Walk up to an outdoor cabinet on a spring morning and you can sometimes hear the problem before you see it. The inside of the door is beaded with water, and the base holds a centimetre of it along with a rust line that was not there in October. The panel passed its ingress test, the gaskets are new, and nobody can find the leak, because there isn't one. The water arrived as vapour riding on air and turned into liquid overnight, when the metal got colder than the air around it could tolerate. Get that sequence wrong and you will keep buying heaters, vents and gaskets for years without ever drying the box.
Before anyone reaches for a heater, three things about the site matter more than the drawing: how big the daily swing is, whether the cabinet gets hosed down or only rained on, and who opens it to check the drain and gasket.
Where the water comes from, and what you notice first
| Driving condition | What happens inside | What you see in the field | Usual control |
|---|---|---|---|
| Day-to-night temperature swing | Air cools past its dew point and lets go of the vapour | Beads on the panel, a rust line low down | Heater switched by a hygrostat |
| Warm humid air pulled in as it cools | Vapour gets in, then condenses on cold metal | Corrosion near entries, fur on plated parts | Membrane vent, sealed entries |
| Hose-down or pressure washing | Water is driven past gaskets that shrug off rain | Standing water, damp terminal rail | IP-rated entries, low-point drainage |
| Sun on one face | The lit wall warms, the shaded wall stays cool | Water on the cool door or side wall | Sun shield or insulation |
| Load cycling | Equipment heats the air by day, sits idle at night | Failures that bunch up in spring and autumn | Heater run-on below the duty cycle |
| Coastal or tropical site | High absolute humidity all year round | Bloom on fasteners, tracking, blisters | Stainless fasteners, shorter inspections |
A Sealed Box Is Not a Dry Box
An ingress rating tells you how a cabinet behaves when water attacks it from outside, under the conditions in the test. It says nothing about water that forms on the inside. We have seen IP66 stainless cabinets with puddles in them, and cheap painted boxes that stayed dry for a decade. The difference was never the seal. Air moves: it squeezes out when the sun heats the cabinet and gets pulled back in at night, and every gland plate, gasket land, breather and conduit run is part of that path. Vapour is a gas, and a gas is far harder to keep outside than a droplet — which is why condensation in electrical enclosures turns up in cabinets that were built and tested properly.
The Temperature Swing Is the Pump
Think of the cabinet as a slow bellows. Warm it and it pushes a little air out. Let it cool and it sucks fresh air back in through whatever openings exist. Whatever humidity was in that outside air is now inside the cabinet, and the surfaces that cool first collect the water: the door skin, the base, the face of any component that is not generating heat. Sun on one wall makes it worse, because the lit side stays warm while the shaded side drops away. Near the coast that film carries salt, which is why corrosion shows up low down long before anything feels wet.
Every Cable Entry Is a Doorway for Vapour
A gland plate that passes a hose test can still pass vapour all day. Water droplets are enormous next to water molecules, so a gasket that stops a jet of water is not a vapour barrier. Compression matters more than thickness. A gasket squeezed flat, or a plate with more M20 holes than the drawing called for, gives the air an easier round trip — which is why a rework that only changed the cable layout can bring on a moisture problem that was not there before. Our ingress testing walkthrough covers how that boundary is checked before shipment.
How Much Water Are We Actually Talking About?
Numbers help, because "a bit of condensation" sounds harmless. Take a 1000 x 800 x 300 mm cabinet. Once the mounting panel, ducts and components take their share, you have roughly 0.24 m³ of free air. Air at 30 °C and 70 % relative humidity holds about 21 g of water per cubic metre; cool it to 15 °C overnight and it can hold about 13 g. The difference is roughly two grams of liquid per cycle — a thin film across a terminal block, enough to lift a label. Run that thirty nights and you have explained most of the corrosion in a cabinet that never leaked: the practical scale of condensation in electrical enclosures.
What Condensation Looks Like Before It Becomes a Failure
You rarely get to watch condensation in electrical enclosures happen. You find the evidence afterwards. A rust line or white bloom at the lowest point. Water staining on the panel under a component. A window that clears once the cabinet warms up. Green fur on plated parts. A gasket that has gone shiny and hard instead of staying springy. Two or three on one cabinet is enough to open a moisture review.
Three Control Strategies, and How They Combine
You can keep moisture out, let the cabinet breathe along a path you control, or keep the surfaces warm enough that the vapour never condenses. Most good designs do two of the three. Heating on its own is common and often oversized, and sealing on its own is the weakest option, because the pressure difference will find a way in eventually.
