Short answer: the screws that come with a weatherproof box are #8 self-tappers with an 8-32 thread, about 3/8 in long, and they drive into the mounting lugs — the cast or stamped ears on the back corners of the box. That is the term you were after: mounting lugs, also called mounting ears. Put every fastener through a lug, a foot or a hole the maker documents, and drill nothing through the box wall. A hole you add is a penetration the rain-tight or hose-down rating never covered, and no bead of sealant restores it. Two screws, two lugs, rating intact — that is the whole trick.
What the hardware is called and what screws it takes
A weatherproof box carries its mounting hardware outside the sealed volume. The cast or stamped tabs on the back corners are the mounting lugs or ears. Stamped steel boxes often use formed mounting holes in the body instead; one manufacturer's Type 1, 3 and 3R catalog puts it plainly: the mounting holes in the back stay accessible with or without the panel in place. Stainless wall-mount enclosures take a third route — separate wall-mount feet that bolt to the wall, with the enclosure hanging on them.
| Hardware | Screw it takes | What it holds |
|---|---|---|
| Mounting lugs or ears (cast boxes) | #8, 8-32 thread, 3/8 in [10 mm] self-tapping | Box to wall; the tip stops inside the lug boss |
| Body mounting holes (stamped boxes) | #8 sheet-metal screws | Box to wall through the maker's documented holes |
| Wall-mount feet (stainless boxes) | Sized to the wall: #10 structural screw into a stud, 1/4 in sleeve anchor into block | Feet to wall; the fastener never enters the shell |
The 8-32 thread is the electrical-box standard; the same size mounts fixtures to octagon boxes. Keep the length at the 3/8 in the lug boss is cut for. A longer screw bottoms in the boss or pokes through into the wiring space, and both outcomes are bad.
Why the lugs sit outside the sealed volume
The rating stamped on the box — Type 3R rain-tight, Type 4X hose-down — is evaluated on the enclosure as manufactured, under NEMA 250 and UL 50E. The NEC requires listed equipment to be installed in line with its listing and the maker's instructions. The lugs are part of that instruction set, and a screw through the back is in neither document. Weakest-link logic governs the rest of the assembly too: a box with a solid shell and an unrated hub at the top is an unrated box. For how listing and type ratings interact, see the UL 50E enclosure standard; for what rain-tight means in practice, the NEMA 3R guide.
What a screw through the back actually costs
Take the shortcut that keeps reappearing on job sites: a 1-1/4 in [32 mm] self-drilling screw driven straight through the back into the masonry behind it. Assume a 1/8 in [3 mm] wall, a working figure for a cast box this size; replace it with your own thickness when you reuse the numbers. The arithmetic: 1.25 in - 0.125 in = 1.125 in [29 mm] of bare thread left inside the box.
Against the 3.91 in [99 mm] interior depth of the catalog box below, that is 1.125 / 3.91 = 29% of the wiring space occupied by threaded steel, right where conductors land. Each thread root wicks any water that finds the fastener, and the sealant bead over the head turns into an annual maintenance item. The supplied lug screw does none of this. It passes through the ear alone, its tip stops in the boss, and nothing enters the wiring space. When a fastener genuinely has to pass through a shell, do it properly, as in mounting screws through an outdoor enclosure without leaks; the same question at panel scale is screws through a control panel wall and the Type 4 rating.
What you need
- The maker's instruction sheet, which names the mounting holes and the rated orientation
- The supplied 8-32 screws, plus stainless spares at the same length
- A #2 Phillips hand screwdriver
- Level, pencil and a strip of masking tape for the marking template
- Wall anchors to suit the structure: sleeve anchors for block, structural screws for studs
- For a stainless enclosure, the wall-mount feet shipped with the box
Mounting steps
- Isolate the circuit and verify absence of voltage at the box before you open it. Boxes in wet locations collect water as well as wire.
- Read the instruction sheet. It names the mounting holes and shows the rated orientation; a Type 3R box hangs upright with its drain low, and laying one on its side turns the weep path into a pool.
- Mark through the hardware. Hold the box or the feet level, mark each hole onto a strip of masking tape on the wall, then check the marks against the level. For height, level and clearances, the enclosure mounting guide carries the numbers.
- Drill the wall only. Take the bit size and depth from the anchor pack; block takes sleeve anchors, studs take structural screws, and wood sheathing has its own fastener table in fasteners for mounting a weatherproof box to OSB sheathing. Masonry detail is in mounting an enclosure on a brick wall.
