Why your driver box runs hot inside a sealed enclosure

A driver box, power supply or compact PLC that runs hot inside a sealed enclosure is almost never a defective part. It is heat with nowhere to go: the box is too small for the watts inside it, or the heat has no path to the outside air. Replace the box and the next one runs hot the same way. This is how to tell how hot is too hot, find the cause, and fix it so it stays fixed. Our enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed.

Symptoms and what each one points to

The clues appear in a predictable order. Work down this list and each symptom narrows the cause:

• You can hold a hand on the housing for about ten seconds. Skin pain starts near 45 to 50 C, so the surface is roughly in that band. That is warm but survivable for many drivers; measure before replacing anything.
• The box is hot only when the load is on. The heat comes from inside, from the driver or supply, not from sun or a neighboring heat source.
• The box is hot even with the load off. Look outside: solar gain on a dark wall, a hot pipe behind the mount, or a nearby machine. The outdoor enclosure hotter than ambient article covers that path.
• Parts fail early, one after another. Electrolytic capacitors inside drivers and supplies roughly halve their life for every 10 C rise. Repeated failures at the same spot mean sustained high internal temperature, not bad luck.
• The box fogs at night or trips on humidity. Heat cycling a sealed box drives moisture in and out of every seam. See the breather vent vs drain discussion for that symptom.
• The door or gasket feels hotter on one side. The hot component sits near that wall with no air gap, and the whole heat load is soaking through one small area.

What counts as too hot

Get a number. Tape a thermocouple or a cheap thermometer probe to the inside air space, an inch or two from the driver, and read it after an hour at full load.

• Under 40 C internal air: normal for a sealed box with a real heat load. No action needed beyond a re-check in summer.
• 40 to 55 C: warm. Acceptable for most industrial parts, but capacitor life drops. Plan spacing and check the driver's rated maximum.
• Above 55 C, or above the driver's rated maximum ambient: the enclosure is undersized or the heat load is too high. Fix it now, before failures.

Most LED drivers and power supplies carry a rated maximum case or ambient temperature printed on the label, commonly 70 C at the case. The air around the part, not the label claim, is what you measured, so leave margin.

Causes in order of likelihood

The enclosure is too small for the heat load. This is the dominant cause. A sealed box sheds heat only through its walls, and the walls can only be so hot. A rough planning figure for a painted steel or stainless box in still air is about 1 W of dissipation for every 10 to 15 square inches of surface area per 10 C of rise above ambient. A small box has very little area.

Everything is piled in one corner. Drivers touching a wall, wrapped in wiring, with the supply stacked on top, creates a local hot spot even when the average air temperature looks fine. The component layout inside a sealed enclosure article covers the spacing rules.

The box is mounted to something hot. A wall in full sun, an uninsulated roof deck or a warm machine frame feeds heat in faster than the walls can shed it.

The heat load is higher than you think. An LED driver at 90 percent efficiency still dumps 10 percent of its load as heat. A 30 W driver puts about 3 W into a box the size of a fist; a 150 W supply puts out 15 W, and two of them in one small enclosure double that. The enclosure heat calculation walks the full worked example.

A failing driver drawing extra current. Least likely, but real: a driver with failing capacitors runs hot and inefficient. It is the exception, so prove it last, by measuring the input power against the rated output.

How to check each cause

1. Measure internal air temperature at full load, as above, and record ambient at the same time. The difference, the rise, matters more than the absolute number.
2. Add up the heat load. Take each part's rated watts times (1 minus its efficiency), or use 10 percent of rated power as a planning figure. Note the total.
3. Estimate the enclosure's surface area and divide: total watts divided by area gives W per square inch. Anything above about 0.1 W per square inch means a large rise above ambient is inevitable in still air.
4. Measure input power at the supply with a clamp meter or power meter. Reading well above rated output plus expected losses points at a failing driver, not a sizing problem.
5. Feel the mounting surface with the load off on a hot afternoon. If the wall itself is hot, the heat is coming in, not going out.

The fix

Match the fix to the cause you proved:

• Undersized box: go bigger, or split the load. Doubling the surface area halves the temperature rise for the same watts. The enclosure selector sizes by inside height and width so you can check what fits before ordering.

• Layout: move the hottest part to the middle of the back panel with an air gap around it, and keep supplies off the lid. Hot air stratifies; the top of any sealed box is its hottest zone.

• Heat load: use a higher-efficiency driver, or move the driver outside the sealed volume. Many LED fixtures allow a remote driver in a cooler, ventilated location.

• Still too hot sealed: an internal circulation fan equalizes the air and lets the whole wall work, worth a few degrees; the fan on a NEMA 4X enclosure article explains why an exhaust fan through the wall is the step that costs you the rating. For big loads, the fan vs air conditioner vs heat exchanger comparison picks the sealed cooling method.

If the box lives somewhere that does not actually need a washdown rating, do not seal it. The enclosure vent or sealed box article gives the decision rule.

How to stop it coming back

Do the heat math at ordering time, not after the first failure. Write the total internal heat load, in watts, on the panel documentation. Check it whenever a part is added, because heat loads creep upward one retrofit at a time. Re-measure internal temperature after any change and each summer. Specify the enclosure from the heat load first, then from the cutouts, because cutouts can go anywhere on a bigger box but heat cannot leave a small one.

For a deeper treatment of wall-mounted heat path design, the sealed enclosure heat sink design for electronics cooling reference covers spreading heat to the enclosure wall.

When to re-specify the enclosure

Re-specify when the measured rise cannot meet the parts' ratings in the worst ambient, or when the layout fix has run out of room. A custom enclosure lets you set the width, height and depth and every cutout from one piece, in 304 or 316L stainless, powder-coated steel, aluminum or fiberglass, delivered in 10 business days from your approval of the drawing. If your environment is mild, dropping from a sealed NEMA 4X design to a vented Type 1 or 3R box of the same size may solve the heat with no cooling hardware at all.

Enclosures for this job

Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.

Questions