Draw the line at mounted components. A junction box holds splices and nothing else — twist-on connectors, crimps, split bolts — with slack left on every conductor so you can pull a joint out and remake it. A terminal enclosure carries mounted terminal strips, so conductors are cut to length and land on the blocks instead. The two take different sizing math, and only one normally earns 3 ft [900 mm] of clear space in front of it. Here is the line, the math and what goes on your order.
What puts a box on each side of the line
Open the cover and look at the connections. If every joint is a splice device — twist-on connector, crimp sleeve, split bolt — and nothing is screwed to the back, you have a junction box. If terminal strips, fuse holders or relay sockets sit on a rail, you have a terminal enclosure; the same hardware travels under the names terminal box, marshalling box and distribution box.
The code books never define the second box. The NEC names enclosures by construction — junction box, pull box, cutout box, cabinet — and gives cabinets and cutout boxes (Article 312) their own rules. The industry line runs through what is mounted inside, because that is what changes the sizing math and the clear space you owe in front.
Re-pullability is the practical test. In a junction box every conductor keeps 6 in [150 mm] of free length at the splice point, the minimum the NEC sets at junctions and splices, so any joint remakes without shortening the circuit. In a terminal enclosure you trim to the terminal, and the slack lives in the duct loop or a spare coil on the panel.
A terminal block is a connection point like a splice, so one box can serve both duties, and many panels do. The design call is which duty dominates: see where a junction box earns its keep for the five places one pays for itself. Outdoors the splice device carries weather as well as current, and sealing it is its own subject — how 600 V conductor splices stay sealed outdoors.
What the code asks of each box
A splice-only box gets the simplest set of rules. The NEC sizes junction and pull boxes by conductor volume, and its fill table counts each #14 at 2.0 in³ [32.8 cm³] and each #12 at 2.25 in³ [36.9 cm³]. Add the free conductor per splice and the box is fully determined: volume for the conductor count, depth for the folded loops. With nothing mounted, no terminal bending-space table applies.
That last point settles the removable-cover question. NEC 110.26(A) asks for clear working space in front of equipment likely to be examined, adjusted, serviced or maintained while energized. A junction box is equipment — the code's own term sweeps in fittings and the like — but a blank cover over splices is not something you service live. You owe it access, not 3 ft. The same test governs a box above a ceiling tile, as the working-clearance case above a ceiling works through.
A terminal enclosure is a cabinet or cutout box with terminals, so the Article 312 rules land on it. NEC 312.6 sets the minimum wire-bending space at each terminal by conductor size: Table 312.6(A) where the conductor enters the same wall as its terminal, Table 312.6(B) where it crosses to the wall opposite. NEC 312.8 also limits how much of a cabinet holding switches or overcurrent devices may be filled with conductors, splices and taps — the rule that constrains circuits passed through a breaker panel.
When you test at the terminals live, the box is exactly the equipment 110.26(A) has in mind. Give it at least 3 ft [900 mm] of clear depth for 0-150 V to ground in Condition 1, 30 in [760 mm] of width or the equipment width, 6.5 ft [2.0 m] of headroom, and room for the door to swing 90 degrees; above 150 V to ground the depth grows (Table 110.26(A)(1)).
How sizing differs, side by side
| Junction (splice) box | Terminal enclosure | |
|---|---|---|
| Mounted parts | none — splice devices only | terminal strips, sometimes fuses or relays |
| Conductor length | slack at every splice (NEC minimum) | trimmed to the terminal |
| Size set by | conductor fill: 2.25 in³ [36.9 cm³] per #12 | terminal count x pitch + 1 in [25 mm] end gaps |
| Depth driver | the re-pull loop | rail, wiring duct, removable panel |
| Working space in front | access only | 3 ft [900 mm] deep when tested live |
| Cover | blank, gasketed | blank or windowed, gasketed |
The sizing driver is the tell. A splice box grows with the wire count alone; a terminal enclosure grows with the layout — terminal count, pitch, group spacing and the gap from the last block to the wall. The fill math is identical at heavier wire: sizing a splice box for 8 AWG three-phase splices runs the same steps, and the enclosure sizing guide carries the full method. Decide by asking who opens the box at 2 a.m.: if they need to measure a live voltage at a labeled point, build the terminal enclosure; if they need to remake a joint and re-pull, build the splice box. A pass-through with no splices at all is a pull box, and its minimum length keys to the raceway size rather than the fill.
Sizing a 13-conductor conveyor feed both ways
Inputs, with their sources: 12 #12 AWG THHN circuit conductors and 1 #12 equipment ground, spliced from a conduit feed to six motor whips in a washdown corner — a typical small conveyor job, so swap in your own counts. The fixed values: 2.25 in³ of fill per #12 (the NEC fill table), 6 in [150 mm] of free conductor per splice (the NEC), a 600 V / 25 A screw-clamp terminal on a 10 mm pitch (the common power block in terminal catalogs), and 1 in [25 mm] from the last block to the wall — the common shop rule, with the method in setting the terminal gap at the enclosure wall.
