A sealed stainless enclosure can act as its own heat sink. You bond the heat-producing components to the wall or to a plate tied to it, and conduction carries the heat to the outside surface. Ambient air then takes it away. Nothing is vented, no fan is added, and the gasketed sealing scheme survives intact. This is the first option to price for low-heat-load boxes in sheltered positions, before you consider venting or active cooling. The rating fundamentals are covered in the guide to NEMA enclosure types.
What conduction cooling actually means
Every sealed enclosure already cools by conduction to some degree. Heat generated inside moves through the air, the mounting plate and the wall. It then leaves the outer surface by natural convection and radiation. The question is never whether the box is a heat sink, but whether it is good enough for your load.
The wall of a metal enclosure is a good conductor over its own thickness. A 0.06 in (1.5 mm) stainless wall adds a negligible temperature drop. The real bottlenecks are the interfaces: component to plate, plate to wall, and wall to outside air. Design those three and the box does the rest.
The outside surface is the limiting step in still air. As a rule of thumb for a metal box in free air, expect roughly 10 to 15 W per square metre of exposed surface per degree C of rise above ambient. Work backwards from your heat load to find the rise you will get. Or work forwards from an allowed rise to find how much load the box can absorb.
Worked sizing example
Take a 16 x 12 x 8 in (400 x 300 x 200 mm) wall-mount box. Its exposed surface is roughly 0.6 square metres. Inside it holds a 12 V supply, a controller and a small amplifier with a combined steady dissipation of about 15 W. The box sits in a sheltered outdoor cavity where the air peaks at 30 C.
The load is 15 W over 0.6 square metres, which is 25 W per square metre. At 12 W per square metre per degree, the inside runs about 2 C above the air around the box. Add the coupling losses from component to wall, perhaps 3 to 5 C with good thermal pads and bolted contact. The hot component then sits 5 to 7 C above local ambient. For most industrial electronics, that is comfortable.
The same arithmetic shows when conduction is not enough. A 60 W load on the same box is 100 W per square metre. The outer surface must then run 8 C or more above ambient just to reject it, and internal hot spots climb past that. Past roughly 50 to 100 W in a box this size, start pricing active options. Compare a fan, a heat exchanger and an air conditioner, or run the sizing method in the guide to sizing an electrical enclosure before you choose.
Building the conduction path
The wall can only carry what reaches it. Inside a sealed box there is no airflow to speak of, so conductive and radiative coupling from components to the wall is all you have.
- Mount the hottest components, typically the power supply and any amplifier stage, directly to the enclosure wall or to a metal backplate in firm contact with it.
- Use thermal pads or thermal paste at each interface; a bolted joint without them can have ten times the thermal resistance of a joint with a pad.
- Keep the hot components low or centre-mounted on the wall so heat spreads before it reaches the outside surface. Keep heat-sensitive parts such as electrolytic capacitors and controllers away from them.
- Leave air space around the box outside. Concrete, insulation or a tight cavity against the wall blocks the outer half of the path, which is the half that matters most.
If the box sits inside a larger structure, do not let it touch the structure where that contact would block convection or cause corrosion. A frame that holds the box with an air gap on all faces gives the outer surface the airflow it needs. The backplate and DIN rail layout guide shows how to arrange the plate so components couple to the wall instead of blanketing it.
When to choose this over venting or a fan
Conduction cooling is the right first choice when the load is low, the environment is hostile or hard to reach, and the box sits in a sheltered but unconditioned space. Venting trades rating for airflow. Any penetration is a path for moisture, dust and, over years, corrosion. A fan adds a moving part, a filter and another penetration.
| Approach | Rating kept | Moving parts | Typical load range in a 16 x 12 x 8 in box |
|---|---|---|---|
| Conduction through the wall | Yes | None | Up to roughly 30-50 W |
| Membrane vent, no fan | Reduced | None | Little cooling benefit; mainly moisture control |
| Filter fan | No | Yes | 100 W and up |
| Air-to-air heat exchanger | Yes | Yes | 100-500 W |
| Air conditioner | Yes | Yes | 500 W and up, or high ambient |
The rough numbers in the last column assume still air, a 20 C ambient allowance and ordinary industrial electronics. Treat them as an order-of-magnitude screen, not a design result.
