out. That is why the AC is sized for the internal load first and the outside-driven share second. In the example the foam cuts the outside-driven 250 W to about 90 W and the AC from 1,000 W to 750 W. On a passively cooled box, where the walls are the only cooling, skip the foam.
Can a vortex cooler replace the enclosure air conditioner?
Yes for panels up to a few hundred watts; no for the example above, whose internal load alone outruns a 1,500 Btu/h [440 W] cooler. A 1,500 Btu/h cabinet vortex cooler runs on about 15 scfm at 100 psig, holds the Type 4X rating because nothing opens the box, and cools below ambient like the AC. Bigger 3,400 Btu/h [1,000 W] sizes cover heavier panels when plant air is already paid for. Fit a filter and water separator on the air line: wet air puts moisture inside your sealed box.
My PLC is rated 55 C, not 40 C. Does the AC get smaller?
By about one frame. Target 122 °F [50 °C] inside instead of 95 °F [35 °C]: the shaded walls flip to -6 W outward, only the sun side still pushes in at about 80 W, and the load lands near 570 W, which a 600-750 W unit holds at the design ambient. Read the manual's ambient table first, because 55 °C ratings often carry derating fine print.
How much heat does a PLC panel actually add?
Sum what the devices consume and subtract what leaves on the field wires. A 24 V supply drawing 240 W that feeds mostly signal I/O leaves 150-200 W in the box; a mid-range processor adds 30-60 W and a 10 in HMI another 50-100 W; relays and spares close the gap to the example's 400-500 W. The worked example carries 500 W as a round-up. Metering the supply input for one shift replaces the guess with a number.
