Wire a Zone 22 field run in listed 300 V PLTC with a dust-tight gland at each end; anything that moves takes a listed extra-hard-usage cord. The junction box itself is ordered as listed dust-location equipment — that approval, not an ingress rating, makes the box legal in the zone. Cable tray is optional on a short run: a 12 ft [3.7 m] drop, strapped at both ends and mid-span, installs under the NEC's signaling-circuit rules. Our stainless boxes carry a manufacturer's NEMA 4X / IP66 rating and are not UL listed; the Zone 22 box itself comes from the equipment maker's listing.
What Zone 22 asks of the wiring
Zone 22 is the dust zone where an ignitable cloud is unlikely in normal operation and, if one forms, it clears in a short time — the flour, grain, sugar and polymer-powder rooms of a plant and most of the floor around them. Two failure modes drive every rule there. Fine dust finds each cable entry, settles on terminals and tracks current across insulation until a 120 V pilot circuit faults. And a hot surface, from a pilot lamp to a loose termination, ignites a cloud or the layer beneath it.
Your wiring method therefore closes the box at every cable entry, keeps temperatures where the equipment marking puts them, and bonds every metal part back to ground. The zone adds no hose duty of its own; water protection comes from whatever washdown routine the room has.
The wiring method: PLTC, cord or conduit
| Run | Wiring method | What closes the joint |
|---|---|---|
| Fixed device to box, pilot or signaling duty | Listed PLTC, 300 V, shielded where noise lives | Dust-tight gland at each end |
| Device that moves, tilts or vibrates | Listed extra-hard-usage cord (SOOW, STOOW) | Listed cord grip at each end |
| Exposed run across traffic or through a wall | Threaded metal conduit with the cable pulled inside | Gasketed conduit hubs and device covers |
PLTC — power-limited tray cable — is a listed 300 V cable for signaling and remote-control circuits, and the NEC's Zone 20/21/22 article names it among the cables that may run in tray. On a 24 V DC pilot loop the voltage margin is wide: 24 V is 8 % of the 300 V listing. In a gas atmosphere the same run answers differently, and the Class I Division 2 PLTC article covers conduit and seals on that side. What 'listed' means next to manufacturer-rated is the subject of the UL 50E vs NEMA 250 guide.
Cords answer the moving-device case. The NEC's Zone 20/21/22 article requires a flexible cord to be of a type listed for extra-hard usage, and in the NEC's cord table that is the full hard-service grade: SOOW and STOOW are the everyday types. The junior hard-service SJ grades sit one step below and fall short, as does a plain SPT cord. Read the jacket letters before you buy: the double O adds oil resistance, the W adds a 90 C wet rating and outdoor weathering, so a SOOW cord shrugs off a washdown floor that cracks an indoor-grade cord in its first sanitizer bath.
Is cable tray required for a short run?
Cable tray is one permitted home for PLTC, not the only one. A point-to-point drop — a limit switch 12 ft [3.7 m] from its box, a pushbutton cluster 15 ft [4.6 m] up a chute — installs under the NEC's signaling-circuit rules with no tray: support the cable, protect it from damage, and hold it 2 in [50 mm] from power conductors or behind a grounded barrier.
Shop practice is a fitting at each end and a strap every 6 ft [1.8 m] in between; sleeve the run in flexible conduit where carts, hoses or people pass below 8 ft [2.4 m]. Tray earns its keep when the job grows: once you are collecting a fourth cable or passing 50 ft [15 m], tray or threaded conduit returns, because loose strap is how a cable gets kicked loose and dust finds the entry you fished.
Glands and cord grips: the dust seal at both ends
Every cable entry in the zone takes a gland marked for dust service, matched to the protection concept the box listing names. The seal is an elastomer ring the dome nut compresses onto the jacket, so the gland's clamping range must sit around the cable OD — mid-range, not at the edge. Hand-tighten the dome nut, then a firm turn with a wrench; the ring should hold the cable against a steady pull. Cords take the same logic in a listed cord grip sized to the cord OD. If a gland bottoms out on the box thread before its seal bites, the next hub length is the fix, and the short-gland sizing article walks that repair.
Threaded gland hubs bond into a metal wall; keep the threads bare of paint at the joint, which NEC 250.12 calls for on grounding and bonding contact surfaces. The gland bonding article for stainless junction boxes carries those details, and outdoors you point the gland down or loop the cable drip-tight, as the outdoor gland orientation article shows. The seal ranges, elastomers and markings worth comparing sit in a guide to selecting Ex cable glands for stainless junction boxes.
Worked example: a 12 ft PLTC run to a limit switch
A limit switch sits 12 ft [3.7 m] from its Zone 22 junction box, switching a 24 V DC dry contact that draws 0.1 A.
- Cable: 18 AWG [1.0 mm²] shielded PLTC, jacket OD 0.31 in [8.0 mm], 300 V listing. 24 V / 300 V = 8 % of the cable rating, with the shield worth having beside a drive cabinet.
- Glands, both ends: M16 dust-tight glands with a seal range of 5-10 mm [0.20-0.39 in]. The 8.0 mm cable sits mid-range, so the ring compresses evenly; the same cable in a 9-14 mm gland falls below the seal and works loose of its grip.
- Supports: strain relief at the switch, gland at the box, one strap at mid-span — fittings at 0, 6 and 12 ft [0, 1.8, 3.7 m].
- Box side: shield onto its terminal, gland body bonding through the hub, threads bare metal.
The cost of skipping the sizing: a seal held at the edge of its range relaxes as the cable flexes, dust walks in, and flour on an uninsulated terminal starts tracking a 120 V circuit within a season.
