Splicing wire in a Class I area is a hazardous-locations job, and the division decides what you buy. In Class I, Division 1 you need an explosion-proof junction box identified for Class I, Division 1 and your gas group. Feed it through threaded rigid conduit, seal every run that enters the box, and engage five full threads at each joint. In Division 2 a general-purpose enclosure suited to the environment, with sealed splices and nothing inside that arcs, usually serves; conduit seals enter only at trade size 2 in [53 mm] and larger. We build the stainless general-purpose boxes for the Division 2 splice and for the non-classified areas around it. We do not build explosion-proof enclosures: the Division 1 box comes to you listed by its maker for the Class, Division and Groups your area classification names.
Which rating does a Class I Division 1 splice box need?
Buy an enclosure identified for Class I, Division 1 and the gas group present — an explosion-proof box marked for Class I, Division 1, Groups A through D or your specific Group. The NEC requires every box and conduit fitting in a Division 1 area to carry that identification.
Division 1 means ignitable concentrations of gas or vapor can exist under normal operation, or frequently during repair and maintenance. Your splice sits in that atmosphere, so the NEC treats the box like any other piece of electrical equipment there. In Division 2 the gas shows up only when something fails, which is why the two divisions buy different boxes.
An explosion-proof box carries its rating in the construction: heavy cast walls, threaded hubs and a bolted cover, with no gasket doing the protective work. The threaded joints and the seals hold back the flame. NEMA 250 defines the type numbers behind the marking: Type 7 for indoor use in Class I, Division 1, Groups A through D, and Type 9 as its indoor Class II dust twin. NEMA publishes the definitions; it does not test, certify or endorse enclosures.
NEMA 7 is an indoor definition. Outdoors, the listed box carries a Type 4 or 4X weather rating on the same nameplate, and you order both lines off that one marking. Purged and pressurized enclosures are the other common technique on the NEC's protection-technique list; the purged-enclosure article for Class I Div 1 walks through that route.
What do Class I Groups A, B, C and D mean?
The Group letter matches the box to the gas: Group A is acetylene, B is hydrogen and hydrogen-rich fuel gases, C is ethylene and ethyl ether, and D is propane, gasoline and methane. Buy the box whose stamped groups include yours.
| Group | Gases and vapors covered |
|---|---|
| A | Acetylene |
| B | Hydrogen, and fuel gases with more than 30% hydrogen by volume |
| C | Ethylene, ethyl ether, cyclopropane |
| D | Gasoline, propane, butane, methane (natural gas), hexane |
The groups come from the NEC's gas-group table. Match the stamp to the gases at your site. A mixed hydrocarbon area usually takes Groups C and D, hydrogen service takes Group B on its own, and a Groups A through D stamp covers every Class I gas at the cost of a heavier box than most sites need.
The nameplate also carries a temperature class, T1 through T6, capping the hottest surface the enclosure can reach: T1 at 450 °C [842 °F], T2 at 300 °C [572 °F], down to T6 at 85 °C [185 °F], per the NEC temperature classes. Pick the class whose ceiling sits below your vapor's ignition temperature. Most hydrocarbon vapors clear T1 or T2 with room to spare.
Does a Class I Division 2 splice box need to be explosion-proof?
Division 2 usually takes a general-purpose enclosure: with only sealed splices inside, no switches or breakers, the NEC allows an ordinary box rated for the environment. Conduit seals enter the picture at trade size 2 in [53 mm] and larger.
Division 2 assumes a leak or a containment failure, not gas on an ordinary day, so the Division 2 wiring rules let you use ordinary boxes and fittings wherever the box holds no arcing parts. Your splices qualify when they are made with sealed, insulated connectors. Any device you do mount inside must carry its own Division 2 identification on the marking.
Pick the environment rating from the area itself: stainless NEMA 4X for washdown, rain, salt air or chemical splash, Type 12 for a dry indoor wall. Our stainless boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Where the job demands listed equipment, buy a box that carries the NRTL mark, because a manufacturer rating does not stand in for one. The judgment call — when a 4X stainless box is enough and what to specify beside it — is worked through in whether a NEMA 4X junction box is enough in Class I Division 2 and in Class I Division 2 enclosure selection.
The wiring method feeding the box matters as much as the box itself; wiring methods into a NEMA 4 enclosure compares the EMT, PVC and cable choices.
How many threads and conduit seals does a Class I splice box need?
Make every threaded joint up wrench tight with at least five full threads engaged. Run threaded rigid metal conduit or threaded steel intermediate metal conduit from trade size 3/4 in up, and fit a seal in each conduit run that enters a Division 1 splice box.
A seal is a packed fitting that stops gas migrating along the conduit and contains the pressure of an internal explosion. The NEC's Class I sealing rules set where it goes. In Division 1, every run entering a box that holds splices, terminals or taps gets a seal, at any trade size, and a run crossing out of the classified area is sealed within 10 ft [3.0 m] of the boundary. In Division 2 the seal duty starts at trade size 2 in [53 mm] and larger. The five-thread rule covers every threaded joint in a Class I location, Division 1 or 2. The smallest trade size in a Division 1 threaded run is 3/4 in.
