Yes: NEC 312.6 sets the height. With every conduit entering the bottom, each conductor turns ninety degrees from its entry up to a terminal, so the vertical run from the bottom wall to the block row carries the bending space. Table 312.6(A) sizes it by conductor: 1½ in [38 mm] for 8 AWG, 3½ in [89 mm] for 2 AWG, one wire per terminal. Stack that with the block row and a service gutter, and an 8 AWG submersible-pump splice needs about 6½ in, which a 12 x 12 x 6 in stainless box covers with a spare row to spare.
What the height is made of
A bottom-entry box has three stacked layers between its bottom wall and its top wall, and the inside height must hold all three at once:
- Bending gutter. The clear vertical space between the bottom entries and the lowest wire entry, sized from Table 312.6(A) for your conductor.
- Block row. A through-terminal for this wire size on 35 mm DIN rail stands about 3 in [75 mm] tall; blocks for 2 AWG run about 4 in [100 mm] (typical block-maker dimensions).
- Service gutter. Plan 2 in [50 mm] above the row so you can dress a wire, tug it and re-land it without stripping the block.
Measure against inside height, never the outside number. Our 12 in box gives you 11.91 in of it, nominal from the CAD. For how rails and duct share the panel across the width, see the backplate and DIN rail layout guide.
The bending-space numbers that set it
Use Table 312.6(A) when the conductor comes in through an adjacent wall — your bottom wall, with the blocks on the back panel. Table 312.6(B), with larger values, is for conductors that pass through the wall directly opposite their terminal. Below 8 AWG the table has no rows; box fill and the block maker's spacing take over.
| Largest conductor | One wire per terminal | Two wires per terminal |
|---|---|---|
| 8 AWG | 1½ in [38 mm] | — |
| 6 AWG | 2 in [51 mm] | — |
| 4 AWG | 3 in [76 mm] | 3 in [76 mm] |
| 3 AWG | 3 in [76 mm] | 3 in [76 mm] |
| 2 AWG | 3½ in [89 mm] | 4 in [102 mm] |
| 1 AWG | 4 in [102 mm] | 4½ in [114 mm] |
| 1/0 AWG | 4½ in [114 mm] | 5 in [127 mm] |
NEC 312.6 prints these as the minimum bending space at terminals and the minimum width of wiring gutters. Keep the space in front of each block clear by the same distance; the 5.91 in depth leaves room to spare. For the full measuring method, read how to measure wire bending space in a junction box, and for box volume, the enclosure sizing guide.
The extra rule that arrives at 4 AWG
Once a box over 1650 cm³ (100 cu in) holds conductors 4 AWG or larger, the NEC's pull- and junction-box distances join in. A straight pull needs 8× the trade diameter of the largest raceway between entries. A bottom entry turning up to a terminal is an angle pull: count 6× the trade size of the largest raceway from the entry to the top wall, and add the trade size of every other entry in that same row.
Run those numbers before you order the hub. A 1½ in hub needs 6 × 1.5 = 9 in of height, which the 12 in box clears with its 11.91 in inside. A 2 in hub needs 6 × 2 = 12 in, and 11.91 in falls 0.09 in short, so that hub belongs on the 16 in box.
Six steps, worked on a pump splice
The job: a 5 hp submersible pump at a wellhead, 8 AWG feeders from the controller (sized for voltage drop on the long run), factory pump cable at about 0.7 in [18 mm] outside diameter, three conductors and a ground.
- Count the terminals. Three line blocks, one ground block and two spares make six through-blocks with end brackets on one rail.
- Pull the bending space. Table 312.6(A) gives 1½ in [38 mm] for this conductor, one wire per terminal. Keep that clear from the bottom entries to the lowest wire entry, and clear in front of each block.
- Stack it. 1.5 in of gutter + 3 in of block row + 2 in of service gutter = 6.5 in.
- Check the box. 11.91 in inside − 6.5 in = 5.4 in spare. A second rail row adds 3 in and still leaves 2.4 in, so future spares have a home.
- Check width and depth. Six blocks at about 0.4 in [10 mm] plus 1 in of brackets take 3.4 in of the 11.91 in width. The blocks stand about 2.5 in [64 mm] off the panel inside the 5.91 in depth.
- Put it on paper. Mark hub and gland centers from the bottom edge, rail height and block count. Delivery runs 10 business days from your approval of the drawing.
