Nested enclosure heat buildup with multiple power supplies

Nested plastic boxes can overheat even when each power supply serves only a small load. Calculate the heat released inside each box, then evaluate the temperature rise across both thermal barriers. Five converters create five loss sources, but their output loads may contribute more heat than the converters themselves. Do not drill ventilation holes or glue modules into place before resolving the thermal path, mounting method and mains insulation requirements.

What you need before calculating

Collect the following information for the complete assembly, not just the five supplies:

• Actual output voltage and current for each supply at maximum sustained load.
• Efficiency or input-power data at that operating point, plus no-load consumption where relevant.
• Supply temperature limits, derating curves, permitted orientation and mounting instructions.
• Inner and outer enclosure dimensions, wall materials and installed orientation.
• Heat from transceivers, isolators, indicators and other electronics, separated by location.
• Maximum surrounding air temperature, including heat from nearby equipment or sunlight.
• Required electrical insulation, ingress protection and end-equipment approvals.

Allow space for sensors and wiring without reducing required electrical clearances. A gap of 0.4 in [10 mm] between modules does not, by itself, establish safe insulation or adequate cooling.

Mains wiring, protective devices and insulation coordination require qualified electrical design. For the underlying supply arrangement, use the principles in mains power supply installation inside a small enclosure, while checking requirements for the actual supply voltage.

Calculate the heat inside each box

1. Find each supply's conversion loss at its actual load. For output power Pout and efficiency η, calculate Ploss = Pout × (1/η − 1). Use efficiency as a decimal. A supply delivering 3 W at an assumed 80% efficiency dissipates 0.75 W internally. Five identical supplies at that operating point dissipate 3.75 W altogether. These are example operating values, not specifications for a particular module.

   Do not assume efficiency remains constant at light load. Use manufacturer data for the relevant input voltage and output current. If reliable data are unavailable, obtain suitable measurements using an electrically safe test arrangement. At low power, input measurements must account for power factor rather than simply multiplying AC voltage by current.

2. Assign every load to its physical location. Supply conversion losses are only part of the heat budget. If the example supplies deliver all 15 W to circuits inside the inner box, that box may dissipate approximately 18.75 W. If the output power leaves the entire assembly, only the 3.75 W conversion loss remains there, apart from wiring and other internal losses.

   Power delivered to electronics in the outer box heats that space instead. Do not count the same output power twice. For ordinary control circuits, nearly all consumed electrical power eventually becomes heat locally; subtract significant power exported through cables or other useful outputs.

3. Build separate inner and outer heat totals. Label heat generated inside the inner box as Qinner. Label heat generated between the boxes as Qouter. The outer enclosure must ultimately reject Qinner + Qouter to the surroundings. Record normal operation, maximum sustained load and any permitted standby condition. An assembly with lightly loaded converters can still run warm because each converter has its own operating losses.

Trace heat through both enclosure walls

4. Model the inner box as an additional thermal barrier. In a simplified steady-state model, inner heat crosses the inner enclosure before crossing the outer enclosure. Heat passes through internal air, walls and surrounding air films. The gap between boxes is not an unlimited cooling reservoir: its temperature rises as the inner box rejects heat.

   Use these approximate relationships when effective thermal resistances are known:

   Outer-space rise = (Qinner + Qouter) × Router

   Inner-air rise above outer space = Qinner × Rinner

   Inner-air temperature = ambient + outer-space rise + inner-air rise

   Thermal resistance is expressed in K/W; a temperature rise of 1 K equals a rise of 1 °C. These effective values depend on geometry, orientation, airflow and mounting. They are not simply the thermal conductivity of the plastic. The distinction is explained in thermal resistance inside sealed enclosures.

5. Check a complete numerical case, then verify it experimentally. Suppose 3.75 W is released inside the inner box and another 0.5 W in the outer space. Assume effective resistances of 6 K/W from inner air to outer space and 3 K/W from outer space to ambient.

   The outer-space rise is 4.25 W × 3 K/W = 12.75 °C. The additional inner rise is 3.75 W × 6 K/W = 22.5 °C. At 35 °C ambient, estimated inner air reaches approximately 70 °C. Component hot spots can be hotter still.

   Those resistance values are illustrative assumptions, not design allowances. Real assemblies have parallel heat paths through supports, wiring and radiation, and the effective resistance changes with temperature. Use the model to expose the extra penalty from nesting, then measure the assembled design. A larger outer box alone may provide little improvement when the inner box remains the dominant restriction.

Mount modules without changing the insulation design

6. Use the supply's documented mounting method. Prefer modules with a defined PCB footprint or enclosed supplies with specified mounting provisions. Support the carrier board or mounting assembly mechanically, and provide strain relief so cable movement cannot load electrical joints. Follow orientation and spacing requirements, especially around heat-producing parts.

