Short answer: count the free area after mesh and hood, then make the inlet and outlet free areas equal — an outlet up to 1.5x the inlet spends the last useful pascal, and beyond 150% you buy single-digit percent more flow. An 0.8 m [31.5 in] tall enclosure running 65 K warmer inside than out develops about 1.2 Pa of draft, so the smallest restriction on your path, usually the mesh or the rain-hood gap, sets the airflow, and no outlet ratio rescues it. That is the arithmetic under outdoor brake resistor enclosure selection. If the box must take hose-down, venting is off the table: a sealed build with an enclosure heat exchanger answers it, and the vent question goes away.
How much draft a warm enclosure makes
Your draft is a weight difference. The warm column inside the box weighs less than the cold air outside it, and the push comes to Δp = 9.81 x H x (ρ_out − ρ_in), with H the height between the middle of the inlet and the middle of the outlet. Air at 35 C [95 F] weighs 1.15 kg/m³; the same air at 100 C [212 F] weighs 0.95 kg/m³. A 65 K rise hands you 0.20 kg/m³ of difference to work with.
| Inside-outside rise | 0.6 m tall box | 1.0 m box | 2.0 m box |
|---|---|---|---|
| 20 K | 0.4 Pa | 0.7 Pa | 1.4 Pa |
| 40 K | 0.8 Pa | 1.3 Pa | 2.6 Pa |
| 65 K | 1.2 Pa | 2.0 Pa | 3.9 Pa |
| 90 K | 1.5 Pa | 2.5 Pa | 5.1 Pa |
A working rule from that table: draft ≈ 0.03 x H(m) x ΔT(K) pascals at 35 C ambient, good from a 20 K to a 90 K rise. Height is your cheapest lever — a 1.0 m box doubles the draft of a 0.5 m box at the same temperature. At 1500 m elevation the air is about 17% thinner and mass flow drops by roughly a quarter, so go one vent size up for high sites. Wind dwarfs all of it: a 5 m/s [11 mph] breeze carries about 15 Pa, ten times your draft, which is why both openings wear hoods.
Why the smaller opening sets the airflow
The two openings sit in series, and series areas combine as 1/A² = 1/A_in² + 1/A_out². A 0.040 m² [62 in²] inlet feeding a 0.067 m² outlet behaves as one 0.034 m² hole, much closer to the smaller opening. That is why a fat roof vent cannot rescue a starved inlet: the inlet caps the flow, and the outlet trims only the last few percent.
The faster-air argument runs backwards. A smaller inlet does raise inlet velocity and does pull the inside pressure down, because you spent your 1.2 Pa of draft on the inlet's resistance. The budget is fixed by height and temperature; every restriction spends it, and none adds to it. Halve the inlet free area with the outlet at 1.67x and the flow falls to about 56%. Holes in one wall add instead: two 0.02 m² vents equal one 0.04 m² vent, the same parallel addition behind combining multiple vents for natural draft in boiler stacks.
Mesh, louvres and hoods hide more area than the drawing shows
The cutout on the drawing is not the area the air sees. Mesh, louvre blades and the rain hood each take their share before any air moves:
| Hardware | Free area you get | What moves it |
|---|---|---|
| Woven mesh, 2.1 mm aperture | 45-65% of the hole | Wire diameter: (aperture ÷ pitch)², so 0.5 mm wire gives 65%, 1.0 mm gives 46% |
| Blade louvre | 30-50% of the face | Blade depth and pitch; the maker's datasheet prints the measured figure |
| Perforated sheet | about 50% | The hole pattern |
| Rain hood | the escape gap sets it | Keep the gap at least 1.5x the vent free area, with one turn of the air, not two |
Compare free areas, never cutout sizes. If your 2.1 mm mesh runs 65% open with 0.5 mm wire instead of 50% with heavy wire, the flow rises about 30% — wire gauge moves more air than any outlet ratio will. Write the mesh on the order as aperture plus wire diameter, with the open-area percentage beside it.
Does a bigger outlet help? The 150% rule on trial
One honest reason exists for a bigger outlet: the leaving air is lighter. At a 65 K rise the exhaust density sits 21% below the intake, and equal pressure sharing between the openings lands at A_out/A_in = √(ρ_in/ρ_out) ≈ 1.10. The balanced box carries an outlet about 10% larger than the inlet, not 50%.
Then the curve flattens. With a fixed inlet, measured against the flow an open roof would give:
| Outlet free area vs inlet | Share of maximum flow |
|---|---|
| 100% — equal | 71% |
| 120% | 77% |
| 150% | 83% |
| 167% | 86% |
| 200% | 90% |
Equal free areas cost about 29% of the flow your inlet could ever deliver — a modest price when wall space is tight. The 150% convention lands where the curve is already flat: from 150% to 200% you gain about 7%, and from 150% to an open roof about 20%. When more panel space exists, add area to whichever free area is smaller right now; that habit keeps the pair near the top of this table with no ratio arithmetic at all.
Sizing the vents for a 2.2 kW brake resistor in an 0.8 m box at 35 C ambient
Inputs, from the drawing and the site:
- Enclosure 800 x 400 x 400 mm [31.5 x 15.7 x 15.7 in]; inlet 400 x 200 mm low on one side; roof outlet 365 x 365 mm
- 2.1 mm mesh at 50% open on both openings, per the mesh datasheet
- Ambient 35 C, interior estimate 100 C, average resistor dissipation 2.2 kW
- Sea level; exhaust near interior temperature
Rules in words: draft equals 9.81 x height x density difference; the two free areas combine as a sum of 1/A²; flow equals 0.6 x combined area x √(2Δp/ρ); heat carried equals mass flow x 1.005 kJ/kg·K x rise. The 0.6 is the discharge coefficient building-ventilation practice gives a sharp-edged opening.