Terminal strip or power distribution block
A terminal strip is one-for-one: one conductor lands on each side of each block, which is exactly what a splice needs. A power distribution block takes one large feed on one side and several smaller taps on the other; it earns its place when one feeder serves several loads. Where the NEC allows a power distribution block into a junction box, it sets three conditions: the block is listed, the box is larger than 1650 cm³ (100 cu in) — the 12 in box holds about 840 cu in inside — and the box dimensions match the block's own installation instructions. Bending space at its terminals still follows 312.6.
For this splice, six through-blocks beat a distribution block on simplicity and keep both sides of the rail readable. The terminal layout article: grouping rows and spares shows the arrangement that ages best when the panel grows.
Keep the water at the bottom
Bottom entry is the right call at a wet site: the jacket sheds water and the gland faces down. Hang a 6 in [150 mm] drip loop below the gland before the cable rises into it, or water will walk up a jacket that touches the fitting. An M25 gland, or its 1 in NPT equivalent, seals a typical 13-20 mm [0.5-0.8 in] cable range, which covers the pump cable. The motor lead splices outdoors article covers gland choice and terminals for the same duty.
Land the pump's grounding conductor on a ground bar bonded to the bare metal of the box, the way the enclosure grounding and bonding guide shows. A sealed box still breathes with the day-night swing; if condensation becomes a nuisance, a breather drain fixes it, and the assembly rating then follows that part's rating, so pick one that matches the box. Tacky insulation on the blocks means water is already inside — the sticky wire insulation in a terminal box article tells the two causes apart.
On material: a wellhead sees chlorides in the water and, on a farm, fertilizer runoff. Order 316L there, because 304 can pit at room temperature under chloride deposits; keep 304 for dry indoor rooms.
What to put on the order
| Line | What to order |
|---|---|
| Body | 12 x 12 x 6 in 316L wall-mount enclosure (H x W x D), hinged gasketed door, back panel |
| Rating | rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| Bottom entries | one 3/4 in hub for the feeder conduit; one M25 gland for the pump cable |
| Interior | 35 mm DIN rail, six through-blocks with brackets, ground bar |
| Spares | two spare blocks now, rail space for four more |
| Paper | cutout drawing approved; delivered in 10 business days |
For the dry room indoors, the same box in 304 does the job. When the wire grows to 1/0 or the hub to 2 in, move up to the 16 x 16 x 6 in 316L box with 15.88 in of inside height. More sizes sit in stainless wall-mount enclosures.
Checks when the wiring is done
- Measure from the bottom wall to the lowest wire entry: at least 1½ in [38 mm], with nothing standing in it.
- Torque every block screw to the value marked on the block, then check it again after the pump's first full-load run.
- Lock out the controller and check absence of voltage before you re-land anything.
- Hose the closed door and bottom entries for five minutes, then open and look for a wet track at the gasket or gland seat.
- Leave field cutting out of it: order the gland and hub holes on the drawing, so the seal and the paper agree. The cutouts and glands guide lists what that drawing should carry.
Your decision in one line: terminal blocks on this wire size, bottom entry, the 12 in box; at 4 AWG and larger, check the 6× entry distance as well, and step to the 16 in box when a 2 in hub needs its 12 in.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 12 × 6 in | 316L stainless | 12 x 12 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 12 × 6 in | 304 stainless | 12 x 12 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 16 × 6 in | 316L stainless | 16 x 16 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does wire bending space apply inside a junction box, or only in cabinets?
Apply it at the terminals wherever the blocks sit. The NEC rules that allow power distribution blocks in junction boxes set their terminal bending space by 312.6, and inspectors apply the same table at ordinary terminal strips. For one wire per terminal at 8 AWG that is 1.5 in [38 mm]; landing two 2 AWG wires in one terminal raises its figure to 4 in [102 mm].
How tall must the box be for 2 AWG conductors on terminal blocks?
Plan 3.5 in [89 mm] of bending space, about 4 in [100 mm] for the block row and a 2 in [50 mm] top gutter: 9.5 in of stack against 11.91 in of inside height, so a 12 x 12 x 6 in box covers it. The pull-box distance for a 1½ in bottom hub adds 9 in to the top wall, which the same box also clears.
Do I need a listed power distribution block for this splice?
For one pump cable landing on one feeder set, through-terminal blocks do the job. A power distribution block earns its place when one large feed splits to several loads; then it must be listed, sit in a box larger than 1650 cm³ (100 cu in), match the box dimensions in its installation instructions, and still give 312.6 bending space at its terminals.
Should I cut the gland hole in the field or order it cut?
Order it on the cutout drawing: a factory hole keeps the gasketed door and gland seal as one documented assembly, and delivery runs 10 business days from your approval of the drawing. A field hole needs a hole saw to suit the gland thread, full deburring, and a gland installed to its instructions to hold the same seal.