   Do not use transformer wrapping, component bodies or adhesive alone as an improvised mounting interface. Wrapping is not a structural mounting surface. Ordinary hot-melt glue and wooden spacers do not establish long-term retention, suitable flammability performance or electrical clearances. Potting is also a design change, not a general substitute for proper mounting.

   Keep hot supplies away from temperature-sensitive electronics and avoid blocking natural convection paths. Apply the placement principles in component layout inside a sealed enclosure, accounting for the inner box's restricted air volume.

7. Evaluate insulation independently of the enclosure count. Two plastic boxes do not automatically provide double insulation or establish Class II construction. The complete insulation system includes barriers, creepage, clearance, wiring, terminals, connectors and mechanical retention. Its suitability depends on working voltage, environmental conditions, materials and the applicable end-equipment standard.

   Accessible metal connector shells also need assessment. Plastic walls do not automatically isolate them from a loose mains conductor. Review metal penetrations in plastic enclosures when planning exposed hardware and cable entries.

   Select input protection from the supply instructions and complete circuit design, including inrush and conductor protection. A guessed fuse rating does not establish shock protection or prevent every overheating fault. Likewise, combining five supplies into one requires checking whether independently isolated outputs are essential. Do not join outputs or signal references without reviewing the isolation architecture.

Check temperatures in the completed assembly

8. Test the final geometry under controlled conditions. Install suitable sensors with power isolated, then close both boxes before thermal operation. Route sensor wiring without compromising seals or electrical separation. Use an electrically safe measurement arrangement that does not require reaching into energized mains circuitry.

   Measure inner air, the space between boxes and manufacturer-designated supply case points. Include sensitive adjacent components. Reproduce the intended orientation, mounting surface, cable arrangement and maximum sustained load. Test at the maximum specified ambient or use a justified thermal assessment covering it.

Continue until temperatures stabilize; a short powered check may miss heat stored in both enclosure walls. Compare each result with the correct manufacturer limit. An ambient rating, case-temperature limit and internal semiconductor limit are not interchangeable.

Confirm these points before accepting the design:

• Each supply remains within its operating limits and derating curve.
• Nearby components, wiring, supports and enclosure materials remain within their temperature ratings.
• Output voltages remain within requirements at full load and during permitted operating transitions.
• Both boxes close without pinching wiring or moving modules.
• Cable movement cannot reduce electrical spacing or damage insulation.
• Thermal margin covers load, efficiency and ambient uncertainties rather than merely matching a maximum limit.

A thermal test does not establish electrical safety compliance. Required insulation and abnormal-operation evaluations remain separate parts of the complete equipment assessment.

Mistakes that remove protection without fixing heat

Drilling holes through only the outer box may do little when heat remains trapped inside the inner box. Drilling both boxes can improve air exchange, but changes ingress protection, contamination exposure and access to live parts. It may also invalidate an insulation barrier relied upon by the design.

An internal fan redistributes heat; it does not remove the total heat load from sealed boxes. It can reduce local hot spots, but its own electrical consumption adds heat. Compare the available heat-rejection methods using enclosure fans, air conditioners and heat exchangers before adding openings.

Do not treat a metal plate inside the inner box as an automatic solution. Unless it has a designed heat path outward, it mainly spreads heat within the same restricted volume. Direct thermal connections to a supply also require checking electrical insulation and manufacturer mounting permissions.

Finally, do not infer internal component temperatures from the outside wall alone. A moderately warm outer surface can coexist with an overheated inner compartment.

When to change the enclosure or power architecture

Change the design if acceptable temperatures require removing a necessary barrier, exceeding component limits or relying on uncertain mounting. Options include lower-loss supplies, reduced sustained loading, a larger inner box or eliminating unnecessary nesting after a safety review.

One mains supply feeding separately isolated DC-DC converters may simplify mains distribution. It is not automatically cooler: calculate the losses through both conversion stages and verify the required isolation ratings.

If enclosure selection is the next step, the enclosure selector can narrow dimensional and environmental requirements. A dimensional match does not establish thermal suitability.

For custom enclosures, send the following with the quote request:

• Inner and outer dimensions, wall material and mounting orientation.
• Heat generated in each compartment and maximum ambient temperature.
• Supply mounting drawings, required clearances and cable routing.
• Connector cutouts, entry locations and sealing requirements.
• Required insulation classification, approvals and service access.

Specify a verified thermal target alongside the physical dimensions. The enclosure must support the electrical safety design, not substitute for it.

Enclosures for this job

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