Short answer: a control transformer in Class I Division 2 needs no purge and no explosion-proof enclosure. The Division 2 rules let it sit in a general-purpose, ordinary-location enclosure that suits the environment; indoors, that is a Type 12 box. Size the box on the transformer's losses, about 1-3% of rating. A 5 kVA unit sheds 50-150 W, which lifts the inside of a 24 × 20 × 12 in painted-steel box about 21 C at full load. The door-operated disconnect is the different animal. It opens load current by design, so buy it identified for Class I Division 2, use an interlocked isolation-only switch, or put the combined panel on Type Z purge. Ratings below are manufacturer ratings for the specified design; ours are not UL listed.
What the Division 2 rule asks of the transformer
Division 2 is the lenient zone of the classified areas. The NEC's Class I rules let control transformers sit in general-purpose enclosures — the industry term is ordinary-location — as long as the enclosure suits the rest of the environment. NEC Table 110.28 lists which enclosure type fits which conditions, so the room you have decides the type:
| Condition at the box | Ordinary-location type that suits |
|---|---|
| Indoor, dry, dust and dripping liquids | Type 12 |
| Outdoor rain, sleet and snow | Type 3R |
| Hose-directed water, washdown | Type 4 |
| Washdown plus salt or corrosives | Type 4X |
Every row is ordinary-location equipment; none of it is hazardous-location gear. NEMA 7 and 9 enclosures are the hazardous types, and we do not build them — the classified devices come to you as rated hardware from your panel-device supplier. What each type in the ladder covers, type by type, is laid out in the NEMA enclosure types guide.
The transformer's ordinary installation duties keep applying inside Division 2. A dry-type transformer maker's Class I Division 2 brochure states that its units comply with the NEC's transformer installation rules as well as the hazardous-location article, so secondary protection, grounding and heat get designed rather than skipped. Equipment in a classified area also carries a maximum-surface-temperature marking; order the transformer and every device with a temperature class below the autoignition temperature of the gas present.
Why the disconnect is a different device
The NEC's Division 2 switch rule draws one line by function. A switch intended to interrupt current in the normal performance of its job must sit in an enclosure identified for Class I Division 2. A switch used only for isolation, opened after another device has already cleared the load, is not required to be identified for the location. The test is the design function, so operating frequency changes nothing: a disconnect opened once a year still interrupts by design.
A door-operated panel disconnect is an under-load switch. It is the handle the operator reaches for to shut the line down, and the NEC's general rule asks every switching device to carry an interrupting rating at least equal to the current it must interrupt. Specify the door handle as a switch identified for Class I Division 2, with an interrupting rating at or above the available fault current at its terminals.
Isolation-only schemes work when the interlock is real. A switch that cannot open until the upstream breaker has cleared — mechanical interlock or key exchange — is true isolation, and general-purpose hardware covers it. Shut the panel down with the upstream breaker and confirm the load is dead before anyone opens a door or pulls fuses; a fuseholder opened under load performs the same interruption the switch rule exists for.
Three layouts that pass
Separate boxes. The transformer lives in the ordinary-location Type 12 box. The door-operated disconnect is a Division 2-rated device in its own Division 2 enclosure, mounted adjacent on the same wall. This is the cleanest answer when the handle must break load current, and the rest of the panel follows the ordinary Division 2 cabinet layout described for panels without purge.
Isolation at the door. The upstream breaker, outside the classified area, does the interrupting. The classified box holds only an interlocked isolation switch, the transformer and terminals, and general-purpose hardware covers all of it. The interlock is what makes the switch honest.
One sealed panel for everything. When the transformer, the door disconnect and the contactors must share one enclosure, run it on Type Z purge and follow the purged-panel specification. Contactors break load current every start and stop, so they are interrupting devices — Division 2-rated hardware unless the purge covers them. Pilot operators get the same read: choose ones marked for Class I Division 2. Terminal blocks and pilot lights that only carry current belong in an ordinary-location box, as the Division 2 terminal-box case shows.
The worked example: heat from a 5 kVA transformer
Take a 5 kVA dry-type unit, 480 V primary to 120 V secondary, feeding control circuits in an indoor Division 2 room.
- Secondary current: 5,000 VA ÷ 120 V = 41.7 A. The smallest copper conductor that covers it is 8 AWG, rated 50 A at 75 C in the NEC ampacity table.
- Losses: dry-types shed about 1-3% of rating, so 50-150 W here. Design the box on 150 W, the top of the range; assume a split of 50 W core plus 100 W copper and replace it with the maker's test data when you have it.
- Box: a custom 24 × 20 × 12 in painted-steel enclosure presents about 1.30 m² of surface, and painted steel transfers heat at 5.5 W/m²K.
- Rise: 5.5 W/m²K × 1.30 m² = 7.15 W per kelvin, and 150 W ÷ 7.15 = 21 C. A 30 C room puts the inside near 51 C; 40 C ambient puts it near 61 C.
- Half load: the copper share follows the square of current and falls to a quarter — 50 W + 25 W = 75 W — so the rise drops to about 10 C.
Stainless walls transfer at 3.7 W/m²K, so the same 150 W in the same box rises about 31 C. Each added square foot of panel lowers the rise in proportion. The coil insulation is built for far more than 61 C — this check protects the terminal blocks and anything else sharing the box. The partial-load arithmetic is worked further in the transformer heat-loss guide.
