Fit the gland, and the box keeps its IP65. A rubber grommet slit open over the cable is a splash seal: dust-protected, splashing water held, nothing more. The jet test behind IP65 drives a 6.3 mm nozzle at 12.5 L/min at the enclosure, and that jet walks through a slit grommet. Count a grommet as IP54-class at best, two grades short of what the box is sold for. A gland's clamping seal, matched to the cable within about 1 mm of a clamp limit, is what holds the manufacturer's rating. Flat twin and earth tails through M16 rear knockouts need a membrane gland or a gasketed entry frame instead, because a straight M16 gland cannot close on a flat face. This comparison sets the two parts side by side, then runs one fit check end to end. It ends with the entry to order for each room your box lands in.
What each entry actually gives you
The two parts look interchangeable, and they are not. IEC 60529 grades the second digit of the code: a 4 holds splashing water from every direction, a 5 holds jets from a 6.3 mm nozzle at 12.5 L/min. A grommet passes splashing and loses to the jet. A gland closes on the cable's outside diameter and carries the rating with it, which is why each digit in the IP rating chart is its own test.
| Grommet, slit over the cable | Cable gland | Membrane gland or entry frame | |
|---|---|---|---|
| Claim you can make | Splash and dust: IP54-class, never the full IP65 | The manufacturer's IP65 or IP68 when the cable sits in the clamp range and the box maker lists the gland for that hole | IP65 to IP68 as the entry is rated, one membrane per cable |
| Cable it takes | Round cable; flat cable badly | Round cable, flat cable with a shaped oval insert | Round and flat, each membrane piercing to the profile |
| Where it seals | The hole wall only | The cable OD, plus a gasket under the locknut | The gasket face, plus every membrane around its cable |
| Tools | None | Wrench and a hole at the knockout size | Drill or punch, screwdriver, a flat landing |
| Opens again | Yes | Yes | Yes, membranes re-close on the same cable |
| Best job | Blanking spare holes, edge protection indoors | Any rated entry, and the armored cable | Retrofit rated entries over existing knockouts |
One weak entry sets the level for the whole assembly: the rating you can claim is the weakest entry's rating. Fit a slit grommet into an otherwise perfect IP65 box and the assembly reads IP54-class to anyone who tests it. A blanking plug in a spare knockout holds the level only when it is the accessory the box maker documents for that hole and rating, installed as its instructions set out; the same holds for every gland you fit.
Where a grommet wins
Dry indoors - behind a fitted kitchen unit, inside a stud wall, up a loft run - the enemies are dust and sharp edges, and a grommet handles both without a wrench. On a plastic body the molded knockout edge cannot cut a sheath, so there the grommet is tidy work and blanking rather than protection. On a steel body the same grommet saves the insulation from a punched edge. Speed is the third win: no thread cutting, no locknut, and a spare knockout blanks in seconds. A blind grommet, pressed in flat until it snaps past the wall, is the right part for a knockout you will not use; it seats with the same move described for square tube in blind grommet fitting guide.
The picture changes with panel thickness. A thick steel plate grips a grommet differently from a 1 mm box wall, and the hole size starts to decide the fit; the thick-panel comparison works that case through.
Where the gland wins
Any hose, any jet, any wall that sheds rain: fit the gland. Its clamping seal grips the sheath, so the entry also carries the cable's weight, where a grommet carries none. Three checks keep the rating, and the gland fitting guide walks them: hole at the gland's knockout size, gasket landing flat, locknut pulling the seal square. Match the seal to the cable within about 1 mm of a clamp limit; the clamping-range guide shows how the published ranges are read.
Steel-wired armor (SWA) goes into an armored gland, never a grommet: the armor terminates there and the clamp takes the strain, as the armored gland guide explains. The usual failures are short and physical - an oversized hole, a gasket on a weld bead, a cable under the range minimum - and the cable-entry hazards guide lists them so you can check your own work. Isolate the circuit before you open a live box, then fit.
The fit check: a 6 mm² twin and earth tail against an M16 gland
- The box. An IP65 plastic junction box with M16 knockouts on the back face and a 20 mm knockout for the armored cable: the layout most small sealed boxes follow.
- The cable. A 6 mm² twin and earth tail. The example uses a caliper check of 14.0 mm across the flats and 8.0 mm through the thickness; measure your own length before ordering, because brands vary by about a millimetre.
- The gland. A typical M16 gland clamps 4-8 mm of round cable, the range metric gland tables publish; the datasheet packed with yours sets the final numbers.
The rule: every dimension that crosses the seal sits inside the clamp range, within about 1 mm of a limit. A flat cable presents its wide face to the bore, so the face width decides.
- Face width against the top of the range: 14.0 mm - 8.0 mm = 6.0 mm beyond the largest cable the seal is made to close on.
