Size a non-ventilated stainless enclosure by heat dissipation

Short answer: divide the heat by k times the allowed rise. At 150 W of losses and a 10 K rise, a non-ventilated stainless enclosure needs 4.05 m² (43.6 ft²) of usable wall surface. Stainless in still air sheds 37 W per square metre at that rise (k = 3.7 W/m²K). A 48 × 60 × 16 in wall-mount cabinet delivers 4.09 m² of usable surface and carries 151 W, holding the load at a 9.9 K rise. Painted steel sheds 55 W/m² under the same conditions and needs only 2.73 m². That is why a painted design rebuilt in stainless rises about 15 K instead of 10 K. The wall alloy barely matters; the finish and the two air films set the number.

This page sizes a sealed box for a 5.5 kW drive at 4.6 kW output plus controls. It scales the result to other loads and ends with the checks and the order line.

How much surface does the load need at a 10 K rise?

Set the area from Q = k × A × ΔT: 150 W ÷ (3.7 W/m²K × 10 K) = 4.05 m² (43.6 ft²) of usable stainless surface. The basis is still air, indoors, with no sun on the box — 37 W per square metre, or 3.4 W per square foot, for every 10 K of rise.

Each symbol is an input you set. Q is the heat the contents release at working load. ΔT is the rise you allow over ambient. k is the combined surface coefficient for convection plus radiation. In still air, enclosure heat-sizing works at 3.7 W/m²K for stainless, 5.5 W/m²K for painted steel and 3.5 W/m²K for plastic; the enclosure sizing guide carries these values with the rest of the method. IEC 60890 turns the same surface balance into a calculation method for low-voltage assemblies. The enclosure heat dissipation formula and surface-area calculation page runs the same balance step by step with painted-steel numbers, if you want a second worked pass.

Two habits keep the number honest. Add margin for fouling and future losses; 10% more area costs little on a custom build. And hold the basis. 37 W/m² assumes still air, an indoor box and a finish close to brushed stainless. Sun or wind rewrites it.

What numbers do you need before you size the box?

Three inputs fix the answer: total losses at working load, the rise the hottest component tolerates, and k for the wall finish. At the drive design load with a 10 K rise they set 4.05 m² of usable surface before any deduction.

Work them in this order:

1. Tally the losses. Read each component's dissipation at working load. A 5.5 kW drive running 4.6 kW at 97–98% efficiency sheds 110–165 W; that range is what small drive datasheets publish. With a PLC, a 24 V supply and terminals, 150 W total is the realistic planning figure. The tally method sits in the component-loss heat load guide.
2. Set the rise from the hottest part. Most drives and electronics carry a 40 °C ambient rating unless their datasheet allows more. At a 30 °C site, a 10 K rise lands the interior at 40 °C. At a 35 °C site the budget is 5 K and the area doubles.
3. Pick k from the wall finish: 3.7 W/m²K stainless, 5.5 W/m²K painted steel, 3.5 W/m²K plastic, all in still air.
4. Divide: A = Q ÷ (k × ΔT).
5. Convert to dimensions and deduct the mounting face before you commit a size; the rules are in the usable-area section below.
6. Check the contents still fit. Heat binds this design, but DIN rail length, bending space at terminals (NEC 312.6 sets it by conductor size) and door swing still decide the depth. The drive case is worked end to end in the VFD enclosure sizing article.

Why does stainless shed less heat than painted steel?

Because k tracks the finish rather than the alloy. Brushed stainless works at 3.7 W/m²K against 5.5 W/m²K for painted steel, so the stainless box needs about 49% more surface. A 10 K painted design runs at roughly 15 K in stainless.

The instinct that bare metal beats paint comes from conductivity tables, and they mislead at enclosure scale. Austenitic stainless conducts about 16 W/mK and mild steel about 50 W/mK. Yet across a 1.5 mm sheet carrying the full load over 4 m², the drop through the metal is 150 × 0.0015 ÷ (16 × 4) = 0.004 K. Nearly the whole 10 K lives in the two air films, one on each face. Paint's high emissivity strengthens its radiating film, and that film is the difference the two k values carry. The practical consequence: keep the stainless bare and brushed for washdown. Buy area instead of chasing a coating that chlorides will undermine.

What counts as usable surface area?

Every face with free air counts, and the mounting face comes off. The 48 × 60 × 16 in cabinet has 5.95 m² gross, its 1.86 m² back sits on the wall, and 4.09 m² of door, sides, top and bottom does the cooling.

• Door, sides and top count in full at any mounting height.
• The bottom counts when air can pass beneath it; at head height on a wall-mount, air passes under it freely.
• The back is the mounting face. Subtract it. Standoffs let a little heat off the rear panel, but the gap chokes the film, so count none of it and keep the deduction.

A cutout list matters less than you would think. Rated glands and a sealed window still pass heat through their own surfaces, so the wall keeps working roughly as sized.

