Short answer: a round gland seal cannot seal flat cable. The rubber bottoms out on the two flats first, its travel is spent on two contact lines, and the side troughs stay open for water to follow. Use an entry made for flat or pre-terminated cord instead: an oval cable gland, a slotted-seal gland, or a split sealing frame. Size it so the clamping range covers the cord's width and its thickness. The worked example below seals a cord 10.0 mm wide by 5.0 mm thick, and sets a drip loop 50 mm below the gland. If the box is a printed shell carrying permanent outdoor lights, replace the shell with a rated metal enclosure: the gland seals the hole, and the box earns the rating.
Why a round seal leaks on flat cable
A round gland works by squeezing a rubber seal down onto a round sheath until it grips all the way round. On flat cord the seal reaches its travel limit on the two flats, and the sides never close. Those open troughs run the full length of the seal, so hose water and wind-driven rain find a path in seconds, whatever the torque.
Every entry lives by one rule: the seal must contact the outer sheath over its whole perimeter. That contact is also the grip, so the cord's pull stops at the gland instead of reaching the terminals. The principle is the same one behind why a gland seal must touch the cable's outer sheath.
Pick the entry that matches the cord
Three entry types seal flat cable, and the choice turns on what else has to come through the wall.
| Entry type | Best for | Check before ordering |
|---|---|---|
| Oval (flat-cable) gland | One flat cord in a permanent run | Clamping range covers width and thickness |
| Slotted-seal gland | A flat cord through a rectangular slot | Slot width and height, corners included |
| Split sealing frame | Pre-terminated cords, several cables on one plate | Each split slot's range covers its cord; body sizes run M12 to M63 |
| Wraparound multi-cord grip | Several cords through one large hole | The grip is listed for wet locations |
Split frames exist for pre-assembled cords: the split seal closes around the cable without cutting off the molded connector on the far end, and it grips for strain relief at the same time. The sizing discipline is the same as matching a gland clamping range to the cable: both flat dimensions sit inside the range, not only the thinner one.
What you need
- The cord, measured across its largest point, ribs and moldings included
- An oval gland, slotted seal or split frame whose clamping range includes both measured numbers
- A hole saw or step drill in the gland datasheet's hole size
- The gland's o-ring and locknut; fast-fit glands that seal without a locknut are the exception, and the datasheet says so
- A deburring tool, a paper towel and a garden hose for the water check
Fit the gland, step by step
- Measure the cord across the flats where the seal will sit and write both numbers down. The example cord reads 10.0 mm wide and 5.0 mm thick; a molded rib can add to either figure.
- Match both numbers to the entry. A range that covers only the thickness fails, because the wide dimension is what opens the troughs. A seal rated 4 to 8 mm thick by 8 to 14 mm wide takes the example cord with margin on each axis.
- Mark the hole on a flat face and keep at least 10 mm of flat sheet around it. Cut to the datasheet hole size, 20.5 mm for the M20 example, and deburr both sides so a burr cannot nick the seal. Lay the whole face out in the cutout designer first; the fundamentals of entries and openings are in the cutout and gland guide.
- Fit the body with its o-ring against a clean, dry face and run the locknut up. Slide the seal over the cord, tighten the compression nut hand-tight plus half a turn, and look for an even bulge into both flats.
- Form the drip loop below the gland: bring the cord down until its lowest point sits at least 50 mm under the entry, then rise to the terminals. Water running down the cord drips off at the loop instead of pooling at the gland. The full pattern for a wall box is in glands, drip loops and drains for an outdoor junction box.
- Pull the cord firmly by hand. It should barely move and spring back; a sliding cord takes another quarter turn, and a seal that starts to split is the wrong range, not a torque problem.
Checks before you close the box
- Compression: the seal bulges evenly into both flats, keeps a round cross-section, and shows no cuts or white stress lines.
- Strain relief: a firm hand pull moves the cord about 1 mm or less, and nothing at the terminals carries the load.
- Water: close the box, run a garden hose over the entry for 5 minutes from above and both sides, then open it and confirm the paper towel inside is dry. An immersion depth is a maker's test condition, never a hose result.
- Entry direction: the gland points down or sideways, with the loop dressed below it.
Mistakes that cost the seal
- A round seal torqued onto flat cord is the classic failure: the two troughs stay open at any torque. When the cord on hand is thinner than the range, the workarounds are in sealing a gland around a smaller cable.
- Sealing against a printed rib or a layer seam: the seal copies the print's surface, and the layer lines wick water along the wall.
- Caulk as the only barrier: it fills the troughs once, but it adds no strain relief and peels off the first time the cord is handled.
- Skipping the soffit fact: warm, damp house air rises into a soffit and condenses on any cold surface, so the box sees real moisture even under cover.
The box around the gland
Under a soffit, the gland is the small half of the problem. Permanent outdoor wiring in the US must use approved parts. OSHA 29 CFR 1910.303(a) requires electrical equipment to be approved, and 1910.399 defines approved as accepted, certified, listed, labeled or otherwise determined safe by a nationally recognized testing laboratory. A printed shell carries no such evaluation; at most it is outer clothing over a box that is, and when a printed outdoor shell needs a rated box draws that line.
A listed enclosure has been evaluated by UL to UL 50E, and how enclosure listing to UL 50E works covers that side; where a specification calls for the mark, buy a box that carries it. Our stainless bodies are rated NEMA 4X / IP66 by the manufacturer, not UL listed. We build them as custom enclosures to your drawing, every cutout placed before the sheet is cut, delivered in 10 business days from your approval of the drawing.
The decision, by job:
- Permanent outdoor lighting, rain and frost: a rated metal enclosure, an oval or split entry sized to both cord dimensions, and a drip loop under the gland. Kill the circuit and confirm it dead before opening the existing wiring.
- Portable or temporary work in dry weather: a printed shell with a split entry is fair weatherproofing, and it stays a shell, not an enclosure.
- Indoors and dry: any correctly sized gland on a standard interior box; flat-cable entries earn this effort only where water is.
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 |
Questions
Can one entry take two flat cables?
Yes, when each cord gets its own split slot. A split-seal frame or wraparound multi-cord grip seals and grips each cord separately, and every slot's clamping range must cover that cord's width and thickness; the 10 by 5 mm example cord needs a slot sized for 10 mm, not one for 5 mm. A single round seal never seals two cords, and opening its hole to try just splits the seal along the flats.
Is an IP68 flat-cable gland submersible?
Read the code in two digits. The 6 means dust-tight; the 8 means continuous immersion on conditions the manufacturer and user agree, so the depth and the time come from that datasheet. IP67 is temporary immersion, 1 m for 30 min. For a soffit or wall box, second digit 6 already covers powerful jets from a 12.5 mm nozzle at 100 L/min, which is beyond anything rain delivers.
Which seal material survives a freezing winter?
Nitrile stiffens toward its -30 C floor, so on a soffit that sees -20 C nights pick EPDM, good from about -50 to 150 C, or silicone, about -60 to 200 C. EPDM also suits hot water and caustics best; nitrile remains the pick where oil or coolant touches the cord. The temperature limit belongs to the seal, not to the plated brass body.