- Drive the lugs. Start each screw square to the ear, snug it with the hand screwdriver and finish a quarter turn past contact. Power drivers shear soft zinc screws in about half a turn, so a snapped one is replaced with the stainless spare.
- Land the entries. Make up rain-tight connectors or hubs at the top and side openings per the maker's sheet, and set gasketed covers in front of any devices.
- Leave the drain alone. The weep path at the bottom of a rain-tight box is part of the design, and plugging it traps the water it exists to shed.
- Hang a stainless box on its feet. Bolt the wall-mount feet to the wall, lift the enclosure onto them and swing the door through its full arc. The shell takes zero field holes, which is how the box keeps its manufacturer rating after mounting.
Checks when done
- Every fastener passes through a lug, a foot or a documented hole — count 2 at the lugs or 4 at the feet.
- No bare thread inside the sealed volume; sweep the interior with a light before you close up.
- Gasket seated evenly, with the door pulling flat against it all round.
- The drain or weep path at the bottom runs free.
- Top and side entries made up with rated connectors; the assembly carries its weakest entry's rating.
- The box hangs level in both axes.
Mistakes that cost the rating, and the decision
- A screw or anchor through the back. The marked rating is spent the moment the bit breaks through. Sealing a wrongly drilled hole stops the drip, and what a panel keeps after you cut it and mount devices maps how far a modification can go. A box with a field penetration no longer matches its Type number, so a specification that calls for the rating gets a new box.
- A screw too long for the lug. Past the 3/8 in the boss is cut for, the tip bottoms or breaches the shell. Keep spares at the size the maker ships.
- Sealant doing a fitting's job. A bead over a knockout is a maintenance item, not an entry system. Fit the rated hub or connector at every opening.
- A weep hole drilled into a hose-down box. On a Type 4X shell, drainage belongs on the drawing, where it is part of the tested design. After the drill, the box is unrated.
The decision, by location. Covered and dry: mount on the lugs and stop, because nothing reaches the box. Exposed rain is Type 3R duty — lugs plus the factory drain, and where standing water genuinely collects, a drainage opening of 1/8 in to 1/4 in at the lowest point is what NEC 314.15 permits in a listed box in a damp or wet location. The wet-location duty then moves to the weatherproof cover in front of the box.
Washdown and hose spray are Type 4X duty a cast bell box never had. Step up to a stainless shell and keep every hole on the drawing: an 8 x 6 x 4 in [203 x 152 x 102 mm] box in 304 stainless or 316L stainless, or the one size larger 10 x 8 x 4 version in 316L when the devices need the room. These are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Order the cutouts you draw through custom enclosures; the wall-mount enclosures range runs from 6 x 4 x 4 in to 60 x 36 x 16 in and is delivered in 10 business days from approval of the drawing.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 8 × 6 × 4 in | 304 stainless | 8 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 8 × 6 × 4 in | 316L 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 |
| 10 × 8 × 4 in | 316L stainless | 10 x 8 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
What size are the screws on a weatherproof box's mounting ears?
8-32 thread, pan-head or countersunk self-tapping, 3/8 in long. The 8-32 size is the electrical-box standard, used for the lugs on weatherproof boxes and for mounting fixtures to octagon boxes. Replace a snapped zinc screw with a stainless spare of the same size and drive it with a hand screwdriver; a power driver shears the soft zinc shank in about half a turn.
Can I seal a bolt through the back and keep the waterproof rating?
No. A Type rating covers the box as manufactured, and a field penetration is outside that design; a sealant bead stops drips but does not return the marked rating. Mount through the lugs instead: the supplied screw costs a few cents against a callback and a replacement box. If a hole already exists, plug and seal it for the weather, then swap the box when the specification calls for a rating.
Do I need to drill weep holes in the bottom of an outdoor box?
Usually no: a rain-tight Type 3R box carries its own drain, and both plugging and adding drains change the design. Where standing water genuinely collects, NEC 314.15 permits drainage openings of 1/8 in [3 mm] to 1/4 in at the lowest point of a listed box in a damp or wet location. Take the trade knowingly: the hole turns the box from sealed to drained, and the wet-location duty moves to the weatherproof cover in front of it.
What actually holds a stainless wall-mount enclosure to the wall?
Its wall-mount feet. Bolt the feet to studs with #10 [5 mm] structural screws or to block with 1/4 in sleeve anchors, then hang the enclosure on them; the gasketed shell takes no field holes at all, so the manufacturer rating survives the installation. On the 8 x 6 x 4 in box that means four fasteners, all in the feet, none in the shell.
