The short answer
- Heat the walls can shed: Q = k x A x ΔT. A is the exposed surface in m², ΔT the inside-minus-outside temperature in K, and k about 5.5 W/m²K for painted sheet steel or about 3.7 W/m²K for stainless.
- Heat load: add the losses of every device from its datasheet, then a safety margin; one maker's guide recommends 25%.
- If the load is higher than the walls can shed: a bigger enclosure, a filtered fan (open loop), an air-to-air heat exchanger (closed loop, needs cooler ambient air) or an enclosure air conditioner (closed loop, can cool below ambient).
- Outdoors, add the sun. On a small gray enclosure it can be larger than the equipment heat.
The method in one line
In steady state, the heat the equipment produces equals the heat the enclosure gets rid of. For a sealed enclosure with no cooling, that means through the walls, by natural convection and radiation. Enclosure climate-control catalogues write the wall term as:
Qs = k x A x ΔT
Qs = heat given off by the enclosure surface (W); k = heat transfer coefficient (W/m²K); A = effective surface area (m²); ΔT = inside air temperature minus outside air temperature (K, same as °C difference).
Rearranged, the temperature rise for a given heat load Qv is ΔT = Qv / (k x A). The sizing guide uses this as a first check on a 24 x 20 x 8 in enclosure. This guide takes the same enclosure further: a real heat load, a design margin, and the cooling you would need.
Step 1: add up the heat inside
The heat input has to come from the makers of the components: their datasheets give power loss or dissipation in watts. Where only efficiency is given, as for most power supplies, the loss is:
Loss = output power x (1 / efficiency - 1)
For our example we assume these datasheet values. They are illustrative; use your own.
| Device | Basis | Heat (W) |
|---|---|---|
| 24 V DC power supply | 120 W output at 90% efficiency: 120 x (1/0.90 - 1) | 13.3 |
| PLC with I/O | Datasheet dissipation | 10.0 |
| Ethernet switch | Datasheet dissipation | 6.0 |
| Small industrial PC | Datasheet dissipation | 25.0 |
| Relays, terminals, wiring | Estimate | 5.0 |
| Total | 59.3 | |
| With 25% margin | 59.3 x 1.25 | 74.1 |
One enclosure maker's thermal guide says its temperature-rise method is only an approximation, because layout, internal fans and air movement around the enclosure all change the result, and recommends a 25% safety margin for critical applications. Here we apply it to the load, so the design heat load is 74.1 W.
Step 2: set the allowed temperature rise
The allowed rise is the maximum inside temperature minus the maximum outside temperature. The inside limit comes from the lowest-rated device in the enclosure, and it pays to stay well below it: the same thermal guide notes that for every 18 °F (10 °C) rise above normal room temperature of 72-75 °F (22-24 °C), the reliability of electronic components is cut in half.
Example: the lowest-rated device is rated for 50 °C; we design for 45 °C inside to keep a margin. The room reaches 35 °C on the hottest day. Allowed rise: 45 - 35 = 10 K (18 °F).
Step 3: what the walls can shed
The surface area of an A x B x C in enclosure is 2[(A x B) + (A x C) + (B x C)] / 144 in ft², counting all six sides. Leave out any side that cannot transfer heat, such as the back of a wall-mounted enclosure. Volume cannot be used in place of surface area. More refined methods, such as the IEC 890 surface-area method used in climate-control catalogues, also weight the surfaces by how the enclosure is installed: free-standing, against a wall, or side by side.
The coefficient k is where published values disagree, so check which one you use:
| Material | Climate-control catalogue / heater guide | Heat exchanger selection guide |
|---|---|---|
| Painted steel | 5.5 W/m²K (0.51 W/ft²K) | 0.22 W/°F·ft² (about 4.3 W/m²K) |
| Stainless steel, unpainted | about 3.7 W/m²K (0.344 W/ft²K) | 0.10 W/°F·ft² (about 1.9 W/m²K) |
| Plastic / non-metallic | about 3.5 W/m²K (0.325 W/ft²K) | 0.22 W/°F·ft² (about 4.3 W/m²K) |
One maker's thermal guide explains the direction: unfinished stainless steel and aluminum radiate heat less efficiently than painted surfaces, so they run hotter for the same load, while non-metallic enclosures behave much like painted metal. For bare aluminum the published coefficients disagree widely, so ask the enclosure maker. We use 3.7 W/m²K for stainless, as in the sizing guide, and show the lower value as a check. Read conservatively, one maker's temperature-rise chart also implies a lower figure, so treat 3.7 as the optimistic end and keep the margin.
