Short answer: a 2.5 A stepper drive dissipating up to 13 W will overheat in a sealed 120 × 120 × 80 mm [4.7 × 4.7 × 3.1 in] cast-aluminum box if it rides on standoffs. That mount leaves the whole heat load to cross two air films. The interior climbs about 52 K above ambient, to roughly 87 C, against a drive rated for 40 C (104 F) ambient. Bolt the drive's base plate flat to the metal wall through a thermal pad and the rise halves, to about 26 K. That saves burst duty: your box's 1.5 kg of aluminum holds a 10-minute job to about 6 K of rise. Continuous full load at 35 C ambient is a different story. It needs about 0.35 m² of bolted wall, a 300 × 300 × 150 mm box or larger. The small box carries only about 2.5 W continuously.
What overheating looks like
The fault behind all of this is the one the driver-box piece describes: heat with nowhere to go. When your sealed box runs hot, the symptoms sort into five patterns, and each points at a different part of the path:
- Thermal trip 30-90 min into a run, and it resets once the box cools. The wall has reached steady state and the rise is over the drive's budget. Short bench tests hide this behind the casting's thermal mass.
- Drive case far hotter than the box wall. The base-to-wall joint is the culprit. With a proper pad the case sits within about 2 K of the wall.
- Wall barely warm, air inside hot. The drive is on standoffs or on the mounting panel, so both air films are in the path.
- Torque loss or missed steps late in a shift, with a cold motor. The drive is drifting or folding back, so measure it before you blame the motor.
- Fine on the bench, faults on the machine. A 10-minute test warms a 1.5 kg cast box by about 6 K; a two-hour run takes it to the full rise.
Why the heat gets stuck, most likely first
- No metal path. The drive sits on standoffs or spacers, so the full 13 W crosses the inside air film, the wall and the outside film in series. It is the default mount and the most common cause. Check it with an IR gun after two hours: a wall within a few kelvin of ambient under a hot drive means the heat never reached the metal.
- Bolted but not coupled. Paint, anodize or a dry gap does the same damage more slowly. Over a 64 × 64 mm [2.5 × 2.5 in] base, a 0.5 mm air gap is worth about 4.7 K/W — close to 60 K at full load. Check the case-to-wall difference with the IR gun and aim for under 2 K.
- More heat than you assumed. Output-stage losses follow the square of motor current: half the current, a quarter of the heat. The 13 W figure belongs to full current at heavy load. Clamp one motor phase during the hardest part of your cycle, and log the drive case through a complete duty cycle.
- Ambient or sun above 35 C. Every kelvin of ambient eats a kelvin of budget, and full sun on a dark face adds on the order of 10-20 K more — the solar-load article puts numbers on that. Log the shaded ambient beside your box at the worst hour of the day.
- The wall is too small for the duty. Even a perfect joint leaves 26 K of rise at full heat in this box. The arithmetic below turns that into a size you can order.
The worked example: a 120 × 120 × 80 mm box at 35 C ambient
Inputs, with their sources:
- Heat: up to 13 W — the manual's heavy-load figure for this class of 2.5 A microstepping drive, and it leaves through the base plate.
- Box: 120 × 120 × 80 mm cast aluminum, the case at hand.
- Wall value: 3.7 W/m²K, the still-air figure this site's sizing guides use for stainless walls. The air films set that value, not the metal, and a thin cast-aluminum wall behaves the same.
- Bolted-base wall value: about 7.4 W/m²K. The bolted base takes the inner air film out of the chain, and in still air the two films carry roughly equal shares.
- Budget: a drive rated for 40 C ambient — the usual rating for drives and control electronics unless its datasheet allows more — at 35 C ambient gives 40 − 35 = 5 K.
The rule, in words: temperature rise equals heat divided by wall value times area.
Surface area: A = 2 × (0.12 × 0.12) + 4 × (0.12 × 0.08) = 0.0288 + 0.0384 = 0.067 m².
On standoffs: rise = 13 ÷ (3.7 × 0.067) = 13 ÷ 0.248 = 52 K. The interior sits near 35 + 52 = 87 C, which puts a 40 C-rated drive 47 K over budget. That is a shutdown, not a margin.
Base bolted to the wall: rise = 13 ÷ (7.4 × 0.067) = 13 ÷ 0.496 = 26 K, and the pad adds under 1 K, as the fix below works out. The wall sits near 35 + 26 = 61 C — 21 K over budget for continuous duty, and fine for bursts.
The decision the arithmetic gives: continuous duty at this heat needs area = 13 ÷ (7.4 × 5) = 0.35 m². This box, bolted, carries 7.4 × 0.067 × 5 ≈ 2.5 W continuously. Bursts live: 1.5 kg of aluminum (typical for a casting this size) at 900 J/(kg·K) stores 1,350 J/K, so 13 W warms it at 0.58 K per minute — about 6 K in a 10-minute job. The time-to-overheat article runs this start-up arithmetic for other boxes. That is why the small cast box survives short jobs and dies on shifts.
Bolt the base plate to the wall
- Pick the wall, not the extras. Bolt the drive base to the enclosure's back wall itself. The removable mounting panel hangs on standoffs and puts an air film right back in the chain. Keep other heat sources off the same wall patch — the layout article spreads what is left around.
