Short answer: for the last exposed 10 ft [3 m] to a machine-mounted pressure transmitter or RTD, run PLTC-ER, power-limited tray cable with the exposed-run marking. Land it with fittings listed for Class I Division 2. Inside threaded rigid metal conduit, any cable the NEC permits in conduit stays acceptable. The Ethernet jacket stays on through the boundary seal-off: you seal around it, and no special wire is needed. NEC Article 501 governs all three calls, and every one of them turns on a single question: does the cable run in raceway, or open?
What cable does Class I Division 2 allow for 4-20 mA and RTD signals?
NEC Article 501 permits a short list of cable types to run without raceway in a Class I Division 2 area: PLTC and PLTC-ER, ITC, and Type TC or MV in tray. Each must be terminated with fittings listed for the location.
PLTC is the workhorse. It is a 300 V power-limited tray cable built under the Class 2 and Class 3 circuit rules of Article 725, so a 4-20 mA loop fed from a Class 2 source fits it exactly. A listed 24 V DC supply inside the 30 V, 100 VA Class 2 limits counts as that. Where the source sits outside those limits, ITC is the instrumentation cable for the circuit, and both types go in with fittings listed for Class I Division 2. The -ER suffix earns its price on a machine: it marks the cable as listed for exposed runs, off the tray and out of the raceway.
| Circuit | Cable to specify | Where it may run | Termination |
|---|---|---|---|
| 4-20 mA loop, Class 2 source | PLTC-ER, 18 AWG twisted shielded pair, 300 V | Tray, raceway, strapped exposed runs | Fitting listed for Class I Division 2 |
| RTD, 3-wire | Same construction, 3-wire | Same | Fitting listed for Class I Division 2 |
| Instrument circuit outside Class 2 | ITC or ITC-ER | Industrial plant, tray or exposed (ER) | Fitting listed for Class I Division 2 |
| Ethernet, kept in raceway | Any Cat 6 the conduit rules permit | Permitted raceway only | Gland or sealed connector |
| Ethernet, partly exposed in the area | Cat cable also marked PLTC or ITC | Open run, strapped | Fitting listed for Class I Division 2 |
One branch changes the rules: intrinsically safe loops answer to the intrinsically safe wiring rules rather than this cable list. Whether IS barriers must sit inside a purged C1D2 enclosure is a separate question, covered in purged-enclosure barrier discussion. A purged enclosure protects what is inside it; the field wiring still follows Article 501.
Can ordinary cable be used inside threaded rigid conduit?
Yes: the Article 501 wiring rules reach back to the Division 1 raceway list, so any wire or cable the NEC permits in conduit keeps running there inside threaded rigid metal conduit or threaded steel IMC.
Your existing THHN pulls and multiconductor runs in rigid conduit already meet Division 2; the classified area changes nothing about the cable inside the pipe. Division 2 then widens the raceway choices: six permitted conduit types against five in Division 1 on Article 501's list, adding EMT and rigid or intermediate conduit installed with listed fittings. So the cheap answer for Ethernet is to keep it in the conduit and let the raceway carry the wiring method, which is exactly what a seal-off at the boundary assumes.
What cable is allowed for the exposed drop?
PLTC-ER: the ER marking means the cable itself is listed for exposed runs, so you strap it along the machine for the final stretch. Land it on the sensor through a fitting listed for Class I Division 2.
Plain PLTC without the ER marking stays in tray or raceway. Strap PLTC-ER at the support interval its listing prints. Where the sensor moves, take that last part through liquidtight flexible conduit with fittings listed for the location.
Worked example: a 4-20 mA transmitter sits 200 ft [61 m] away in tray, plus the exposed drop. That is 210 ft [64 m] one way, so an 18 AWG pair at 6.4 Ω per 1000 ft [21 Ω/km] puts 420 ft [128 m] of copper in the loop: 420 x 6.4 / 1000 = 2.7 Ω. At 20 mA it costs 2.7 x 0.020 = 0.054 V. Pay the 250 Ω input resistor its 5 V and the transmitter its 12 V minimum, and 24 - 5 - 0.05 = 18.95 V still arrives, about 7 V of headroom.
Run RTDs three-wire. Two wires over 250 ft [76 m] of that pair add 250 x 6.4/1000 x 2 = 3.2 Ω, which on a Pt100, at 0.385 Ω per C, reports as 3.2 / 0.385 = 8.3 C [15 F] of phantom temperature. The third wire lets the receiver subtract the lead instead of reporting it as heat.
Does the Ethernet jacket come off at the seal-off?
No: where a multiconductor cable crosses out of a Division 2 area in conduit, Article 501 has the boundary seal made around the outer jacket as it arrives. Stripping it is a Division 1 treatment, and no special wire is called for.
The seal exists to stop gas migrating through the conduit into the unclassified area, and Article 501 pins down three things about it:
- The sealing fitting sits within 10 ft [3 m] of the point where the conduit leaves the Division 2 area, on either side of the line.
- Between fitting and crossing point the conduit runs straight: no union, coupling, box or fitting.
