Size a VFD enclosure from the drive's heat loss and its mounting clearances, never from the drive's footprint. A 2.2 kW spindle drive loses about 66 W at rated load. At a 25 C shop ambient a 30 x 24 x 10 in stainless box sheds 90 W, which holds the air at the drive near 36 C, inside the 40 C limit most drives carry. Pick a box by footprint alone and the drive fits but cooks: the same 66 W in a 20 x 16 x 10 in box lifts the interior about 20 K, to 45 C that day. The steps below run from the datasheet figures to a box size, a layout, and the sealed-versus-ventilated call, then to a stainless box built to your drawing.
What you need before you size
Five figures decide the box. Three come from the drive's documentation, two from the site.
| Figure | Typical value | Where it comes from |
|---|---|---|
| Heat loss at rated load | 2-6% of the drive's power rating; 3% is the working estimate | The datasheet's power loss line; 2-6% is the common sizing range for drives |
| Drive ambient limit | 40 C (104 F) | The drive's rating; some datasheets allow 45-50 C with output derating |
| Site ambient | 25 C conditioned shop; 35 C hot plant or sunlit wall | Your site record; a sunlit wall runs above shade temperature |
| Mounting clearance | Commonly 4 in [100 mm] above and below the heatsink | The installation drawing in the drive manual |
| Other loads | Supply, PLC, HMI, each in watts | Each component's datasheet |
In a sealed box the walls do all the cooling. The wall value this method uses is k = 3.7 W/m²K for stainless, the figure behind this site's heat guides; painted steel runs 5.5 W/m²K and plastic 3.5 W/m²K. Stainless sheds the least of the three, so sizing on it is the safe case.
The sizing steps
- Add the heat. Take the drive's loss from its datasheet, or estimate 3% of its rating. Add every other energized component: a 240 W, 90% efficient 24 V supply loses about 27 W, and a compact PLC adds about 20 W.
- Set the allowed rise. Subtract site ambient from the drive's 40 C limit: 15 K at a 25 C ambient, 5 K at 35 C.
- Size the wall surface. The walls shed k × surface × rise, so the surface you need is heat ÷ (3.7 × rise), in m², for stainless walls.
- Pick the box. Match that surface against the table below. Every size is orderable; wall-mount stainless runs from 6 x 4 x 4 in to 60 x 36 x 16 in (H x W x D).
- Lay the drive out. Upright, fins vertical, with the manual's air space above and below, glands on the bottom wall, and the shielded motor cable clear of the signal duct.
- Call it sealed or ventilated. Seal the box when the walls shed at least 1.2 times the heat at the allowed rise. A filter fan drops the assembly's rating to the fan's own, so on washdown and outdoor boxes the alternatives are a bigger box or a sealed air-to-air exchanger; the fan-on-a-4X walkthrough tallies what the hole costs, and the fan airflow arithmetic gives the m³/h figure. Our stainless boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed.
Heat a stainless box sheds, by size
Watts = 3.7 W/m²K × total outside surface × allowed rise, counting all six faces; the wall-mount feet hold the box off the wall, so the back still sheds. The rise is the drive's 40 C limit minus ambient: 15 K at 25 C ambient, 10 K at 30 C, 5 K at 35 C.
| Box, H x W x D (in) | Surface (m²) | Sheds at 5 K | Sheds at 10 K | Sheds at 15 K |
|---|---|---|---|---|
| 16 x 12 x 8 | 0.54 | 10 W | 20 W | 30 W |
| 20 x 16 x 10 | 0.88 | 16 W | 32 W | 49 W |
| 24 x 20 x 10 | 1.19 | 22 W | 44 W | 66 W |
| 30 x 24 x 10 | 1.63 | 30 W | 60 W | 90 W |
| 36 x 28 x 12 | 2.29 | 42 W | 85 W | 127 W |
| 48 x 32 x 16 | 3.63 | 67 W | 134 W | 202 W |
| 60 x 36 x 16 | 4.77 | 88 W | 176 W | 265 W |
Surface grows with the square of the linear size and volume with the cube. Doubling every dimension quadruples the heat the walls shed, which is the lever the worked example pulls when the ambient climbs.
Worked example: a 2.2 kW spindle drive at 25 C and 35 C ambient
The case is the common CNC build: one 2.2 kW spindle drive, its 24 V supply, room for a PLC later. The inputs:
- Drive: 2.2 kW output; heat loss 3% × 2,200 W = 66 W (the common 2-6% rule; the datasheet line replaces it when you have it)
- Drive air limit: 40 C
- Wall: stainless, k = 3.7 W/m²K, six faces counted
- Ambient: 25 C shop, then 35 C hot plant; both are assumed example values, so take yours from the site
The rule: the walls shed k × surface × rise, so find the surface that sheds the heat, round up to a real box, and check the 1.2 times margin.
At 25 C ambient: one size up
Allowed rise: 40 − 25 = 15 K. Surface needed: 66 ÷ (3.7 × 15) = 66 ÷ 55.5 = 1.19 m², about 1,843 in². The 24 x 20 x 10 in box offers essentially that, 1,840 in², and sheds 66 W: the mathematical minimum, zero margin. Order the 30 x 24 x 10 in box instead. Its 1.63 m² sheds 90 W at a 15 K rise. With 66 W inside, the rise is 66 ÷ (3.7 × 1.63) = 11 K, so the drive air sits at 25 + 11 = 36 C. That is 4 K under the limit, with a 1.36 times margin (90 ÷ 66).
