Multi drive VFD enclosure sizing for a large lineup

Short answer: size a 30-drive lineup from the summed losses on the drive data sheets, never from the motor kW. A realistic mix of twenty 4 kW, seven 22 kW and three 75 kW drives releases about 15.3 kW of heat. No sealed box sheds that. The enclosure that works is a bayed floor-standing lineup, one cooling zone per bay, with filter fans in a clean room and sealed Type 4X bays with air conditioners where dust, washdown or sun rules filtered air out. The steps below work the numbers: drive losses, wall conduction, the bay split, airflow, cooler sizing, commissioning checks and the drawing you send for a quote.

How much heat does a 30-drive lineup release?

Sum the losses printed in each drive's data sheet: a lineup of twenty 4 kW, seven 22 kW and three 75 kW drives rejects about 15.3 kW of heat, because losses run 2-6% of drive rating and larger drives sit near 3%.

Drive makers publish heat loss as a share of drive rating, commonly 2-6%: small frames sit near the top of that band and large frames near 3%. Efficiency gives the same number from the other side. A 75 kW (100 hp) drive at 95% efficiency turns 3,729 W into heat inside the cabinet; the same drive at 97% loses about 2.3 kW. Those two figures bracket most lineups. For cooling, take each drive's data sheet loss at your duty.

The three loss shares are worked-example assumptions. Replace every one with its data sheet figure. Two heat sources get forgotten: a braking resistor dumps its deceleration energy in or beside the bay, and a dry-type transformer or line reactor adds roughly 1-3% of its own rating. Control gear counts too; a 24 V DC supply delivering 240 W near full load dissipates about 27 W at 90% efficiency. Size the cooling for the worst duty the machine can actually run. If interlocks make some combinations impossible, cool for the worst allowed combination and write that on the drawing. One drive alone follows the same arithmetic in the single-drive VFD sizing article.

Can one sealed enclosure take the whole lineup?

No — pushed through the wall of the largest catalog box, a 60 x 36 x 16 in stainless enclosure, 15.3 kW would need a temperature rise near 870 K, so a lineup this size lives in a bayed floor-standing lineup, one cooling zone per bay.

The wall check settles it in one line of arithmetic. Stainless steel passes about 3.7 W through each square metre of wall for every kelvin of difference, the figure the enclosure heat calculation guide uses. The largest catalog stainless box, 60 x 36 x 16 in, has about 4.77 m² of surface. Its wall therefore carries 3.7 × 4.77 ≈ 17.6 W per kelvin. The full lineup would need a rise of 15,340 ÷ 17.6 ≈ 870 K. A single 75 kW drive at 2,250 W lifts the inside 2,250 ÷ 17.6 ≈ 128 K, taking a 30 C room to about 158 C. Even one 22 kW drive adds 770 ÷ 17.6 ≈ 44 K, so the box sits near 74 C. Losing 15.3 kW through stainless walls at a 25 K rise takes 15,340 ÷ (3.7 × 25) ≈ 166 m² of wall, some 35 boxes that size.

Wall conduction is the right tool for single boards and small sealed controllers, which is exactly what the conductive cooling article covers. At fifteen kilowatts the answer is a lineup, not a bigger box.

How do you split the lineup into bays?

Give every bay a loss budget and a width budget: the example split puts 3.2 kW with the small drives, 5.4 kW with the mid drives and 6.75 kW with the large ones, in six 1,000 mm-wide sections.

Work in widths from the dimensional drawings of the drives you selected. Example envelope widths for 400 V frames are about 100 mm for a 4 kW drive, 200 mm for a 22 kW and 350 mm for a 75 kW. The mounting width alone comes to 20 × 100 + 7 × 200 + 3 × 350 = 4,450 mm.

1. Give each group a loss budget: 3.2 kW, 5.4 kW and 6.75 kW in the example.
2. Add non-drive heat where it sits: braking resistors, line reactors at 1-3% of their rating, the 24 V supply.
3. Place drives with about 100 mm [4 in] of free space above and below, the clearance drive installation manuals typically ask for.
4. Run two 100 mm wire ducts per row and keep fill at or below 50% of the duct cross-section, the fill the wiring-duct tables use under NFPA 79 section 13.5.2.
5. Set the section count from width: six 1,000 mm sections hold this mix, plus one 800 mm section for controls and network gear.
6. Check the aisle: 900 mm of working space, doors opening 90°, and the highest breaker handle at or below 2.0 m (6 ft 7 in).

NEC 110.26(A) sets that 900 mm figure for equipment at 0-150 V to ground in Condition 1 and asks for more depth at higher voltages; NEC 240.24(A) sets the 2.0 m handle limit. At the terminals, NEC 312.6 sets the wire bending space in two tables: Table 312.6(A) where the conductor does not enter the wall opposite its terminal, Table 312.6(B) where it does. The enclosure sizing guide collects these clearance rules, and the backplate and DIN rail layout guide covers duct and rail planning in detail.