The arithmetic:
- Densities from ρ ≈ 353/T: 1.15 kg/m³ at 35 C, 0.95 at 100 C, difference 0.20 kg/m³
- Height from mid-inlet (0.2 m) to roof (0.8 m): H = 0.6 m; draft = 9.81 x 0.6 x 0.20 = 1.2 Pa
- Free areas: 0.08 m² x 50% = 0.040 m² in; 0.133 m² x 50% = 0.067 m² out
- Combined area: 1/0.040² + 1/0.067² = 625 + 223 = 848, so A = 0.034 m²
- Flow: 0.6 x 0.034 x √(2 x 1.2 ÷ 1.15) = 0.6 x 0.034 x 1.44 = 0.029 m³/s ≈ 105 m³/h [60 cfm]
- Heat: mass flow 1.15 x 0.029 = 0.033 kg/s; 0.033 x 1.005 x 65 = 2.2 kW
Decision: at a 2.2 kW average the interior sits at 100 C, the vents carrying exactly the load with nothing spare. Hold the average near 1.1 kW and the same openings keep a 40 K rise, 75 C inside, which 90 C wiring tolerates. Above 2.2 kW, buy inlet free area or height — doubling both free areas doubles the flow, while the outlet ratio buys percent. Stand the box in the sun and add that load first: the solar heat load on an outdoor enclosure puts a few hundred watts on a box this size. Brake duty comes in pulses, so size on the duty-cycle average and let the thermal mass coast between them — how fast an enclosure heats up covers that estimate. Isolate the supply and prove it dead before opening the door; a 100 C resistor burns skin long after the drive stops.
What open vents do to the enclosure rating
A meshed vent under a rain hood sheds rain and falling water, which is Type 3R territory: the weather of an outdoor pad, with no hose resistance at all. Write a ventilated build as a 3R-class design, because any open vent fails the Type 4 hose test. Our catalog enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed — and the weakest part sets the rating, so an open vent drops your whole assembly to the vent's level. The NEMA 3R vs 4 comparison prints what each type faces, what cooling louvers cost the rating covers the sealed-plus-cooling route, and vent or sealed box outdoors covers the moisture side of the same choice.
| Where the box lives | Vent choice | What you can claim |
|---|---|---|
| Outdoor pad, rain and sun | Meshed inlet low, hooded outlet high | 3R-class: rain and falling water, no hoses |
| Washdown or hose-down area | No vents; sealed build with a heat exchanger or air conditioner | 4X-class by design |
| Indoor, dry plant | Meshed vents low and high | As dust-tight as the mesh |
| Coast, brine, chemical wash | Sealed 316L build with cooling | 4X-class by design; vents corrode and clog first |
What to specify
Put numbers on the drawing, not adjectives:
| Line item | What to write |
|---|---|
| Inlet | Free area in m², position low on the wall, mesh aperture and wire diameter |
| Outlet | Free area 1.0-1.5x the inlet, at the top; upper side wall when the roof runs out |
| Mesh | 2.1 mm aperture, wire diameter, 316L near salt water |
| Hood | Escape gap at least 1.5x the vent free area |
| Inside | Resistor in the bottom third, clear line from inlet to outlet |
Bolt the resistor low: the lower it sits, the taller the warm column and the more draft the same box makes. Keep 50 mm [2 in] of solid roof around any roof cutout, because an outlet nearly the footprint turns the lid into a flap. Lay both vents out in the cutout designer and send the layout with your quote; custom enclosures take your free-area figures with the mesh and hood spec, delivered in 10 business days from your approval of the drawing. On the first hot day, hold a vane anemometer across the outlet and compare the reading with the design flow.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 10 × 6 in | 316L stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 316L stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does adding a stack on the roof vent increase the airflow?
Yes, and it is the cheapest upgrade you can make. Draft scales with height, so 1 m of 150 mm [6 in] duct on the outlet of the example box takes the effective height from 0.6 to 1.6 m and the draft from about 1.2 Pa to about 3.1 Pa. Flow rises with the square root of that, about 1.6x, less a little duct friction. Keep the duct free area at least equal to the vent and cap it against rain.
How much does wind change the ventilation?
A 5 m/s [11 mph] breeze carries over ten times the stack draft of a warm enclosure, so wind can double your flow or reverse it. Hood both openings and set them so the prevailing wind meets neither face square. On an exposed site, steady wind usually moves more air than extra vent area would.
Should the openings still differ when the box runs only 20 K warmer inside?
Barely. The balanced outlet-to-inlet ratio is the square root of the density ratio: about 1.03 at a 20 K rise and 1.10 at 65 K. At small temperature differences the two free areas want to sit within a few percent of each other, so make them equal and spend your panel space making both bigger. The 150% convention earns its keep only at large rises.
How do I check the airflow once the box is built?
Hold a 100 mm [4 in] vane anemometer across the outlet on a hot day: the example box should read about 0.4 m/s over the free area, 0.2 m/s over the gross hole, roughly 60 cfm. A smoke pen at the inlet shows direction and dead corners. If you read under half of that, clean the mesh and open the hood gap first, because those clog years before the holes are wrong.
What mesh material should an outdoor vented enclosure use?
Stainless, matched to the site: 316L within about 1 km [0.6 mi] of salt water or where chlorides wash the panel, 304 for general outdoor work. Mesh corrodes first because its fine wires carry huge surface per unit of metal, and a rusted mesh both restricts flow and stains the box below it. Plastic mesh holds up where the vent stays under about 80 C [176 F].
