In a sealed enclosure, layout is the cheapest cooling you will ever get. Heat-producing devices go high and toward the sides, temperature-sensitive devices go low and central, and nothing blocks the vertical air path. Done right, this holds internal temperatures down without vents, fans or extra gland holes, so the enclosure keeps its rating. Every box we build, from a 304 wall-mount enclosure to a 316L custom unit, relies on convection and wall conduction when it is sealed, so the layout decisions below apply to all of them.
How heat moves in a box with no airflow
A sealed enclosure has two heat paths. Inside the box, air moves only by natural convection: warm air rises along hot surfaces, spills across the top, cools at the walls and sinks. Outside the box, the steel or polycarbonate wall conducts heat to the ambient air. Both paths work best when hot spots are near the walls and the internal air column is unobstructed.
The physics gives you a rough rule: internal air stratifies, with the top third of the box noticeably warmer than the bottom third. In a 20 x 16 x 8 in (500 x 400 x 200 mm) box dissipating 50 W at a 25 C ambient, expect the internal air several degrees above ambient on average and 10 to 15 C warmer at the top than at the bottom. Component surfaces run hotter still. That gradient is not a problem to fight; it is a resource. Put devices where the temperature suits them and the gradient does the sorting for you.
If the numbers get away from you, layout alone will not save the panel. Work the load properly with how to size an electrical enclosure before you finalize the drawing, and check that the box you picked through the enclosure selector is thermally suitable, not only dimensionally.
What you need
- The heat dissipation of each device, in watts, from its data sheet.
- The temperature limits and required mounting orientation of each device.
- The enclosure inside dimensions and the mounting panel size.
- A plan of the backplate showing every device, terminal block and duct run. The rules in the backplate and DIN rail layout guide fit alongside the zoning below.
- The enclosure drawing, so cutouts and internal mounting features can be specified before the box is built.
Steps
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List every device with its wattage and its limits. Mark each as a heat source (power supplies, drives, transformers), heat-sensitive (PLCs, network gear, sensors) or neutral (terminals, contactors at low duty). Note the mounting orientation each data sheet requires, because a device mounted on its side loses convection performance even if the box is cool.
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Draw three horizontal zones on the backplate. Top zone: heat sources. Middle zone: neutral devices and most terminal space. Bottom zone: heat-sensitive electronics. This single rule fixes most hot-spot complaints, because the rising heat from a power supply never passes through anything that minds.
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Push heat sources toward the side walls. A drive mounted 1 in (25 mm) off the side wall heats mostly itself and the wall; the wall conducts the heat out. The same drive mounted dead center heats every device around it, because its plume rises through the middle of the box where nothing can escape sideways.
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Leave a clear vertical channel. Keep at least 2 in (50 mm) of clear backplate from the bottom device row to the top row, and keep wire duct out of that channel. A duct packed full of cables across the middle of the box acts as a shelf that hot air piles up under. Route the main duct runs along the sides instead.
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Give each device its data sheet clearance. Most convection-cooled devices want 2 in (50 mm) or more above the case. In a sealed box there is no forced airflow to forgive tight spacing, so treat the stated clearance as a minimum. Devices with cooling fins need the fins vertical, with free air above and below.
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Keep sensitive devices low, even when it is inconvenient. A switch-mode supply might tolerate 70 C while a compact PLC or a network switch is happiest well below that. If the wiring is easier with the electronics on top, you are buying convenience with MTBF.
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Space wiring so it does not blanket devices. Bundles laid across the top of a power supply trap its heat. Dress the incoming and outgoing conductors along the sides and enter the box through glands or a gland plate positioned low, where the warm internal air will not ride the cables into the terminations. For the entry itself, see the guide to cutouts, glands and HMI windows.
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Check the panel-to-door and panel-to-wall clearances. Devices mounted on the removable panel still need their clearance respected once the panel is installed. A drive with 4 in of clearance on the bench may have 1 in once the panel sits in the box. Do the check with the panel in place, in the drawing, before ordering.
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Re-check the total. Add up the wattages. As a rough check, a sealed stainless box in still air can typically dissipate on the order of 10 to 20 W per square foot of uncovered surface for a 10 to 15 C rise at a 25 C ambient, depending on size, finish and airflow around the box. If your load is well above what the surface supports, plan closed-loop cooling now, not after the first summer. The sizing steps in how to size an electrical enclosure include the load check.
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Specify the layout on the order. Mark cutout positions, panel mounting holes and any standoffs on the enclosure drawing for your custom enclosure. We build to the approved drawing, so a layout worked out on paper becomes holes and standoffs in the right places, delivered in 10 business days from drawing approval.
Checks when the panel is built
- Run the panel at full load and measure air temperature at three heights: near the top, middle and bottom of the box. More than about 15 C difference between top and bottom means a blocked channel or a source mounted too low.
- Touch-check (with appropriate protection) or use a non-contact thermometer on each power device. A device far above its expected temperature rise usually lacks its stated clearance.
- Confirm every finned device is vertical and has free air above it.
- Check that no duct or cable bundle crosses the middle of the backplate.
Mistakes that cost you temperature and rating
- Drilling vent or drain holes to let hot air out. Any field-drilled opening threatens the water tightness the rating depends on; manage heat with layout instead, or fit a rated cooling unit through a factory cutout. See NEMA 4 vs 4X vs IP66 for what the rating is tested against.
- Mounting the power supply low because the terminals are convenient. You warm everything above it.
- Packing wire duct full. Fill above roughly half and you choke the channel and the terminations both.
- Mounting a finned device horizontally or against a wall with no gap. The fins do nothing without air over them.
- Ignoring the sun. A box in direct sunlight gains a large additional load; site or shade the box before you blame the layout.
When layout is not enough
If the measured internal temperature still exceeds the hottest device's limit at worst-case ambient, move to active cooling. Because the box is sealed, that means a closed-loop unit through a proper cutout, never a vent. Order the cutout with the box: send the cooler's opening dimensions with your drawing through the custom enclosure builder so the rating stays intact. In a humid plant, a cooled sealed box will also see condensation on interior surfaces, and layout alone will not fix that either.
Questions
Where should a power supply go inside a sealed enclosure?
High and toward one side wall. Heat rises, so a power supply mounted low warms every device above it. Mounted high, its heat lifts into the top zone and the side wall carries some of it out through the steel. Leave clearance above it so the hot air is not trapped against the roof.
How much clearance does a device need for natural convection?
Most convection-cooled devices want at least 2 in (50 mm) of clear space above and around the case, and more if the data sheet says so. In a sealed box there is no airflow to compensate for tight packing, so treat the data sheet clearance as a minimum, not a target.
Does a sealed enclosure always need active cooling?
No. If the total dissipation is modest and the enclosure surface is large enough, good layout plus conduction through the walls holds the temperature down. Calculate the internal temperature rise first; if it exceeds what the hottest device tolerates at your worst-case ambient, then look at closed-loop cooling options.
Can I drill drain or vent holes to help hot air escape?
Not without consequences. Every hole in a NEMA 4X or IP66 enclosure is a potential water path, so the rating depends on the manufacturer's tested design; ours are rated by the manufacturer and not UL listed. Do not add field-drilled openings to a sealed box; manage heat with layout and, if needed, a rated cooling unit.