The junction box approval
The box itself must be listed for the location. In Division language the mark is dust-ignitionproof; in Zone language the certificate names an Ex t protection concept. Either way the enclosure keeps dust out of its own interior and holds every internal surface below the temperature the listing states.
A manufacturer's ingress rating sits a step below that approval. Our stainless boxes are rated NEMA 4X / IP66 by the manufacturer; they are not UL listed, and no ingress test asks whether a fault inside the box can ignite the dust outside it. NEMA types are designations the manufacturer applies — the association itself tests nothing — and we build no NEMA 7/9 or zone-rated boxes. NEC 110.28 keeps IP codes and enclosure type numbers separate, so a US specification wants the type number on the box, not only the IP code.
So you order the Zone 22 junction box under the equipment maker's listing: the machine's control documentation names the box and its markings. Where a listed panel gains a compartment, the enclosure listing approval path article walks the paperwork, and the unlisted-enclosure article covers your box as a component inside listed equipment. Terminals inside that box count as part of the listing, which the DIN rail terminal assembly article explains. For the unclassified rooms around the zone — MCC rooms, control rooms, washdown corridors — the stainless box picker article sorts 304 from 316L.
Heat, dust layers and housekeeping
Temperature marks in dust zones come from the dust's own numbers. Published corn dust data, for example, puts cloud ignition near 470 C [880 F]; a T4 device mark (135 C [275 F] maximum surface) sits far under that. Dust layers ignite below cloud temperatures, which is why the listed equipment mark — not a hand calculation — is the limit you install to.
Keep the layers off: vacuum spilled dust with a vacuum rated for that dust, because compressed-air blow-down raises a cloud and NFPA 652/654 allow it only where the site's dust hazard analysis sets the precautions. De-energize the circuit and verify absence of voltage before you open any box in a dusty room.
What to put on the order
| Item | What the spec says |
|---|---|
| Field cable | Listed PLTC, 18 AWG [1.0 mm²] shielded, 300 V; sunlight-resistant jacket outdoors |
| Cord-connected devices | SOOW or STOOW listed extra-hard-usage cord, oil-resistant, 90 C wet where washdown meets the floor |
| Glands | Dust-tight glands marked for the box's protection concept; M16 or M20 body; seal range spanning the cable OD mid-range; stainless bodies at washdown boxes |
| Cord grips | Listed extra-hard-usage cord connectors with the same seal logic |
| Junction box | Zone 22 listed box (dust-ignitionproof / Ex t), sized for terminal fill and NEC 312.6 bending space, sourced under the equipment listing |
| Supports | Stainless straps at both ends and every 6 ft [1.8 m]; flexible conduit sleeve below 8 ft [2.4 m] in traffic |
| Marking | Area classification label on the box and circuit ID at both ends |
The decision, case by case. Fixed pilot and signaling devices on short runs: PLTC with a dust-tight gland at each end, supported and protected, no tray. Moving or vibrating devices: a listed extra-hard-usage cord in listed cord grips. Distribution across a room or a whole building: tray or threaded conduit returns, and the box still carries the zone listing on its nameplate.
The box itself is ordered from the equipment maker's Zone 22 listing. Our stainless work covers the rooms around the zone — from custom stainless enclosures built to your cutout layout to custom enclosures in any of our grades — delivered in 10 business days anywhere in the world.
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 |
|---|---|---|---|
| 6 × 6 × 4 in | 304 stainless | 6 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 8 × 6 × 4 in | 304 stainless | 8 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 10 × 8 × 4 in | 304 stainless | 10 x 8 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Is cable tray required for PLTC in a Zone 22 location?
No. Tray is one permitted route, and a short point-to-point run of 15 ft [4.6 m] or less installs under the NEC's signaling-circuit rules when you fit each end, strap at least every 6 ft [1.8 m] and hold the cable 2 in [50 mm] from power conductors. When the run collects several cables or passes 50 ft [15 m], tray or conduit returns.
Which cord types meet the extra-hard usage listing?
The full hard-service grades: types SOOW and STOOW are the everyday picks, rated 600 V with oil-resistant jackets, and SOOW carries a 90 C wet rating. The junior hard-service SJ grades sit one step lower and fall short of the requirement, as does a plain SPT cord.
Can a stainless NEMA 4X box serve as the Zone 22 junction box?
Not on that rating. A manufacturer's NEMA 4X / IP66 mark — ours are not UL listed — proves hose-directed water and dust ingress resistance; IP66 is a 100 L/min jet test on the second digit. A Zone 22 box needs the dust-ignitionproof or Ex t approval on its nameplate, and we build no zone-rated boxes.
Do both ends of the PLTC run need a dust-tight gland?
Yes, at the field device and at the box. Size the gland so the cable sits mid-range in the seal: an 8.0 mm jacket in an M16 gland spanning 5-10 mm [0.20-0.39 in] grips evenly, while the same cable below a 9-14 mm range leaks dust within weeks of vibration.
How hot can equipment surfaces run around grain dust?
Let the dust data sheet and the equipment mark decide. Published corn dust data puts cloud ignition near 470 C [880 F], so a T4 mark at 135 C [275 F] maximum surface leaves a wide margin. Layers ignite below clouds, which is why the listed mark, not a hand calculation, sets the installed limit.
What separation does 24 V PLTC need from power wiring?
Hold Class 2 and Class 3 conductors 2 in [50 mm] from power conductors inside an enclosure, or put a grounded barrier between them — the spacing the NEC's signaling-circuit article sets. The 24 V pilot loop also runs at 8 % of the 300 V PLTC rating, a 12.5:1 voltage margin.
