Short answer: box the 480 V drives and contactors, and leave at most the 24 V DC logic in the open. The NEC lets live parts at 50 V to ground and above be guarded by location — a room, vault or similar space accessible only to qualified persons — so an open control room is legal. It spends your whole guarding allowance on one door, though. You keep the same NEC 110.26(A) working space either way: 3.5 ft [1.0 m] of clear depth in front of 480 V gear, at least 30 in [760 mm] wide per section. What changes is fault behavior, lockout, heat and who can touch what. A Category 0 arc-flash label settles none of that.
The two layouts side by side
Both layouts put the same code on you; the difference is which rules you satisfy with steel and which with a door policy. These are the numbers that move the decision:
| What you carry | Open subpanel in a locked room | Gasketed enclosures |
|---|---|---|
| Guarding 480 V live parts | The room door is the guard: qualified persons only, warning sign at the access | Box and door are the guard, at every hour |
| Working space, NEC 110.26(A) | 3.5 ft [1.0 m] deep by 30 in [760 mm] wide per 480 V section | Same depth and width, plus a 90-degree door opening |
| Access control | Key discipline on one door, forever | Tool or keyed latch on each box |
| Arc flash | One boundary at the room line | Label per box; the number moves with box size |
| Fault behavior | An arc burns in the open, along the whole wall | Tools, clippings and pests hit steel; the fault burns one box |
| Lockout | Common disconnect for the row | Disconnect per box: one machine down, the rest running |
| Heat | Every watt goes to room air | Each box sheds its own watts (3.7 W/m²K for stainless) |
| Comm wiring | Open runs needing 12 in [300 mm] from drive output | Ethernet switch lives in the PLC box |
Where the open room wins
On a troubleshooting night, the open room is the better shop. Every terminal is visible, meter leads land anywhere, and you can thermograph terminations with the line running. You suit up once at the room boundary instead of putting a door at your chest for each panel. The room's HVAC already absorbs the losses, so there is no cooler to buy, and backpanels with rail cost less than boxes. A layout change is drill-and-tap rather than a new drawing. In a narrow plant room you also keep the aisle, because with boxes every door swings its 90 degrees inside the working band.
All of that is worth exactly as much as the room discipline. Guarding by location stands only while the door stays locked and the key list stays short.
Where the boxes win at 480 V
Guarding by construction needs no discipline at all. The box qualifies as the guard at every hour of the day, for the cleaner with a mop as much as for the apprentice with a meter.
NFPA 70E helps you here too. With doors closed and the equipment in normal condition, it counts handle operation, meter reading and observation as normal operation, with no energized-work case to document each time. An open wall gives you no such line; every task on it is work on exposed live parts.
Lockout gets easier per machine. A disconnect on each drive box lets you lock out, test for absence of voltage, and work on one machine while the row keeps running. On an open wall you fall back to a common disconnect and tag-and-distance for everything beside it.
Fault behavior is the quiet winner. A dropped wrench, a stray offcut or a mouse hits steel instead of copper, and when a drive lets go the event burns one box rather than the wall of equipment next to it. Treat a standard gasketed box as finger-safe and fault-tolerant, never as arc-resistant; arc resistance is its own tested construction, covered in what an arc flash rated enclosure actually is. The arc-flash study changes with your choice anyway: IEEE 1584 takes enclosure dimensions as inputs, and a shallower box calculates a lower incident energy, so rerun the study once the sizes are set.
The code numbers that decide it
Guarding comes first. The NEC wants live parts at 50 V to ground and above guarded against accidental contact, and it accepts three routes: an approved enclosure, location in a room or vault accessible only to qualified persons, or permanent, substantial partitions or screens. Where live parts are exposed at 1,000 V or less, a warning sign goes at the access. In the open room that sign hangs on the room door, and the whole guarding case rides on that lock.
Working space comes second, and it is identical in both layouts. A 480 Y/277 V system sits at 277 V to ground; an ungrounded 480 V delta sits at the full 480 V. Both land in the 151-750 V row of NEC Table 110.26(A)(1): 3.5 ft [1.0 m] of depth, and a grounded block wall behind your subpanel keeps that same figure. Live 480 V parts facing the section from across the aisle push the depth to 4 ft [1.2 m]. Width is 30 in [760 mm] or the equipment width, height is 6.5 ft [2.0 m] or the equipment height, and doors must open through 90 degrees. Breaker and disconnect handles stay at or below 6 ft 7 in [2.0 m] above the floor (NEC 240.24(A)).
That depth stays empty in both layouts. The open wall invites cable tray and pipe racks at ankle height, and working space when a pipe sits below the panel shows why an obstruction in the band breaks the rule. OSHA and NFPA 70E both expect the shock and flash analysis on file before anyone touches the gear energized, whatever you mount it in.
The heat you take back when you close the door
Your cooling non-problem is a room statement. In the open room, every watt of drive loss goes straight into air the HVAC already handles. Close the boxes and the room air never reaches your components, so each box must shed its own load through stainless, which passes about 3.7 W per m² per K of inside-to-outside difference.
Run a typical bay: two 10 hp [7.5 kW] 480 V drives, a PLC, a 24 V DC supply and an Ethernet switch. A drive is about 97 % efficient, so each 10 hp unit at full load sheds roughly 250 W — about 3 % of the 8.3 kW it draws. Call it 500 W of drive heat and 100 W of control heat.
A 36 x 24 x 10 in [0.91 x 0.61 x 0.25 m] stainless box has 1.9 m² of wall: 3.7 × 1.9 = 7 W per K. All 600 W in that one box means 600 ÷ 7 = 86 K of rise, which is 111 C inside a 25 C room — far past the 40 C ambient limit on most drives and electronics.