Example: the 24 x 20 x 8 in enclosure on a wall exposes its front (24 x 20), top and bottom (2 x 24 x 8) and sides (2 x 20 x 8): 480 + 384 + 320 = 1,184 in² = 8.22 ft² = 0.764 m².
- Wall dissipation at k = 3.7: 3.7 x 0.764 x 10 = 28.3 W.
- With the lower published value of about 1.9 W/m²K: 1.94 x 0.764 x 10 = 14.8 W.
Worked example: is a sealed box enough?
No. The design load is 74.1 W and the walls shed 28.3 W at the allowed rise: a shortfall of 45.8 W.
- What the sealed box would actually reach: ΔT = 74.1 / (3.7 x 0.764) = 74.1 / 2.83 = 26.2 K, so 35 + 26.2 = about 61 °C inside, above the 50 °C device rating. Even without the margin, 59.3 / 2.83 = 21.0 K gives about 56 °C.
- The surface a sealed box would need: A = 74.1 / (3.7 x 10) = 2.00 m² (21.5 ft²), about 2.6 times the 0.764 m² this enclosure exposes.
So this load needs active cooling, a much larger enclosure, or some of the heat moved outside, for example a power supply mounted elsewhere.
Step 4: size the cooling
| Option | Formula | Result | Keeps the enclosure sealed? |
|---|---|---|---|
| Circulating fan | Mixes inside air | Up to about 10% more dissipation; removes hot spots | Yes |
| Fan and filter | V = f x Qv / ΔT, f = 3.1 m³K/Wh up to 100 m altitude; or CFM = 3.16 x W / ΔT (°F) | 3.1 x 74.1 / 10 = 23 m³/h; 3.16 x 74.1 / 18 = 13 CFM | No: open loop |
| Air-to-air heat exchanger | qw = (Qv - k x A x ΔT) / ΔT | (74.1 - 28.3) / 10 = 4.58 W/K (2.55 W/°F) | Yes: closed loop |
| Enclosure air conditioner, 45 °C room, 35 °C inside | QK = Qv - k x A x (Ti - Tu) | 74.1 - 3.7 x 0.764 x (35 - 45) = 74.1 + 28.3 = 102 W | Yes: closed loop |
Fans and filters move outside air through the enclosure, so the inside sees whatever the outside air carries. The fan formula ignores what the walls shed, which makes it conservative. The two versions agree: 23 m³/h is about 13.5 CFM. Air is thinner at altitude, so the factor f rises to 3.5 m³K/Wh at 750-1,000 m. Put the inlet low on one side and the outlet high on the opposite side, make the outlet at least as large as the inlet, and prefer a fan blowing in at the inlet, which raises the inside pressure and helps keep dust out.
Heat exchangers keep the inside air separate from the outside air. A climate-control catalogue recommends them where the ambient air carries dust, oil or aggressive substances that must not get in. They only work while the outside air is cooler than the inside, and the catalogue calls a 10 K difference ideal. A cross-check in US units gives the same answer: 74.1 / 18 = 4.12 W/°F, less the walls' 0.191 W/°F·ft² x 8.22 ft² = 1.57 W/°F, leaves 2.55 W/°F. With the lower k value the requirement rises to (74.1 - 14.8) / 10 = 5.9 W/K, so choose with margin.
Air conditioners are the only option here that can hold the inside below the outside temperature. When the room is hotter than the enclosure, the walls bring heat in, and that heat adds to the load. They also dehumidify the air inside; the condensation guide covers what to do with the water.
The rating: anything mounted in the enclosure wall becomes part of the enclosure. NEMA's rule is that an assembly's rating is limited by its least-rated component, so a fan, heat exchanger or air conditioner on a NEMA 4X-type enclosure must itself carry a 4X rating, or the enclosure is no longer 4X. In washdown areas that usually rules out fans and filters; see washdown enclosures.