Terminals, working space and handle height
Give the conductors room to bend and the operator room to stand. NEC 312.6 sets the bending space at terminals by wire size — for a 2 AWG lead with one wire per terminal, 2.5 in — and the table grows for larger conductors, so leave that width of wire way at each terminal landing on the transformer. NEC 110.26(A) asks 3 ft (900 mm) of clear working depth in front of equipment likely to be serviced while energized at 0-150 V to ground. The door needs at least 90 degrees of swing, and the 480 V primary side calls for more depth; the enclosure mounting guide covers the clearance and height rules in full. Mount the door-operated disconnect so the center of its handle grip, in its highest position, stays at or below 2.0 m (6 ft 7 in) above the floor, which is NEC 240.24(A).
What to put on the order
Order the box as a custom Type 12 enclosure and buy the classified devices as rated hardware:
| Item | Put this on the order | Basis |
|---|---|---|
| Transformer box | Custom Type 12; painted carbon steel (RAL 7035) for a dry room, 304 stainless where washdown reaches it, 316L under chlorides or brine | Ordinary-location rule, NEC Table 110.28 |
| Box size | 24 × 20 × 12 in for the 5 kVA case; scale surface area to hold the full-load rise near 21 C | 150 W at 5.5 W/m²K over 1.30 m² |
| Transformer | Open core-and-coil, 5 kVA, 480-120 V, bolted to the plate, leads within the NEC 312.6 bending space | Division 2 ordinary-location rule |
| Disconnect | Door-operated switch identified for Class I Division 2, interrupting rating at or above the available fault current | Interrupting-by-design rule |
| Contactors and pilots | Division 2-marked contactors; operators marked for the location | Same functional test |
| Alternative | Type Z purge kit when all interrupting devices share one sealed box | Purged-panel layout |
Size the equipment grounding conductor from the rating of the overcurrent device ahead of it, per NEC Table 250.122, and bond the transformer frame and the enclosure into that path; the grounding and bonding guide walks the connections. Bring every raceway through the bottom wall on hubs that hold the Type 12 seal, because the weakest entry sets the assembly rating. Do not fit a drain or open vent to fight the heat — size the surface instead.
Custom work is delivered in 10 business days, counted from your approval of the drawing. If the environment row is still ambiguous, the enclosure selector sorts by environment, listing requirement and inside size, and carries the result to a custom quote. For sizing help from the component list inward, the guide on how to size an electrical enclosure adds the fill and wiring-space checks.
The decision, case by case
- Indoor dry room, transformer and terminals only: a custom Type 12 box with the open coil bolted to the plate, sized on the 150 W case. No purge, no hazardous-location rating on the box.
- Outdoor site: Type 3R or better from the conditions table; Type 4X where washdown or corrosion joins the weather, as the outdoor enclosure guide sets out.
- Door handle that must break load: a disconnect identified for Class I Division 2 in its own enclosure, interrupting rating at or above the available fault current.
- Isolation-only is honest: when the interlock forces the upstream breaker to clear first, general-purpose hardware covers the classified box.
- Everything in one sealed box: Type Z purge, specified as the purged-panel article sets out.
- Listing on the specification: listed Division 2 devices and, where one is required, a listed enclosure — the UL 50E listing guide explains what listing evaluates. Our boxes are manufacturer-rated, not UL listed; a rating and a listing are different documents, so buy listed hardware where the job calls for it.
Questions
Does the transformer need a purge in Class I Division 2?
No purge is needed for the transformer alone. The NEC's Class I Division 2 rules accept a control transformer in a general-purpose, ordinary-location enclosure that suits the environment — indoors a Type 12 box, outdoors Type 3R or better. Size the box on the losses, 1-3% of rating: a 5 kVA unit gives 50-150 W. Purge enters only when interrupting devices like the door disconnect and contactors must share one sealed enclosure.
What disconnect is acceptable at the door of a Division 2 panel?
A switch identified for Class I Division 2, because a door-operated panel disconnect opens load current by design — and the NEC's test is the design function, not the frequency, so once a year still counts. Its interrupting rating must cover the available fault current. The alternative is an isolation-only switch interlocked so the upstream breaker clears before it can open; that switch may be general purpose.
Can an open core-and-coil transformer go inside the enclosure?
Yes. The same ordinary-location rule covers a non-enclosed coil mounted in the Type 12 box; bolt it to the mounting plate and give it the heat and terminal room. Worked case: a 5 kVA unit losing 50-150 W in a 24 × 20 × 12 in painted-steel box rises about 21 C at full load and about 10 C at half load, and 8 AWG secondary leads carry the 41.7 A.
Is a manufacturer-rated enclosure enough for a classified-area install?
For the transformer box, the Division 2 rule asks only an ordinary-location enclosure, and our boxes carry manufacturer ratings, not UL listings. Where the specification, the inspector or OSHA's approval rule (29 CFR 1910.303(a)) requires listed equipment, a manufacturer rating does not close that gap: order the Division 2-rated devices from listed product lines, and a listed enclosure where one is specified.
What conductors land on a 5 kVA, 480 to 120 V transformer?
The secondary carries 5,000 VA ÷ 120 V = 41.7 A, so 8 AWG copper, rated 50 A at 75 C in the NEC ampacity table, is the smallest conductor that covers it. The primary carries 10.4 A at 480 V, which 10 AWG, rated 35 A at 75 C, covers with margin. Leave the NEC 312.6 bending space at both terminal sets, and size the equipment grounding conductor from the rating of the overcurrent device ahead of it, per NEC Table 250.122.