The placement rules, with the distances and the sizes, are laid out on the explosion-proof conduit seal requirements page.
Worked example, Division 1: you are splicing twelve 10 AWG motor leads into six connectors in a Group D compressor room. Three 1-in RMC runs enter the box, which sits 6 ft [1.8 m] from the Division 1 boundary. Order a box listed Class I, Division 1, Group D with three 1-in threaded hubs. Each run takes a seal at the box, because a Division 1 splice box seals every run at any size. At 6 ft those seals sit inside the 10 ft boundary limit, so one seal serves both duties.
The same splices in Division 2 take a general-purpose box. A 1-in hub clears the 3/4 in minimum, and every joint gets five full threads. At 1 in the runs sit below the 2 in seal threshold, so they carry no seals. The NEC box-fill allowances give 12 conductors × 2.50 in³ = 30 in³, which a 6 × 6 × 4 in box (144 in³) holds with working room.
Lock out the circuit and confirm the absence of voltage before you open any box in a classified area; the arc at the splice is the ignition source the rating exists to contain.
Is a NEMA 7 marking enough without a listing?
A NEMA 7 marking by itself is not the approval. OSHA 1910.303(a) requires electrical equipment in US workplaces to be approved, and OSHA 1910.399 defines acceptable as listed, labeled or otherwise determined safe by a nationally recognized testing laboratory.
The listing mark on a hazardous-location box reads Class, Division and Group — Class I, Division 1, Groups C and D, for instance. The testing laboratories mark these boxes that way instead of with a type number. Read the nameplate against your area classification group by group, because the box works only when the match is exact.
The general-purpose type numbers live in NEC Table 110.28, which lists which enclosure types suit which conditions, and NEC 110.28 states that IP ratings are not a substitute for enclosure type ratings. The code stopped calling them NEMA types years ago; the shop floor never did. What each general-purpose type covers is compared in the NEMA 4 vs 4X vs IP66 guide, and where a listing versus a manufacturer rating sways an inspector is laid out in the UL 50E and NEMA 250 listing guide.
What goes on the order?
For Division 1 splices, order a junction box listed for your Class, Division and Groups. For Division 2 splices with sealed connectors, order a general-purpose enclosure — our 304 or 316L stainless boxes where the area is wet or corrosive.
| Item | Class I, Division 1 splices | Class I, Division 2 splices |
|---|---|---|
| Box | Explosion-proof, listed for the location | General-purpose, environment-rated |
| Nameplate | Class I, Division 1, matched Groups and temperature class | NEMA 4X / IP66 stainless, or Type 12 indoors |
| Entries | Threaded hubs, trade size 3/4 in minimum | Hubs or glands sized to the wiring method |
| Seals | One per run entering the box; boundary runs sealed within 10 ft [3.0 m] | Only for trade size 2 in [53 mm] runs and larger |
| Joints | Five full threads engaged, wrench tight | Five full threads engaged, wrench tight |
| Inside | Splices in listed connectors | Sealed splices; devices identified for Division 2 |
For Division 1, buy the box and its seals from the hazardous-location sections of electrical distributors, matching Group and temperature class to your area classification, and order hubs in the trade sizes you ran. We do not build NEMA 7 or 9 enclosures, and a reworked general-purpose box cannot become one.
For Division 2, we build the box: 304 stainless for indoor washdown and most outdoor sites, 316L for salt air, brine and chemical areas, in your width, height and depth, with hub cutouts where the conduit lands. It is made from one piece and delivered in 10 business days, counted from your approval of the drawing for custom work; the custom enclosure builder takes the details.
Questions
Can a NEMA 4X stainless enclosure be converted into an explosion-proof box?
No conversion does it: the rating lives in the cast walls, the threaded hubs and the bolted cover, tested as an assembly by the listing laboratory, so you order the box already listed for your Class, Division and Groups. A gasketed stainless box rated NEMA 4X / IP66 stays a washdown box in a classified area.
How many threads must be engaged at a conduit joint in a Class I area?
At least five full threads, made up wrench tight, at every threaded joint of a rigid conduit run in Class I, Division 1 or 2. The engaged threads form the flame path; fewer threads shorten it and let an internal explosion escape at full temperature.
Does a small Division 2 splice box need conduit seals?
Only where trade size 2 in [53 mm] or larger conduit enters the box, so three 1-in runs carry none in Division 2. The seal is a fitting packed with a listed sealing compound, and a run arriving from Division 1 stays a sealed run at the boundary, within 10 ft [3.0 m].
Which group covers a natural-gas or gasoline area?
Group D covers methane (natural gas), propane, butane, gasoline and hexane, so a box marked Class I, Division 1, Group D — often stamped Groups C and D for mixed hydrocarbon sites — covers it. Hydrogen takes Group B and acetylene takes Group A; a Groups A–D stamp covers all Class I gases.
Why is an explosion-proof box not airtight?
It is not meant to be: the box contains an internal explosion, and the threaded flame path cools the escaping gases below the ignition temperature of the surrounding vapor before they leave the box. That is why the five-thread joints and the seals carry the rating.
