A cable through a thick panel has two ways through: a rubber grommet or a cable gland. For a panel 12 mm thick or more, and an IP66 target, the gland is the safer choice. It clamps the cable jacket, seals on a defined grip range and holds its rating when the cable gets tugged. A plain snap-in grommet will not do either job on a thick wall; only a threaded liquid-tight type comes close.
What each one actually is
A rubber grommet is a moulded ring that snaps into a hole. The cable passes through the centre and the rubber cushions it against the panel edge. It stops chafing and keeps dust out. It is not designed to seal against hose-directed water, and on a thick panel most snap-in types cannot even seat, because their grooves are cut for sheet metal a millimetre or two thick.
A cable gland is a threaded body with a sealing ring (often called the seal or insert) that compresses around one cable diameter, plus a clamp that grips the jacket for strain relief. Tighten the cap and the cable is sealed and anchored. Glands carry their own ingress rating, so the assembly is only as good as the weakest entry, which is a hole you can control.
There is a middle part worth knowing: the liquid-tight strain relief. It looks like a grommet in a threaded body. It suits thicker walls than a snap-in grommet and gives some clamping, but a true gland still seals and grips better.
Side by side
| Rubber grommet | Liquid-tight strain relief | Cable gland | |
|---|---|---|---|
| Seal against hose-directed water | No | Partly, if clamped | Yes, with matched grip range |
| Strain relief | None (friction only) | Some | Positive clamp on jacket |
| Panel thickness range | Very thin sheet, typically under 3 mm | Stated per part; some suit 12 mm | Stated per part; long-thread versions exceed 12 mm |
| Hole size | Groove diameter, close to cable size | Thread size | Thread size (e.g. M25, 3/4 in NPT) |
| Cables per entry | One, snug | One | One, or a multi-hole insert |
| Rework after cable pull | Usually fine | Good | Good |
| Typical use | Sheet-metal panels, dust, chafe protection | Thick beams, light duty | Washdown and outdoor entries |
Where the grommet wins
A grommet is right when the panel is thin sheet, the entry sees dust and splashes rather than a hose, and the cable never gets loaded. It costs little, installs in seconds and needs no locknut or thread engagement. For an indoor Type 12 style panel with light cables, it is a sensible choice.
It also wins on hole size. A grommet hole hugs the cable, so you can pack more entries into a narrow margin. On a thick panel, though, that advantage disappears because standard grommets cannot seat at all.
Where the gland wins
If water can hit the entry under pressure, the gland wins. IP66 means protection against powerful water jets, and a gland rated IP66 by its manufacturer keeps that seal with the cable installed, torqued and locked. Our enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed, and the same logic applies to every penetrator you add: the rating claim belongs to the entry that is actually clamped and sealed. For how the two schemes compare, see the NEMA 4, 4X and IP66 guide.
The gland also carries the mechanical load. An 18 mm power cable that someone steps on, or a mobile cable on a moving axis, puts real force on its entry. A grommet lets the cable slide and saw against the hole edge; a gland clamps the jacket so the force stops at the panel.
The thick-panel problem, solved either way
Panel thickness is the real constraint. Match the part to the wall:
- Grommet: check the groove width. Most snap-in types seat in panels under 3 mm; a 12 mm wall needs a special thick-panel grommet, and supply is thin. Treat this path as the exception.
- Liquid-tight strain relief: pick a version with a published panel range covering 12 mm. Verify the grip range covers your 18 mm cable; many cover roughly 11 to 18 mm.
- Cable gland: standard threads engage about 6 to 10 mm of panel. For 12 mm and up, specify a long-thread or extended-body gland, or add a gland plate and mount the gland in that instead.
The gland-plate route is often the cleanest on a custom thick wall: a thinner plate bolted over the hole gives every gland a proper thread engagement and a repeatable seal. The cutouts, glands and HMI windows guide covers hole sizes and gland layout in detail.
Hole size and grip range for an 18 mm cable
For an 18 mm cable, size by grip range, not by habit:
- Pick a gland whose grip range includes 18 mm. A typical M25 or 3/4 in NPT gland covers roughly 13 to 18 mm; check the maker's table, since ranges vary.
- Cut the hole to the thread size: about 25 mm for M25, about 28 to 30 mm for 3/4 in NPT. A countersink-free, deburred hole protects the seal.
- Torque the cap until the seal visibly compresses and the cable no longer twists by hand.
For thread sizes and how they map to panel and cable, work through the same sums on paper before you mark the panel; the guide above lists typical hole dimensions per thread.
Mounting into a thick beam
If the penetration is a structural beam rather than an enclosure wall, think about the seal surface too. A rough drilled hole in a 12 mm beam will not seal like a machined enclosure knockout: deburr both faces, keep the area around the hole flat, and use a sealing washer or O-ring under the gland body where the surface is not machined.
Plan the entry position before you drill. Bottom entries shed water; side entries on a thick exposed beam collect it. For entry placement and orientation outdoors, plan the water barrier across all your penetrations before the first hole is cut.
Making the call
Choose by water, load and wall thickness:
- Dust, splashes, thin sheet, light cable: grommet. Cheap and correct.
- Thick wall, occasional wash, cable firmly fixed: a liquid-tight strain relief rated for the panel thickness, if its published range covers 12 mm and your cable diameter.
- Hose-down, washdown, outdoor, or any cable load: a cable gland with the grip range and panel range matched, on a long thread or a gland plate.
When you order a custom enclosure, put every entry on the drawing: position, hole size, thread type and whether each one takes a gland, a strain relief or a blanking plug. The cutout designer lays out holes and glands on any face and checks the spacing before the panel is cut, and every custom enclosure is built to that drawing.
One more check before you commit: a grommet does not ground anything. On a metal panel with metal-clad cable, a non-metallic entry changes your bonding path, which matters for the grounding rules your panel has to meet. A brass gland with a proper locknut keeps the path continuous; the grounding and bonding guide covers the bonding requirements behind that.
For cable bundles and mixed connectors rather than single round cables, give each cable diameter its own sealed entry or use a multi-hole insert, and check which enclosure type your specification actually calls for before you commit to the entry hardware.
Questions
Can a plain rubber grommet keep an IP66 seal?
No. A snap-in grommet is a cushion and an edge guard, not a pressure seal. It seals only where the rubber is compressed, and it relies on friction, so cable movement works it loose. For IP66 you need a grommet-style part that clamps the cable, such as a threaded liquid-tight strain relief with a sealing insert, or a proper cable gland with a grip range matched to the cable diameter.
What hole do I cut for an 18 mm cable?
For a gland, check the grip range, not just the thread. A gland with a grip range of about 13 to 18 mm takes an 18 mm cable; the hole is the thread size, commonly M25 or 3/4 in NPT, roughly 25 to 30 mm. For a grommet, the hole matches the grommet groove diameter, typically a few millimetres wider than the cable, and the panel must match the grommet's stated thickness range.
Does panel thickness change the gland choice?
Yes. Every gland has a thread engagement or clamping range with a maximum panel thickness. Long-thread and extended-body versions accept panels well beyond the standard 6 mm or so; some take 12 mm and more. If the thread cannot fully engage, or the locknut cannot clamp, the seal and the strain relief both fail. Check the published panel thickness range before ordering.
Can I run several cables through one entry?
Not through a single-grommet or single-gland hole with any rating confidence. Both seal on one cable diameter. For a bundle, either give each cable its own entry or use a multi-hole sealing insert in a large gland, matched exactly to each cable diameter. An open or loosely filled hole is the usual reason a sealed panel fails a hose test.