Keep motor cables in the bottom duct and control and network wiring in the top duct. The drive-and-computer wiring separation article gives the spacing that keeps a lineup quiet.

How much airflow or cooling does each bay need?

Size filter fans with airflow in m³/h ≈ 3 × watts ÷ ΔT in kelvin: at a 10 K rise the three bays need about 960, 1,600 and 2,000 m³/h, and a sealed bay takes an air conditioner sized at load plus 25%.

Air near room density carries about 1,206 J per cubic metre per kelvin (1.2 kg/m³ × 1,005 J/kg·K). Each m³/h moving through a bay therefore removes about 0.335 W per kelvin, which folds into the rule airflow (m³/h) ≈ 3 × watts ÷ ΔT (K). With the room at 30 C and the bays held at the 40 C drives are usually rated for unless the data sheet allows more, ΔT = 10 K.

Halve ΔT and the airflow doubles, which is what the 35 C room column shows. Fans in series, one pushing and one pulling, move about the same air as one fan; capacity comes from parallel fans, not from stacking a matched pair. Point every exhaust away from the next drive's intake.

Where filtered air is out, seal the bay and size the cooler with margin: capacity ≈ loss × 1.25. Bay C lands at 6,750 × 1.25 = 8,438 W and takes a 10 kW-class unit; bay B at 6,738 W takes about 8 kW; bay A at 4,000 W takes a 4 kW unit. The cooler sits on a gasketed cutout drawn to its instructions, and only a cooler rated Type 4X keeps the bay's rating; a lower-rated unit sets the assembly. The cabinet air conditioner sizing article works a single overheating cabinet the same way.

Mind the room. Condensers reject the pumped heat plus their electrical input, so at a COP of 3 this lineup dumps about 20.4 kW into the room: 15.3 kW of load plus 5.1 kW of compressor input. The room needs ventilation or HVAC of its own.

When do filter fans beat sealed cooling?

Filter fans win in a clean indoor room, where a Type 12 filter kit moving 960 m³/h costs a fraction of a sealed cooler; dust, oil mist, washdown or outdoor sun push you to sealed Type 4X bays with air conditioners.

Ratings in that table are manufacturer ratings; our enclosures are rated NEMA 4X / IP66 by the manufacturer and not UL listed. The weakest part sets the assembly: a Type 12 filter kit on Type 4X sheet metal leaves a Type 12 bay. The fan, air conditioner and heat exchanger comparison lays out the cost and maintenance side of each choice. Outdoors, add the sun: a sun shield keeps solar load off the steel before the cooler ever sees it. Chlorides change the metal too; brine, salt air and sanitizers call for 316L, as the ocean-side mechanical room article argues.

What checks prove the lineup runs cool?

Commission on numbers: with all drives at rated duty the exhaust-to-room rise should land within about 2 K of the 10 K you designed for, fan filters read clean, and each drive's own overtemperature warning stays silent.

• Run the worst simultaneous duty for an hour and read room temperature plus each bay's exhaust; the rise should sit within about 2 K of the 10 K you designed for.
• Confirm every fan spins the way the frame arrow points. EC and brushless fans reverse by turning the fan round, never by swapping supply wires, which damages them.
• Read the filters after the first week; a loaded filter cuts airflow and the rise climbs within days.
• Scan terminals under load with an IR camera; a joint running 15-20 K hotter than its neighbours wants a re-torque.
• Re-run the loss sum whenever a drive model changes; a different frame can move the heat by tens of percent.

Open a bay only after locking out the feeder and waiting the bus-discharge time the drive manual states; large drives hold several hundred volts on the DC bus for minutes.

If a bay still runs hot, work down this order: loaded filter first, then a stalled or backwards fan, then an optimistic loss figure, then room air warmer than the 30 C you assumed.

What goes on the drawing and the order?

Send the quote a drawing, not a wish: a bay schedule with each drive's loss, width and cooling airflow — 15.3 kW, six 1,000 mm sections and about 4,600 m³/h in the example — plus material, NEMA type and every cutout.

The sheet carries the bay schedule (drive, kW, loss, airflow), section widths, duct routes, the cooler model with its gasketed cutout, gland plate cutouts sized to the cable list, and material plus NEMA type per bay. The cutouts, glands and windows guide shows how a cutout drawing is read, so the holes land where the cooler and glands expect them.

A 30-drive cabinet is an industrial control panel under NEC 409.2, which counts two or more power circuit components; UL 508A is the standard a panel shop lists such a panel to, and NEC 409.110 requires the enclosure type number on the panel marking. Size each equipment grounding conductor from the overcurrent device ahead of it, the pairing NEC Table 250.122 gives; the UL 50E and panel listing guide covers the marking side.

Ordering: send the bay schedule with material (304 for most rooms, 316L where chlorides live), the per-bay NEMA type and every cutout through the custom enclosure builder; you approve the drawing, and the lineup is delivered in 10 business days from that approval. Drive bays at this heat are custom work from the first sketch; the catalog's sealed stainless boxes suit the controls section and single-drive jobs.

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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