Short answer: in a small sealed box, the water almost always comes in one of two ways, and you should treat them as separate problems. Liquid water usually enters through the cable entry: a gland that does not match the cable, a lid that is not fully torqued, or an entry facing up where rain pools. Condensation forms from humid air trapped inside the box on the day it was sealed; no rating stops that. Fix the entry first, then deal with the trapped air. Sealing the box tighter without fixing the entry traps the water you are fighting.
What each symptom points to
Work down this list before you change anything:
- Droplets on the lid or upper walls, no standing water — condensation from trapped humid air, not a leak.
- Water pooled at the lowest point, or a wet trail from one entry — liquid ingress through that entry.
- Corrosion or a white-green crust on connectors near a gland — slow, repeated ingress at that gland; check the seal range against the cable.
- Moisture that appears after rain, not after temperature swings — ingress through the entry, the lid gasket or a mounting screw.
- Moisture that appears on clear cold mornings after sunny days — classic condensation cycling; the entry may be fine.
- Fogging on a window or display inside the box — condensation again, worst where the wall cools fastest.
Why small boxes fog up so easily
The physics is simple. Air sealed inside the box carries water vapour proportional to the humidity on the day you closed the lid. A small sensor box in a garden sees a large daily temperature swing: sun on the lid heats the air, evening cools the walls below the dew point, and vapour condenses on the inside surfaces. Each cycle can pump a little air in and out through any imperfect seal, dragging in more moisture.
A PC or drive in a big panel warms the air enough to keep the interior above the dew point. A sensor board drawing a few hundred milliwatts does not. Its own heat is negligible, so nothing dries the box out. This is why a fix that works for a control panel — just run it and let waste heat do the work — does nothing for a project box.
The broader mechanism is the same one behind condensation in a sealed enclosure: the rating is about water staying out, not about the air inside staying dry. The same physics drives fogging on a window or display inside a bigger enclosure — read what an IP rating actually tests and how outdoor enclosures are specified for the sealing side of the story.
How to check the cable entry
Most small-box failures are here. Check in this order:
- Gland size versus cable. A gland seals only within its stated clamping range, printed on the body. Measure the cable's actual outer diameter, including jacket tolerance, and confirm it sits in the middle of the range. A cable at the bottom of the range, or a slightly oval or braided jacket, will weep. Gland sizing and sealing basics are covered in the guide to cutouts, glands, HMI windows.
- Entry position. Entries in the sidewall or, worse, the top, collect water on the gland nut and wick it in. Every cable entry on a rain-exposed box belongs on the bottom face.
- Drip loops. Outside the box, each cable should dip below its entry and rise back up, so water running down the cable drips off before it reaches the gland. Without the loop, the gland seals against a stream of water.
- Empty holes and loose glands. An unused gland is an open hole unless it has a sealing plug. Check the locknut torque; a gland body that spins with the nut is not compressing the seal.
- Lid and mounting screws. A through-screw can wick water along the thread. The fixes are the same as for mounting screws through an outdoor enclosure: blind holes, seals under the head, or outside the sealed volume entirely — see the enclosure mounting guide.
Fixing the entry
Once you know where the water gets in, choose one of these fixes:
- Correct gland, bottom entry, drip loop. The baseline fix. Right seal range, locknut tight, cable dropping below the entry before it runs anywhere. The gland details are the same as for any outdoor junction box; the drip loop does most of the work.
- Connectorized entry. A paired bulkhead connector, panel-mount half outside and cable half inside, seals on its own mating faces, independent of the cable jacket. That removes the size-matching risk entirely and makes the board removable. For a 12 V or 5 V sensor line carrying little current, the contact resistance of two extra contacts is irrelevant. Keep the mated pair on the bottom face and cap the unmated half when disconnected. The same trade-offs as an antenna feedthrough apply: a connector is the cleaner seal, a single gland the simpler one.
- Potting. For a box you never open, conformal coating plus potting the cable entry turns the cable into a molded seal. It is permanent: no rework, no serviceability. Combine it with a connector elsewhere if the board ever needs to come out.
