For a single VFD, pick the rating from the environment, size the box from the drive's dimensions plus layout clearance, then check the heat. A 5 hp drive sheds roughly 100-150 W. A sealed steel box around 16 x 14 x 8 in (400 x 350 x 200 mm) can dissipate that only if you leave its surfaces clear and give the air inside a path. A bigger box is not automatically a cooler one, and holes you drill for a fan may cost you the rating you paid for. Here is how the three decisions fit together, and what to order.
The short answer
- Indoors, clean and dry: a NEMA 12-type steel or stainless box, sealed, no vents. Most garage and machine-shop drives need nothing more.
- Washdown, outdoors, coolant mist or conductive dust: stainless, manufacturer-rated NEMA 4X / IP66; not UL listed. No vent holes; cool it closed-loop or passively.
- Hot ambient or a drive over about 10 hp: a sealed box plus a heat exchanger or air conditioner, sized from the heat load, not guessed.
The common mistake is buying oversize "to be safe" and drilling a fan hole "just in case". The oversize does little for temperature, and the hole can undo the rating.
Start from the environment, not the box
The rating question comes before any size or cooling decision. Ask three things about where the drive lives:
- Water: hose spray, washdown, rain, condensation on a cold morning?
- Air: coolant mist, grinding dust, wood dust, salt air, fumes?
- Temperature: how hot does the room or the sun-facing wall actually get at peak?
Clean indoor air points to a NEMA 12-type enclosure. Anything wet or dusty points up the scale; a hoseable area needs Type 4X. Outdoors you should also plan for sun load, which adds real heat on a dark wall. If you are unsure how the type numbers map to conditions, the NEMA enclosure types guide covers the full list and what each type is tested for.
Two limits worth knowing. Ratings are manufacturer ratings; NEMA itself does not test or certify boxes, and a UL listed enclosure has been evaluated by UL to UL 50E, which some inspectors and specs require. And on US jobs, NEC 110.28 states that IP ratings are not a substitute for enclosure type ratings, so spec the NEMA type, not only an IP code.
Sizing the box for a drive and its bits
A VFD needs less volume than most people assume and more clearance than the box volume suggests. Work from the drive's own outline dimensions:
- Drive width and height, plus the manual's stated clearance above and below the heatsink, typically 4 in (100 mm) each way for airflow across the fins.
- A line filter or reactor, if you fit one, beside the drive on the same panel.
- Input terminals or a small terminal strip, and a door-mounted potentiometer or run controls if used.
- Gland space at the bottom: leave 2-3 in (50-75 mm) below the lowest device for cable entry and bending room.
- A removable mounting panel, so you can lay out the drive on the bench instead of inside the box.
For a typical 2 hp drive, that lands near 12 x 10 x 6 in (300 x 250 x 150 mm). For 5-10 hp with a filter and terminals, 16 x 14 x 8 in (400 x 350 x 200 mm) is a sensible starting point. NEC 312.6 sets the minimum wire bending space at terminals for cabinets and cutout boxes, so check it if your conductors are large. The enclosure sizing guide works through the full method, and the DIN rail and backplate layout guide covers the internal arrangement.
Our stainless wall-mount line runs from 6 x 4 x 4 in to 60 x 36 x 16 in (H x W x D), all with a hinged gasketed door, removable mounting panel and wall-mount feet. You can set the exact width, height, depth and every cutout per piece; the stainless enclosure size finder filters the whole line by grade and size. Order the size your layout needs, not one size up "for safety".
The heat math that actually matters
Sealed-box cooling works by conduction through the enclosure surface. The useful number is the temperature rise inside over ambient:
A 16 x 14 x 8 in box has about 11 square feet of surface. With a 5 hp drive shedding 120 W, the rise is roughly 120 / (0.17 x 11), about 64 C over ambient. That is far too much for still, sealed air, and it is the calculation most oversize-and-hope installations skip. Two things change it fast:
- A larger box helps less than expected, because surface area grows slowly with size.
- Moving air inside helps a lot. An internal circulation fan that sweeps the drive's heatsink toward the walls can cut the effective rise dramatically, because it improves heat transfer from the components to the shell.
So the honest order of operations is simple. Get the drive's heat loss from its manual (losses, not input power), write down the box's exposed surface area, and run the rise. The enclosure heat calculation guide works a full example with the same arithmetic. If the rise exceeds about 15-20 C over your worst ambient, or the drive's manual states a maximum operating temperature you cannot meet, move to active cooling.