- Bring the joint down to bare metal. A 0.5 mm silicone thermal pad of the typical 2 W/mK grade, over a 64 × 64 mm base, is worth 0.0005 ÷ (2 × 0.0041) = 0.06 K/W — under 1 K at full load. The same 0.5 mm of trapped air is 0.0005 ÷ (0.026 × 0.0041) ≈ 4.7 K/W, near 60 K. Four screws tightened evenly are part of the thermal design, and the plate-and-paste walkthrough covers stepper-specific mounting.
- On one of our boxes, draw the holes instead of drilling blind. We build each stainless enclosure from one piece to your cutout drawing, manufacturer-rated NEMA 4X / IP66; not UL listed — NEMA publishes the type definitions but does not test or certify boxes. A specified mounting hole lands where the base needs it and leaves the gasketed design intact.
- Kill power, then let it run. Isolate the drive and prove absence of voltage before you open the box. Then run the longest duty cycle for two hours; steady state lands about 45 min after a bolted start. Read case and wall with the IR gun. Under 2 K between them means the joint works, and the wall temperature tells you which row of the next table you are in.
Grow the path by wall area, not by holes: a vent or fan opening lowers the whole assembly to that part's rating, so a sealed box gains cooling only by conducting through more wall.
When the box itself is too small
For continuous duty, size the wall instead: allowed heat = wall value × area × rise budget. At 7.4 W/m²K and a 5 K budget that is 37 W per square metre of bolted wall, and the table below falls straight out of it. The same wall-value method scales to any box — it is worked for larger sealed enclosures on this site.
| Box, outside | Wall area | Continuous heat, bolted base | Verdict at 13 W |
|---|---|---|---|
| 120 × 120 × 80 mm | 0.067 m² | 2.5 W | about 5× short |
| 200 × 200 × 100 mm | 0.16 m² | 6 W | about 2× short |
| 300 × 300 × 150 mm | 0.36 m² | 13 W | exact — take the next size up |
| 400 × 400 × 200 mm | 0.64 m² | 24 W | comfortable margin |
Figures are 7.4 W/m²K × area × 5 K at 35 C ambient with a 40 C-rated drive and the base bolted flat; halve them for standoff mounting.
When the wall cannot grow, cut the heat: lower the motor current (a quarter of the heat at half the current), shorten the duty, or fit a through-wall cooler, which keeps the box sealed. The sealed-cooling piece compares those routes. For the cooler itself, a 75 W unit worked through at 25 C shows what such a machine can carry.
When you re-specify, our 304 and 316L stainless wall-mount line spans 6 × 4 × 4 in to 60 × 36 × 16 in outside, built to your width, height, depth and cutouts and delivered in 10 business days from the order (custom timing runs from your approval of the drawing). Start at the stainless wall-mount line or the custom builder; the sizing guide covers the rest of the size decision.
The decision, case by case
- Burst jobs under 15 min at up to 13 W: the 120 × 120 × 80 mm box works with the base bolted flat; the casting holds the first 10 minutes to about 6 K of rise.
- Continuous duty up to 2.5 W at 35 C ambient: the same small box works, bolted, and a 40 C-rated drive stays inside its budget.
- Continuous 13 W at 35 C ambient: move to about 0.35 m² of bolted wall — a 300 × 300 × 150 mm box is the minimum, and the next size up buys margin.
- A datasheet that allows more than 40 C: the budget widens kelvin for kelvin; a 55 C rating turns the table's heat figures up fourfold, to about 10 W in the small box.
- A wall that cannot grow and a duty that will not shrink: a through-wall cooler sized from the heat. A vent or fan opening is the one fix that costs the rating.
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 |
| 60 × 36 × 16 in | 304 stainless | 60 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can I add a vent or a fan instead of sizing the wall?
A vent or fan opening lowers the whole assembly to that part's rating. The second digit of IP66 is tested with powerful jets from a 12.5 mm nozzle at 100 L/min and the first digit 6 means dust-tight, which is exactly what a louver breaks. The sealed routes are more bolted wall area — about 2.5 W continuous in the 120 × 120 × 80 mm box at a 5 K rise budget — or a through-wall cooler sized from the heat.
How long does the box take to reach steady state?
Treat the casting as storage: 1.5 kg of aluminum at 900 J/(kg·K) is 1,350 J/K. Divided by the bolted wall's conductance of about 0.50 W/K (7.4 W/m²K over 0.067 m²), the time constant is about 45 min; on standoffs it is about 90 min. Run the longest duty cycle for two hours before trusting an IR-gun reading — by then the wall is within a kelvin or two of final.
What if the drive datasheet allows more than 40 C ambient?
The budget widens kelvin for kelvin. At 35 C ambient, a 55 C rating gives a 20 K budget instead of 5 K, and allowed continuous heat scales by four because it equals 7.4 W/m²K × area × budget. The bolted 120 × 120 × 80 mm box then carries about 10 W instead of 2.5 W, and the 300 × 300 × 150 mm box carries about 53 W.
Does the removable mounting panel work as the heat path?
The panel hangs on standoffs, so the drive's heat crosses an air film into the panel and a second one out through the wall. Expect close to the 52 K standoff rise rather than the 26 K bolted rise at 13 W. Bolt the base plate to the back wall itself and keep the panel for terminals, low-power supplies and other parts that shed little heat.