At 35 C ambient: the box doubles
Allowed rise: 40 − 35 = 5 K. The 30 x 24 x 10 in box now sheds 30 W, under half the load, while the drive's actual rise stays 11 K. The interior lands at 35 + 11 = 46 C, 6 K over the limit. Surface needed: 66 ÷ (3.7 × 5) = 3.57 m², about 5,530 in². The 48 x 32 x 16 in box (5,632 in², 3.63 m²) sheds 67 W at a 5 K rise. That holds the drive air at 35 + 66 ÷ 13.43 = 40 C, on the limit and short of the margin. The 60 x 36 x 16 in box sheds 88 W, 1.33 times the load, and holds 39 C.
Add the supply and a PLC and the load is 113 W. At 25 C that takes 113 ÷ 55.5 = 2.04 m², so the 36 x 28 x 12 in box sheds 127 W against the 113 W load, a 1.12 times margin, and the 48 x 32 x 16 in box carries the full 1.2 times. Beyond two or three drives the arithmetic changes scale; the multi-drive sizing article works the large lineup, and the one-drive guide pairs this method with the rating and cooling decision.
The call: one 2.2 kW drive at 25 C is a 30 x 24 x 10 in sealed stainless box. The same drive at 35 C is 48 x 32 x 16 in minimum, 60 x 36 x 16 in with margin, or a sealed exchanger on the smaller box.
Clearance and layout rules that decide the fit
- Take the drive's clearance from its installation drawing, commonly 4 in [100 mm] of open air above and below with fins upright. That air feeds the box air the surface calculation budgeted.
- Give large conductors bending room at their terminals: NEC 312.6 sets the minimum wire bending space in cabinets and cutout boxes, table per conductor size, with a second table when the conductor enters the wall opposite its terminal. The enclosure sizing guide walks the tables.
- Fill wire duct to half: duct makers size their fill tables at 50% of the duct's interior cross-section, citing NFPA 79, because round cables never pack tighter. The backplate layout guide carries the fill and DIN rail arithmetic.
- Bring the shielded motor cable in at the bottom, land its shield at the drive's PE terminal, and cross signal runs at right angles; the component layout guide shows the sealed-box arrangement.
- Keep 20-25% of the backplate spare; the next revision always comes.
- Leave the working space outside too: NEC 110.26(A) wants clear space in front of equipment serviced while energized, with the door open at least 90 degrees, as the mounting guide lays out.
Checks after the box is built
- Recompute the heat from the datasheet before the order goes in. The 2-6% spread puts a 2.2 kW drive between 44 and 132 W, and a box chosen at 3% fails at 6%.
- Run the drive at full load for an hour, then read the air temperature at the drive. The 30 x 24 x 10 in box should read near 36 C on a 25 C day, its 11 K design rise. More than 15 K above ambient means the load beat the estimate.
- Point a thermal camera at the terminals after the first run. A hot terminal is a connection problem, not a box problem.
- Close the door and inspect the gasket line: even compression all round, no cable pressing it, and the manual's clearances still open with the wires dressed.
Mistakes that cost the drive or the rating
- Never size from the footprint. A box with no watt budget faults on the first hot afternoon, at 46 C in the 35 C case above.
- Skip the loss spread and you gamble: the box that passes at 3% fails at 6%, where 66 W becomes 132 W.
- Keep sealed boxes sealed. A filter fan hole hands the assembly's rating down to the fan; use a sealed exchanger or upsize instead.
- Mount the drive the way the manual's drawing shows, upright with fins vertical; the clearance figures assume it.
- Add the sun before you fix the size. A sunlit west wall runs well above shade ambient; the real heat gain walkthrough works that case.
What to order
Three lines carry the whole decision:
- Size: the 25 C call, a 30 x 24 x 10 in stainless box; the 35 C call, 48 x 32 x 16 in minimum or 60 x 36 x 16 in with margin.
- Material: 304 for indoor washdown and most outdoor sites; 316L for salt air, brine and chemical areas.
- Cutouts: gland positions on the bottom wall sized to the motor and supply cables, plus any door operators the machine needs.
The size finder filters the catalog by outside size and grade, the custom builder takes any size and cutout from one piece, and the enclosure selector narrows by environment and inside dimensions first. Built to your drawing and delivered in 10 business days.
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.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 30 × 24 × 10 in | 304 stainless | 30 x 24 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 10 in | 304 stainless | 20 x 16 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How many watts of heat does a VFD add per horsepower?
Multiply horsepower by about 22 W. One horsepower is 746 W, and a drive losing 3% of rated power turns 746 × 0.03 = 22 W into heat, so a 5 hp drive adds about 112 W. The honest spread is 2-6% of rating, 15-45 W per hp; the drive datasheet's power loss line replaces the estimate.
How much airflow does a filter fan need for a 66 W drive?
About 20 m³/h [12 cfm] for a 10 K rise: airflow in m³/h is roughly 3 × heat ÷ rise, so 3 × 66 ÷ 10 = 20. A fan-and-filter breathes outside air, so the assembly rating falls to the fan's own IP; on washdown or outdoor boxes, fit a sealed air-to-air exchanger or upsize the box instead.
Can the drive's heatsink sit outside the enclosure?
Yes, when the drive manual documents through-panel mounting: the heatsink plate seals through the back wall and dumps most of the drive's loss outside, so the box air only sees the control electronics' share of the 66 W. Cut the wall to the manual's drawing and keep its gasket, and the assembly holds the NEMA 4X / IP66 rating.
Does a drive that physically fits always fit the box?
No; heat decides first. In a 20 x 16 x 10 in box, 1,360 in² or 0.88 m², a 66 W drive lifts the air about 20 K, so 25 C ambient ends near 45 C, over the 40 C limit. Add a 27 W supply and a 20 W PLC and the rise is about 35 K, near 60 C. Size from heat first, then lay out the manual's 4 in clearances.