Vents, a fan, or sealed cooling
The environment you picked in the first step decides the cooling method; the heat math decides its size.
| Environment | Cooling method | Rating kept? | Notes |
|---|---|---|---|
| Clean indoor air | Sealed, passive; internal circulation fan if needed | Yes | No holes. Fan stirs air inside the sealed box. |
| Warm room, still sealed | Heat exchanger through the wall | Yes | Moves heat without exchanging air. |
| Hot ambient or big drive | Air conditioner or sized heat exchanger | Yes | Size from the heat load and ambient. |
| Dusty but dry, no washdown | Filtered vents or an exhaust fan | No | The box is no longer sealed; the rating drops. |
| Washdown or outdoors | Sealed only | Yes | Never vent; see the note below. |
Venting a sealed box trades the rating for airflow. On a Type 4X box that trade is almost never worth it. Once you drill, you have a ventilated box with a gasketed door, and you own the argument with whoever expected a sealed enclosure. The article on putting a fan on a NEMA 4X enclosure walks through what the hole costs you, and the cooling options comparison picks between exchangers and air conditioners by heat load. For a drive enclosure specifically, the NEMA 4 heat exchanger requirements article covers the sealed-through-wall option in detail.
For most single drives in a shop, the right answer is the plainest one: a sealed box, an internal circulation fan if the rise math says so, and no penetrations at all.
Mounting and sealing the drive in
- Stand the box off the wall on its mounting feet if it is passively cooled; the rear surface is a heat path, and a tight flat mount throws it away.
- Enter cables from the bottom through glands or a gland plate sized for the cable count. Bottom entry keeps drips running past, not into, the seals.
- Do not cycle the drive on and off from a breaker as a routine switch; line-side cycling is hard on the DC bus capacitors. Leave it powered or follow the drive manual's recommended practice.
- Keep the drive's own heatsink fins vertical and clear, so the internal fan's airflow actually crosses them.
- Bond the door and the panel per the grounding practice in the grounding and bonding guide; paint or anodizing must not stand between bonding points.
If condensation is a worry in a cool garage or on an outdoor wall, that is a heater and insulation question, not a ventilation one. The condensation guide explains why sealing alone does not stop it.
What to put on the order
| Item | Spec to write |
|---|---|
| Rating | NEMA 12-type (indoor) or NEMA 4X / IP66, manufacturer-rated; not UL listed |
| Material | 304 stainless indoors; 316L for washdown, salt or sanitizers |
| Size | From your layout: drive plus filter, terminals, 4 in heatsink clearance, 2-3 in gland space |
| Cutouts | None for cooling on a sealed box; gland holes only, bottom face, sized to the glands |
| Mounting panel | Removable, so the drive is laid out on the bench |
| Cooling | Internal circulation fan (sealed) or heat exchanger / air conditioner sized from the heat load |
| Delivery | Delivered in 10 business days, counted from your order or drawing approval |
If choosing between sizes and grades is where you are stuck, the enclosure selector filters by environment, inside dimensions and DIN module count, and the stainless size finder filters the whole catalog by line, grade and size. Send the drive's manual heat-loss figure with the layout and we will check the rise before the drawing is cut.
Questions
How much heat does a VFD give off?
A rule of thumb is 2-3% of the drive's rated power as heat loss at full load. A 5 hp (4 kW) drive therefore sheds roughly 80-120 W continuously. Check the drive manual's stated losses, which are more accurate. Idle losses are far lower, often under 20 W, which is why a sealed box can coast overnight in a garage or workshop.
Can I just buy a bigger enclosure instead of doing the math?
Extra volume helps but less than people expect. Surface area, not volume, moves the heat out of a sealed box, and surface area grows slowly with size. Oversizing also wastes panel space and money. Size the box for the layout first, then check the temperature rise with the heat calculation and add active cooling only if the numbers demand it.
Is a NEMA 4X enclosure overkill for a single drive in a shop?
Usually yes. Indoors, clean air and no hose spray, a NEMA 12-type enclosure protects a drive just as well. NEMA 4X earns its cost in washdown areas, outdoors, or where coolant mist and metal dust are in the air. If your coolant spray can reach the box, the 4X rating and a stainless or plastic body are the right call.
Do I need a disconnect inside the VFD enclosure?
Many drive manuals ask for a means of electrical isolation upstream, and it is good practice, but a disconnect is not usually mounted inside the drive enclosure. Follow the drive manufacturer's installation instructions and your local code. Do not use the drive itself or a breaker as the routine on-off switch; drives last longer when they are not cycled on the line side.
Can I mount a VFD enclosure flat against a wall?
Only if the cooling method allows it. A sealed, passively cooled box loses most heat through its rear surface, so a tight wall mount costs you that path; stand it off on feet or a subpanel bracket. If the box has vents or a fan, follow the drive manual's clearance requirements around the drive's own heatsink regardless of mounting.
What size enclosure for a 2 hp VFD?
A typical 2 hp drive in a metal case fits in roughly a 12 x 10 x 6 in box with room for a line filter and terminals. Its heat loss at full load is around 40-60 W, which a sealed steel box that size can dissipate with a rise of about 15-20 C over ambient in still air. Run the layout and the heat numbers for your exact drive before ordering.