- Dam the fitting and pour the compound around the cable as found, outer jacket on.
Stripping jacket and fillers so the compound surrounds each conductor is what Division 1 asks, and only for cables without a continuous gas/vapor-tight sheath. Special Ethernet cable does exist, Cat cable carrying its own PLTC or ITC marking, and you buy it only where the cable leaves the raceway inside the classified area. Kept in conduit end to end, ordinary Cat 6 rides along with every other cable Article 501 admits.
On the safe side of the boundary, land the run on a switch inside the box. Or bring it through the wall on a sealed Ethernet connector; sealed connectors keep that entry rated instead of opening the box for a patch lead.
What goes through the wall of the NEMA 4X enclosure?
Conduit hubs and cable glands rated for both the location and the environment: for a washdown Division 2 area that means a gland listed for Class I Division 2 with a NEMA 4X / IP66 rating, torqued into a cutout of the diameter its instructions give.
The weakest part sets the rating: one gland marked IP54 makes that hole an IP54 hole whatever the box around it says. Buy glands that carry both the location listing and IP66, and tighten them to the torque printed on the instruction sheet. Our stainless bodies are rated NEMA 4X / IP66 by the manufacturer; not UL listed. If the project spec calls for a listed enclosure, a manufacturer-rated box does not fill that line item, so settle it before the drawing.
Division 2 never asked the box to be explosion-proof; it asked the equipment inside it to suit the location, and the enclosure answers the environment. Whether a NEMA 4X junction box is enough in Class I Division 2 and when Type 4 terminals work in a Class I Division 2 junction box both turn on the parts inside, not the stainless. We build the 4X bodies; we don't build explosion-proof (NEMA 7 or 8) enclosures or purge systems.
For the holes themselves, field-drilled conduit entries that keep a NEMA 4X rating covers the right method. The cutouts, glands and HMI windows guide collects the hole sizes. Inside, keep analog pairs away from drive output wiring; instrument signal cable layout and separation has the spacing practice.
What do you write on the order?
Three lines beyond the box: PLTC-ER 18 AWG twisted shielded pair for each loop (ITC-ER where the circuit sits outside Class 2), glands listed for Class I Division 2 with an IP66 rating, and factory cutouts to each gland's diameter, typically 22 mm [0.87 in] for an M20.
- Cable: PLTC-ER twisted shielded pair, 300 V, for each analog loop; shield landed at the panel end only.
- Glands: listed for Class I Division 2, IP66, with a clamp range covering your cable; a shielded pair typically measures 0.24-0.28 in [6-7 mm] outside.
- Cutouts: cut at the factory to the diameter the gland states, positions marked on the drawing.
- Enclosure: NEMA 4X / IP66 in 304 indoors, 316L where washdown or chlorides are coming; write the type and the IP code both.
Under NEC 110.28, an IP code on its own does not carry a US installation that calls for an enclosure type, so the spec reads NEMA 4X / IP66, never IP66 alone. Custom stainless enclosures and factory cutouts shows what the cutout drawing should carry, and the enclosure selector sizes the body to your terminals. Custom work is delivered in 10 business days, counted from your approval of the drawing.
Questions
Can THHN or ordinary multiconductor cable serve a 4-20 mA loop in Class I Division 2?
Yes, when every metre of it stays inside a permitted raceway: threaded rigid metal conduit and the other Division 2 raceways take any cable the NEC permits in conduit. The exposed drop to the sensor is the part that needs PLTC-ER or ITC. Copper size is rarely the limit; 420 ft [128 m] of loop on an 18 AWG pair drops just 0.05 V at 20 mA.
Can a standard patch cable run exposed in the Division 2 area?
No. CM, CMR and CMP are communications listings, not cable types on the Division 2 list, so patch cable stays inside raceway the whole way. Where an Ethernet run must be open in the classified area, buy Cat cable that also carries a PLTC or ITC marking. Kept in conduit, ordinary Cat 6 does the job for a short crossing.
Where does the sealing fitting go relative to the Division 2 boundary?
Within 10 ft [3 m] of the point where the conduit leaves the Division 2 area, on either side of the line, with no union, coupling, box or fitting between the seal and the boundary. Landing it just inside the unclassified side, next to the enclosure it feeds, keeps the compound pour easy to reach for maintenance.
How far can a 4-20 mA loop run on PLTC-ER before voltage drop matters?
Farther than the plant: a 24 V loop with a 250 Ω sense resistor and a 12 V transmitter minimum keeps about 7 V for copper, which at 20 mA allows 350 Ω of loop, roughly 27,000 ft [8.2 km] one way at 6.4 Ω per 1000 ft [21 Ω/km]. Noise, tray routing and barrier budgets end real runs long before the copper does.
Do the loops need shielded cable?
Yes, twisted shielded pair is the standard: 18 AWG with an overall foil shield and a drain wire, landed at one end only, at the panel. Shields tied at both ends invite ground loops in most plants. Watch the RTD wiring too, because two-wire over 250 ft [76 m] reads 8.3 C [15 F] high on a Pt100.