Splice-box route: the fill is 13 x 2.25 in³ = 29.25 in³. A nominal 4 x 4 x 2 in box gives 32 in³, which clears the fill at 91 percent — 29.25 / 32 — but a 6 in loop folded into 2 in of depth pinches. Step up a depth: 4 x 4 x 4 in gives 64 in³, 29.25 / 64 = 46 percent full, and every loop lies flat behind a blank cover. In steel that is the whole order; in our stainless line the smallest wall-mount size, 6 x 4 x 4 in outside, holds it with slack.
Terminal route: 12 circuit conductors take 12 through-terminals, and the grounds land on 2 ground blocks, so 14 terminals x 10 mm = 140 mm [5.5 in] of rail. The 1 in [25 mm] end gaps add 2 in, and the interior needs 7.5 in [190 mm] of clear width; the stainless size finder filters by nominal inside size. Because you probe these points live, give the box the 110.26(A) space from the last section: 3 ft [900 mm] deep, 30 in [760 mm] wide, 6.5 ft [2.0 m] of headroom.
Same conductors, two orders. The splice box is a 4 x 4 x 4 in blank-cover box with no backplate; the terminal enclosure is a wall-mount stainless box with at least 7.5 in [190 mm] of inside width, a removable panel and duct below the strip. The maintenance habit picks between them, and grouping power, I/O and spares on the backplate shows the layout once you build it.
Which box you order
- Splices that get remade, nothing mounted: a junction box. Size it at 2.25 in³ per #12 and 2.0 in³ per #14, keep the free-conductor minimum on every conductor, and pick depth so the folded loop lies flat — the example lands on 4 x 4 x 4 in.
- Fixed terminations, live testing, marshalling I/O: a terminal enclosure. Size by layout — count x pitch plus 1 in [25 mm] end gaps — and send the terminal sheet; what the terminal drawing needs to show is that sheet.
- Circuits passing a breaker panel: land them on terminals in a separate terminal enclosure. NEC 312.8 limits the conductors, splices and taps a cabinet with overcurrent devices may hold, so the panel stays clean and the through-circuits stay testable.
- Either environment, same hardware: 304 stainless for indoor washdown, 316L for salt air, brine and chemical splash, rated NEMA 4X / IP66 by the manufacturer — not UL listed. The rating survives the wall only if the entries do, so spec every gland with the cable OD; the cutouts and glands guide sets the pairings and mounting a weatherproof box without losing its rating keeps the install tight.
Send the conductor list — count, size, splice or terminal, and each cable's outside diameter — and you can order from one piece with every cutout placed. Delivery is 10 business days from your order, and for a custom layout from your approval of the drawing. Start at the custom enclosure builder; when a catalog box nearly fits but the layout needs one more window, where a catalog box ends and a custom one starts settles which way to go.
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 × 4 × 4 in | 304 stainless | 6 x 4 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 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 |
Questions
Can you splice conductors inside a terminal enclosure?
Yes. A terminal block is a connection point like any splice, so landing circuits on the strips is normal panel practice. One nuance worth knowing: the NEC lets the free-conductor slack at a junction be consumed by the splice itself, so an inline joint that uses up the slack still meets the rule. When the same box holds switches or breakers, NEC 312.8 limits the splice fill, so keep bulk splicing out of panels.
Does a junction box with a removable cover need 3 ft of working space?
Only when you service it energized. The clear-space rule of NEC 110.26(A) applies to equipment you examine, adjust, service or maintain while energized, and a blank cover over splices is not that. A terminal strip you probe live is, and then the space is 3 ft [900 mm] deep, 30 in [760 mm] wide and 6.5 ft [2.0 m] high for 0-150 V to ground, with more depth above that.
What size splice box do twelve 14 AWG conductors need?
Twelve #14 conductors count 2.0 in³ [32.8 cm³] each under the NEC fill table, so 24 in³ in all. A nominal 4 x 4 x 2 in box gives 32 in³ and clears it at 75 percent — 24 / 32. Keep the NEC free-conductor length at every splice, and recount before you order if grounds or clamps share the box.
Does the choice change the NEMA rating or the stainless grade?
Neither — both sides of the line order the same hardware. For a washdown corner that is 304 stainless rated NEMA 4X / IP66 by the manufacturer, not UL listed; move to 316L for salt air, brine and chemical splash. The rating survives only as far as the entries, so spec each gland with the cable outside diameter and place the cutouts before you order.