Outdoors: add the sun
The same thermal guide gives the solar gain as a chart of watts per square foot against the temperature rise you want to hold, by colour. For an ANSI 61 gray enclosure its text gives about 14 W/ft² at zero rise (holding the enclosure at ambient) and about 6 W/ft² at a 20 °F (11 K) rise. Applied to the 8.22 ft² of our example:
- Held at ambient: 14 x 8.22 = 115 W from the sun, plus 74.1 W from the equipment = about 189 W to remove.
- Held at 20 °F above ambient: 6 x 8.22 = 49 W plus 74.1 W = about 123 W.
That is for a gray paint finish; the chart's white and metallic curves sit far lower. Even so, on this small enclosure the sun can add more heat than the equipment. Shade and colour come first; the outdoor enclosures guide covers sun shields and finishes.
Check the answer on site
The same relation works backwards. A climate-control catalogue notes that if the heat loss inside an enclosure is not known, you can measure the inside and ambient temperatures and calculate it from Qs = k x A x ΔT. Measure near the hottest devices, not only in the middle of the enclosure: the calculation gives an average, and a component that produces much of the heat, or sits next to one, can run much hotter. Keep the chilled air from a heat exchanger or air conditioner off active components, and leave room around the devices for air to move; the backplate layout guide covers heat zones and spacing.
Custom builds. NEMA4 Enclosure custom enclosures in 304 or 316L stainless, powder-coated carbon steel and aluminum are built to the size you specify, so you can choose the surface area the heat load needs; polycarbonate and fiberglass come in standard molded sizes cut to your layout. Tell us which fan, heat exchanger or air conditioner you will fit and we cut the opening for it; the rating of the finished assembly is then limited by the rating of that unit. Our wall-mount enclosures in 304 and 316L stainless are manufacturer-rated NEMA 4X-type / IP66 - not UL listed.
Frequently asked questions
How do I calculate the heat load in an electrical enclosure?
Add the power loss of every device from its datasheet. For power supplies, loss equals output power times (1 divided by efficiency, minus 1). Then add a safety margin; one enclosure maker recommends 25%.
How much heat can a sealed enclosure dissipate?
Multiply the heat transfer coefficient by the exposed surface area and the allowed temperature rise. A 24 x 20 x 8 in stainless enclosure on a wall exposes 0.764 m², so at 3.7 W/m²K and a 10 K rise it sheds about 28 W.
How do I size a fan for an enclosure?
Airflow in CFM equals 3.16 times the heat load in watts divided by the allowed rise in °F, or in m³/h, 3.1 times the watts divided by the rise in kelvin at sea level. Fans exchange outside air, so they do not keep a NEMA 4X seal unless the fan unit is rated 4X.
When do I need an air conditioner instead of a heat exchanger?
When the inside must be cooler than the surrounding air. A heat exchanger only works while the ambient air is cooler than the inside; an air conditioner can cool below ambient.
Does a stainless enclosure run hotter than a painted one?
For the same heat load, yes. One thermal guide notes that unfinished stainless steel radiates heat less efficiently than painted surfaces, and published heat transfer coefficients for stainless are lower than for painted steel.
Sources
- Rittal, Technical System Catalogue Climate Control, calculation basis (Qs = A k ΔT, k = 5.5 W/m²K sheet steel, IEC 890 surface area, measuring unknown heat loss)
- Rittal, fan-and-filter units project planning (V = f Qv / ΔT, f by altitude)
- Rittal, air/air heat exchangers project planning (qw formula, 10 K ideal, chilled air not at components)
- Rittal, cooling units project planning (cooling output formula, cooling below ambient, dehumidification)
- Rittal, cooling units selection criteria (installation type to IEC 890)
- nVent HOFFMAN, Spec-00488, thermal management (surface area formula, materials, 25% margin, reliability, circulating fans, solar chart and examples, fan formula and placement)
- Hammond Manufacturing, heating products selection (heat transmission coefficients)
- AutomationDirect Library, "Selecting an Enclosure Heat Exchanger" (W/°F method and coefficients)
- NEMA, "NEMA FAQs: Enclosures" (Q15, the weakest-link rule)