What box size delivers the area?

A 48 × 60 × 16 in (1220 × 1525 × 410 mm) wall-mount cabinet does it. Its 9216 in² gross is 5.95 m², the 2880 in² (1.86 m²) back comes off, and 3.7 × 4.09 × 10 = 151 W at a 10 K rise covers the design load with a whisker over.

The arithmetic, face by face:

• Front and back: 2 × (48 × 60) = 5760 in²
• Sides: 2 × (60 × 16) = 1920 in²
• Top and bottom: 2 × (48 × 16) = 1536 in²
• Gross: 9216 in² = 5.95 m², at 1550 in² to the square metre
• Usable: 9216 − 2880 = 6336 in² = 4.09 m²
• Capacity: 3.7 W/m²K × 4.09 m² × 10 K = 151 W, or a rise of 150 ÷ (3.7 × 4.09) = 9.9 K at the full load

Want margin for fouling or a future second drive? Step up to 48 × 72 × 16 in: gross 10,752 in² (6.94 m²), usable 4.71 m², rise 150 ÷ (3.7 × 4.71) = 8.6 K. Area is what matters, so trade width against height to suit the wall. The drive's terminals, rail length and door swing then fix the depth.

How does the area scale with load or rise?

One-for-one with watts and inverse with kelvin. Each square metre of stainless carries 37 W at a 10 K rise, so 250 W takes 6.76 m². Relaxing the rise from 10 K to 20 K cuts the design requirement to 2.03 m².

Usable stainless surface in m² (k = 3.7 W/m²K, still air):

Read the row for your load and the column for your rise, then convert to a box as the worked example shows. For painted steel, divide the load by 55 instead of 37 at a 10 K rise; for plastic, by 35.

Three things move the column. A 40 °C-rated drive at a 35 °C site has 5 K of budget, and the top row of the table then reads 8.11 m², double the 10 K figure. Direct sun adds load before the electronics release a watt; the solar heat gain article runs that number. And a mounting wall warmer than the air, off a chimney or process line, quietly eats the rise budget — the hot-surface heat transfer article shows by how much.

How do you add cooling without losing the rating?

Move the source or move the heat, with parts documented for the same ingress rating. A drive maker's through-panel heatsink kit, fitted with its gaskets to its instructions, keeps a Type 4X / IP66 assembly whole. A louver or fan-filter drops the entire enclosure to that part's own rating.

Our boxes come rated NEMA 4X / IP66 by the manufacturer; not UL listed. The weakest penetration sets the assembly's rating, so every gland, window and cooler needs its own rating to match. The Type 4X versus IP66 guide explains what each rating tests.

The strongest move with a drive is to put its heatsink through the rear wall. The 110–165 W of drive loss then sheds outside, and only the controls remain. With 35 W left inside, A = 35 ÷ (3.7 × 10) = 0.95 m² usable. A 20 × 28 × 10 in cabinet provides it: 2080 in² gross (1.34 m²) less a 560 in² back (0.36 m²) leaves 0.98 m², a 9.7 K rise at 35 W. Size a through-wall cooler from its W/K rating multiplied by your allowed internal-over-ambient difference.

How do you prove the rise before the panel ships?

Heat the box electrically and log it. Power resistors sized to the design load go on a fused, switched feed, with one thermocouple near the drive heatsink, logging until the trace flattens. For the worked cabinet that means 3 hours, not half an hour.

The box answers slowly. The 48 × 60 × 16 in cabinet carries about 70 kg of 1.5 mm stainless; at roughly 500 J/kgK that is 35,000 J/K of heat capacity. The walls shed 3.7 × 5.95 = 22 W/K, so one time constant is 35,000 ÷ 22 ≈ 1,600 s, about 27 min. Log for 3 hours, near seven time constants, and treat the flat tail as your steady rise. The rise then adds to site ambient: 9.9 K measured at a 20 °C day means 39.9 °C inside on a 30 °C day, right at the 40 °C rating most drives carry.

On site, clip a thermocouple to the drive heatsink, let the panel reach steady state, and compare with the bench number. More than a kelvin or two of gap points to a blocked outer film, packed walls or a hot mounting wall. Wiring the resistor bank is live work: lock out the feed and check absence of voltage before you reach in.

What goes on the order?

Order a custom sealed cabinet: 48 × 60 × 16 in (1220 × 1525 × 410 mm), 304 stainless for washdown or 316L for chlorides, wall-mount, no ventilation openings, cutouts to your layout. Built to order and delivered in 10 business days from drawing approval.

Type 4 and Type 4X share the same k, so the heat sizing holds either way; the X buys the corrosion grade. Put the mounting face, standoffs, gland positions and any heatsink cutout on the drawing, each penetration specified to the wall's own rating. The custom enclosure page takes the dimensions and every cutout from one piece upward, and the enclosure selector turns an environment plus an inside size into a shortlist before you commit.

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.

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