Note the one row where the rating is genuinely lost. A filter fan needs openings on both sides of the wall, so a fan-cooled box is a ventilated box, whatever its nameplate once said. If your specification calls for a Type 4X or IP66 rating, the fan is off the table; conduction or a sealed-path cooler is the answer. In US installations, remember that NEC 110.28 states IP ratings are not a substitute for enclosure type ratings, so carry both on the drawing.
What this changes in a real panel
Designing for conduction changes the layout, not just the cooling choice. The power supply moves to the wall or to standoffs that clamp it against the backplate. Wire routing keeps hot and sensitive circuits separated. The layout leaves the inner wall surface clear, so it can receive heat rather than being blanketed by duct and looms.
It also changes what you order. A box that will work as a heat sink wants good wall contact for the backplate, space for thermal pads and mounting hardware, and enough depth that hot components are not buried behind other parts. Both 304 and 316L stainless walls conduct heat adequately for this duty. The grade choice should follow the corrosion environment, not the thermal one, as the comparison of 304 and 316L stainless explains.
Condensation still needs a plan
Keeping the box sealed solves ingress and corrosion, but not moisture. Air trapped inside at closing time carries water vapour. Every day-night cycle moves it between vapour and condensate. A conduction-cooled box has smaller internal temperature swings than a sunlit sealed one, which reduces the pumping effect, but it does not eliminate it.
For a low-load box in a cold, damp climate, the practical additions are a desiccant pack serviced on a schedule, or a small anti-condensation heater that keeps the interior a few degrees above the dew point. If moisture outranks ingress as the problem, a membrane vent is an option, but treat the rating loss as a deliberate, documented decision. The guide to outdoor electrical enclosures covers why the rating alone cannot stop condensation, and how site conditions change the choice.
What to specify
Before you order, put four numbers on paper: the steady-state heat load in watts, the highest air temperature around the box, the allowed internal temperature for the most sensitive component, and the exposed surface area of the box you plan to use. If the arithmetic gives you 10 C or less of total rise, conduction is your answer.
Specify a sealed stainless wall-mount enclosure with a removable mounting plate, depth enough to keep hot components against the plate, and no cutouts you do not need. Our wall-mount boxes, rated NEMA 4X / IP66 by the manufacturer and not UL listed, start at a 16 x 12 x 8 in (400 x 300 x 200 mm) 304 stainless box and step up to a 20 x 16 x 8 in (500 x 400 x 200 mm) box for larger loads. Set cutouts only where glands actually pass, and leave the rest of the wall to do its second job as the heat sink. The size finder at the stainless enclosure shop narrows the options by grade and inside dimensions if neither size is quite right.
If the sizing comes out marginal, do not split the difference. Either drop the load, move to a larger box for more surface area, or step up to a sealed-path active cooler and keep the rating you paid for.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| 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 |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 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 |
Questions
Does using the enclosure as a heat sink affect the IP or NEMA rating?
No, as long as nothing is cut or drilled through the wall. Conduction cooling uses the wall itself, so the gasket, door and glands stay as the manufacturer built them. The moment you add a fan, a louver or an oversized untightened gland, you give up the rating. Our boxes are rated NEMA 4X / IP66 by the manufacturer; they are not UL listed, and the rating only holds if you leave the sealing scheme intact.
How much heat can a sealed stainless enclosure dissipate through its walls?
A rough natural-convection figure for a painted or bare metal box in still air is about 10 to 15 W per square metre of exposed surface per degree C of rise above ambient. For a 16 x 12 x 8 in box with roughly 0.6 square metres of surface, 10 W of internal load gives about a 1 to 2 C rise. Use the same arithmetic with your real load and ambient before committing.
Is aluminium better than stainless steel for conduction cooling?
Aluminium conducts heat roughly ten times better than stainless steel, so the wall itself spreads heat faster. But the bottleneck in a small sealed box is usually the outside surface releasing heat to the air, not the spread through thin sheet metal. A stainless box with good internal coupling to the wall and free airflow outside performs close to an aluminium one at these load levels, and it resists corrosion far better.
Will a sealed enclosure still get condensation inside?
It can. A sealed box traps the moisture that was inside when the door was closed, and daily temperature swings pump that moisture through condensation cycles. Conduction cooling actually helps by keeping the internal temperature swing small, but in a damp or cold climate you should still plan for it: desiccant, a small anti-condensation heater, or a membrane vent if the rating loss is a deliberate decision.