Split the same load and it works. The PLC, supply and switch at 100 W in the 36 x 24 x 10 in box rise 100 ÷ 7 = 14 K to 39 C, just under the limit. The two drives at 500 W go into a 60 x 36 x 12 in box (4.3 m² of wall, 16 W per K) and rise 500 ÷ 16 = 32 K, past the 15 K budget, so that box gets an enclosure air conditioner or air-to-air heat exchanger rated above 500 W at your temperatures. For scale, the largest stainless box passively sheds about 240 W for a 15 K rise. To run this arithmetic on your own load list, how to size an electrical enclosure walks the same steps.
Comms, drives and one room
The fieldbus survives either layout when you respect the drive output. Keep parallel Ethernet or DeviceNet runs at least 12 in [300 mm] away from drive output cable — accepted panel practice — and cross at right angles where they must meet. Send drive output out on shielded VFD cable with the shield landed 360 degrees at the drive end, and keep the Ethernet switch in the PLC box, out of the drive's electrical noise.
Boxing the PLC on its own buys working room: you troubleshoot logic with the drive boxes shut and locked out. Keep one caution in mind. When a single main feeds the row, live circuits still sit ahead of the PLC box main, so the panel behind that door is live unless you opened the right breaker. If you would rather put the drives, the PC and the HMI in one box, an all-in-one machine control enclosure shows that layout with its heat split.
The decision
- 480 V drives, contactors and machine disconnects in the room — the common case: enclosures. Guarding, per-machine lockout and fault behavior all point the same way.
- A room that cannot stay locked, or one shared with non-electrical trades and deliveries: enclosures with keyed latches. Guarding by location ends the first time the door is propped.
- PLC marshalling and 24 V DC I/O only: an open backpanel inside a locked room holds up, because 24 V DC sits below the 50 V guarding line. A dust-tight box still earns its keep in finger protection; NEMA 12 vs NEMA 4 is the rating pair for a clean indoor room.
- Remember that an open backpanel carries no enclosure type at all; when a NEMA type applies sets out what you give up with an open frame.
- Keep e-stops at the operator station, not in the electrical room. When accidental bumps are the worry, guarding an e-stop beats concealing it.
What to put on the order
- PLC box: 36 x 24 x 10 in stainless wall-mount enclosure, 304 stainless — passive capacity about 105 W at a 15 K rise for your 100 W of control load.
- Drive box: 60 x 36 x 12 in stainless wall-mount enclosure, 304 stainless, with a cooler or heat exchanger rated above the 500 W drive loss.
- Marshalling box: 20 x 16 x 6 in stainless wall-mount enclosure for terminals and interposing relays; with no electronics inside, its 38 W passive cap stays spare.
- 304 stainless is the indoor grade; move to 316L if this room shares air with washdown spray or sanitizer mist.
- Rating wording on every box: rated NEMA 4X / IP66 by the manufacturer; not UL listed. When the project spec asks for a listed enclosure, that is a UL 50E evaluation — what UL 50E listing covers — and the finished panel is built by a UL 508A panel shop working to NFPA 79 and the NEC.
- Plan the cutouts for disconnects, glands and the cooler before the steel is cut; enclosure cutouts, glands and HMI windows carries the dimensions. Custom sizes and cutouts are delivered in 10 business days, counted from your approval of the drawing.
- Pick by inside height, DIN module count and thermal load at the enclosure selector; the stainless wall-mount line runs from 6 x 4 x 4 in to 60 x 36 x 16 in.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 36 × 24 × 10 in | 304 stainless | 36 x 24 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 60 × 36 × 12 in | 304 stainless | 60 x 36 x 12 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 6 in | 304 stainless | 20 x 16 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Is an open control room with 480 V drives against the code?
No. The NEC accepts guarding by location: a room, vault or similar space accessible only to qualified persons, for live parts at 50 V to ground and above. You must keep the door locked, control the keys, post the warning sign for exposed live parts, keep 3.5 ft [1.0 m] of working depth at each 480 V section, and carry the arc-flash labels. Legal, yes — but all of it rides on one door.
What enclosure rating does a clean, dry control room actually need?
Indoors, dust and drip protection is enough — NEMA 12 / IP52 territory. Our boxes are rated NEMA 4X / IP66 by the manufacturer, not UL listed, which covers the room and whatever the panel becomes later. If the project spec asks for a listed enclosure, that is a UL 50E evaluation by a test lab.
Our study says Category 0. Does that end the argument?
It ends the PPE argument, not the layout one. NFPA 70E retired Category 0 in its 2015 edition; under 1.2 cal/cm² [5.0 J/cm²] at the working distance you skip arc-rated clothing, but you still label, train and treat energized work as the exception. An arcing fault on an open wall releases into the room. The same fault in a box destroys one box.
How many watts can a sealed stainless box really shed?
About 3.7 W per m² of surface per K of rise. A 36 x 24 x 10 in box has 1.9 m² of wall, so 7 W per K — 105 W at a 15 K [27 F] rise. The 60 x 36 x 12 in box reaches about 240 W for the same 15 K. Above those numbers, split the load, upsize, or fit a cooler.
Should the PLC share an enclosure with the drives?
Keep them separate when you can. A 24 V DC PLC box lets you work on logic with the drives locked out, keeps the Ethernet switch out of drive EMI, and only has to shed about 100 W. Two 10 hp drives alone shed around 500 W into theirs, which means an enclosure cooler. One box for everything is possible — it just needs the cooling to match.