- Bore circumference at full clamp on an 8 mm round cable: 3.14 x 8.0 mm = 25 mm.
- Perimeter of the flat tail: 2 x (14.0 mm + 8.0 mm) = 44 mm.
- Stretch needed to pass the flats: 44 mm / 25 mm = about 1.8 times.
A seal cannot pass a 14 mm flat and still clamp an 8 mm thickness. Fitted anyway, it rides up the flats and leaves open horns at each edge, and the first jet finds them. The round 6 mm² earth single is the easy case: its measured outside diameter sits inside 4-8 mm, and a straight M16 gland takes it.
The threshold from the rule: more than 1 mm outside the range on any dimension fails the fit, and 6.0 mm is six times that window. Order one of three things instead. A domed-membrane M16 gland closes its membranes around a flat profile. A gasketed membrane entry frame spans both rear knockouts on the flat back face and seals one cable per membrane. Or bring the tails into the bottom knockout behind an oval-insert gland sized to the cable. The armored cable stays on its armored gland either way.
The decision by environment
Where the box lands decides the entry; read the table straight across.
| Where the box lives | Armored cable (armored gland at the knockout size) | Flat tails (M16 rear knockouts) | The claim you can make |
|---|---|---|---|
| Dry indoors, behind a unit | Gland, or knockout with grommet strip | Grommet for speed, membrane gland for tidiness | Dust and splash is the ceiling: IP54-class at the grommet |
| Splash zone, over a sink or boiler | Armored gland, strain relief set | M16 membrane gland, one membrane per cable | The manufacturer's IP65, every entry matched and documented for the rating |
| Outdoor wall, rain and hose | Armored gland | Gasketed membrane entry frame over both knockouts | IP65 across the assembly, sealant bead below |
| Washdown or jet-clean areas | Armored gland on a smooth gasket land | IP68 membrane entry | Step up to a 316L body; the same entry rules carry the rating |
A 4-5 mm hole in the base is the traditional drain on damp walls. NEC 314.15 permits drainage openings of 3 mm to 6 mm in boxes listed for damp or wet locations, and a drained box is a drip box, so schedule it as drained rather than IP65. Rear entries lean on the wall for their last line of seal; the rear-entry guide covers the wall side, so water running down the surface sheds past the box instead of pooling under it.
What to put on the order
- One armored gland per SWA drop, at the knockout's size, clamp range checked against the measured outside diameter of the cable.
- One M16 membrane gland per flat tail, with membranes that close on a flat profile, or one gasketed membrane entry frame sized to span both rear knockouts.
- Blanking plugs, rated to the box, for every knockout left unused at 16 mm and 20 mm - each one the accessory the box maker documents for that hole and rating; one unrated plug drags the whole assembly down.
- A punch at each gland's knockout size, so the gasket lands on flat, deburred material; the field-drilled entry guide sets the same standard for holes you drill yourself.
- For a washdown room or a weather-facing wall, a 316L stainless body: our boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Lay the cutouts out in the cutout designer, check the drawing against the custom stainless cutout checklist, and the finished box is delivered in 10 business days.
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 |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 14 × 8 in | 304 stainless | 16 x 14 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 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 |
Questions
Will a slit grommet keep an IP65 junction box at IP65?
Not on its own. A grommet is a splash-and-dust seal: IP54-class, dust-protected with splashing water held. The jet run behind IP65 sends a 6.3 mm nozzle at 12.5 L/min at the box, and it finds the slit. A gland with the cable inside its clamp range, or a gasketed membrane entry, holds the box at its rating - provided the enclosure maker documents that entry for the rating, because the weakest entry sets the assembly's level.
Does a plastic box need grommets at all?
For sealing, no: the molded knockout edge cannot cut a sheath the way a punched steel edge can, so in plastic the grommet's job is blanking and tidiness. A blind grommet in a spare hole, or a blanking plug in a spare knockout, keeps dust and jets out when it is the part the box maker documents for that hole. On steel bodies, add grommet strip wherever cable crosses a cut edge.
Where does the traditional 4-5 mm drain hole belong?
On damp internal walls where nothing jets or hoses the box: a 4-5 mm hole in the base lets condensation escape, and it ends the sealed rating on the spot, so schedule the box as drained rather than IP65. NEC 314.15 puts compliant drains at 3 mm to 6 mm in boxes listed for damp or wet locations. On a rain-washed wall, seal the base and let the rated entries carry the box.
Does steel-wired armor still need a gland when the tails use membrane entries?
Yes. The armor has to land on a gland that terminates it and takes the pulling strain, and an armored gland at the knockout's size does that in one part. A grommet grips nothing and terminates nothing, leaving the armor floating; a membrane entry over an armor tail only seals around a cable that still needs its gland.