Matching the control to the site
Anti-condensation heaters and hygrostats
A heater does not dry anything. It lifts the surfaces a few degrees above the dew point so the vapour stays vapour a while longer, which is why the switching threshold matters more than the wattage. Size from exposed surface area and the overnight swing rather than cabinet volume, and switch on a hygrostat where site humidity varies. Our heater sizing walkthrough runs through the calculation.
Pressure-compensating breather vents
A membrane vent equalises pressure while keeping liquid water and most dust out, so air moves through a filtered path instead of the gasket land. Where you put it decides how well it works. Near the ground it breathes damp air; facing the prevailing rain it clogs inside a season. Mount it on the shaded side, above anything standing water could reach.
Drainage, low points and drain plugs
If water is going to form anyway, give it somewhere to go that is not the terminal rail. Slope the base or fit a formed channel, put a plug at the low point, and keep the drain clear of the cable route so a blocked outlet cannot fill the wiring. On a cabinet that cycles every night, a drainage channel design with a threaded plug beats another seal.
Desiccant, insulation and shade as supporting measures
Desiccant buys time in a small, well-sealed volume and stops working once saturated, so treat it as a commissioning aid. Insulation and a sun shield go after the cause and cut the solar gain that drives the swing. On a cabinet already bolted to a wall, a sun shield is often the cheapest change that helps.
Sizing a moisture strategy in five steps
- Write down the site: climate, sun exposure, hose-down practice, and how often the door is opened.
- Measure the gap between the warmest internal air and the coldest metal seen overnight.
- Pick the main control: heater, controlled venting, or drainage.
- Keep it as small as the dew point allows. An oversized heater cooks gaskets.
- Put the upkeep in the schedule: drain, vent and gasket checked on a fixed interval.
The pumping station that flooded itself every spring
A municipal pumping station near the coast replaced four outdoor cabinets with stainless-steel units at a higher ingress rating after two seasons of corrosion complaints. Within a year the new cabinets were doing the same thing: water in the base, rust on the mounting panel, a failed supply module every spring. They faced west, the load was intermittent and the door was opened most weeks, so the interior swung hard between warm afternoons and cold nights. Nobody wanted to hear that the IP rating was not the problem. What fixed it was a membrane vent on the shaded side, a small heater on a hygrostat, drainage at the low point, and a spring inspection the crew actually carried out.
Moisture questions buyers ask before specifying an outdoor enclosure
Does a higher IP rating stop condensation?
No. The rating covers water arriving from outside. Condensation is water that was already in the air inside the cabinet.
Why is the inside of my enclosure wet when nothing leaks?
Warm air carries vapour in as the cabinet breathes, and that vapour turns back into liquid once the surfaces cool below the dew point.
Should I seal the enclosure completely or vent it?
Vent it, but vent it deliberately. A membrane gives the pressure somewhere filtered to go, while a sealed volume still breathes through its joints.
How do I size an anti-condensation heater?
Start with the exposed surface area and the overnight swing, not the cabinet volume. Then check the duty cycle.
Where should a breather vent be fitted?
On a shaded face, above the highest level standing water could reach, and out of the prevailing rain.
Is a desiccant bag enough?
In a small, well-sealed volume it buys a few months. Once saturated it simply stops, so pair it with a control that keeps working.
Will a heater keep the inside dry in a tropical climate?
It helps, but with high absolute humidity the dew point sits close to ambient all year, so venting and drainage do more of the work.
How often should gaskets be replaced on an outdoor enclosure?
Replace on condition, not on a calendar. A gasket that has gone hard passes vapour even when it still looks intact.
Can condensation damage electronics before water is visible?
Yes, and that is the usual order of events. A thin film on a terminal block starts corrosion long before water runs out of the base.
Does condensation in electrical enclosures always mean a design fault?
Not always. A sound enclosure still condenses when the site swing is bigger than the specification assumed, so the control strategy is the usual suspect.
The framework behind this is not ours to invent. The IEC 60529 degrees of protection standard sets out how the ingress tests behind a cabinet rating are run, the US National Weather Service dew point explainer covers the air property that drives the cycle, and UK HSE guidance on electrical safety explains why water inside equipment counts as a hazard. None of them approves a particular enclosure, and the site conditions you describe still decide the control.
The Jewein decision boundary
Water in a sealed cabinet is a predictable outcome of air volume, humidity and a daily temperature swing. It is not proof that someone fitted the wrong gasket, and new gaskets will not fix it. Describe the site properly, choose the control that matches the swing, size it from the surface the water forms on, and put the upkeep where it will actually be done. For a second opinion on condensation in electrical enclosures, Jewein can review a marked-up drawing against a site description and confirm the heater, vent and drainage provisions — send it over.