- Moving the electronics instead. If the box sits in standing water or soil contact, no entry fix is enough; re-site the box or move to a larger wall-mounted enclosure with a back plate and proper entries.
Dealing with the trapped humid air
With the entry sealed, address the air already inside the box:
- Desiccant. A sachet of silica gel sized to the internal volume dries the initial fill and absorbs small residual ingress. Use an indicator type and change it at service intervals. This is the cheapest and most effective fix for a small box.
- Breather vent. A membrane vent equalizes pressure across temperature swings, which stops the box from pumping moist air in through imperfect seals. It admits water vapour slowly by design, so pair it with desiccant. Whether you want a vent or a drain depends on whether the problem is pressure cycling or standing water; the environment side is covered in the outdoor electrical enclosures guide.
- Anti-condensation heater. A 3 to 5 W resistive heater raises the interior a few degrees above ambient, keeping the inner walls above the dew point. On a solar or battery system, budget for the continuous draw; even 3 W is around 72 Wh per day.
- A deliberate drain. Some long-lived outdoor designs abandon the sealed approach and put a small downward-facing opening in the bottom, accepting dust and insects in exchange for water escaping. That is a legitimate re-specification for rain-proof-only duty, but it ends the sealing claim for the box.
A humidity sensor inside the box will tell you whether the fix is working; a small heater only helps if the interior actually stays above the dew point.
When to re-specify the box
If you find yourself potting connectors, adding a heater and swapping desiccant every few months, the box itself is wrong for the job. A purpose-built enclosure fixes the hard parts at the factory: entries on the bottom face, cutouts where you need them, a gasketed hinged door, and a removable panel for the board. Our stainless wall-mount enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed, and are made to your size and cutout list, delivered in 10 business days from your drawing approval. For a sensor box that grows into something with a display or a radio, the custom stainless cutouts page shows how to order the openings you actually need instead of drilling a sealed wall after the fact.
Plan the internal layout the same way you would for a bigger panel: board on the removable panel, entries clustered on the bottom face, desiccant and any heater clear of the board. The spacing principles in the backplate and DIN rail layout guide scale down to a project box without change.
Stopping it coming back
After the fix, close the loop:
- Seal the box in dry conditions, or purge the volume with dry air before closing the lid.
- Log the desiccant change date on a label inside the lid.
- Re-check every gland's locknut after the first full temperature cycle; thermal movement loosens marginal ones.
- Inspect after the first heavy rain and the first cold snap, not just on a dry day in the shop.
- If the box opens often, move serviceable connections to a bulkhead connector so the sealed volume is disturbed as little as possible.
Fix the entry, dry the air, and the box stays dry. Fix only one, and you will be back inside it within a season.
Questions
Can condensation happen inside a sealed IP67 box?
Yes. A sealed box holds the air sealed inside it, and that air carries moisture from the day it was closed. Daily temperature swings pump that air against the walls and lid, and when a wall drops below the dew point, water condenses on the inside. The rating stops rain and splash from getting in; it does nothing about the humidity already trapped.
Are cable glands or connectors better for a small outdoor box?
Both work when correctly sized. A gland seals well only when its seal range matches the cable's actual outside diameter, including jacket tolerance. A paired bulkhead connector seals on its own mating faces, independent of the cable, and makes the box modular. For small sensor boxes that get serviced or swapped seasonally, connectors are usually the lower-risk choice.
How much desiccant do I need in a small project box?
Rules of thumb run around 1 to 2 grams of silica gel per litre of enclosed air volume for a season of outdoor service in a box that is opened rarely. A half-litre sensor box needs only a small sachet. Change it at each service interval, and use an indicator sachet so you can see when it is spent.
Is a small drain or breather hole acceptable in an IP67 box?
A true drain hole means the box is no longer sealed and the rating no longer applies as tested. Some designs deliberately run rain-proof rather than sealed, with a small downward-facing opening to equalize pressure and let water escape. That can work for simple, tolerant electronics, but it is a deliberate re-specification, not a fault fix, and dust and insects come in with the air.
