The short answer
- Water condenses on any surface colder than the dew point of the air touching it. Air at 30 °C and 80% relative humidity has a dew point of about 26 °C.
- No enclosure rating covers it: NEMA 250 and IEC 60529 both leave condensation inside the enclosure out of scope.
- Sealed enclosures breathe: cooling pulls the inside below outside pressure and draws moist air in through any small leak path.
- Fixes: a thermostat-controlled heater to keep the walls warmer than the dew point, a vent drain rated to the enclosure type, desiccant for boxes that stay shut, and a sealed, closed enclosure when an air conditioner runs.
Why a sealed enclosure gets wet inside
Air holds water vapour, and the amount it can hold falls as it cools. Cool humid air far enough and it reaches its dew point; below that, water comes out as droplets on the coldest surfaces. In an enclosure those are the walls and the door, and the water then drips onto whatever is below.
A gasketed enclosure keeps rain and hose water out, but it also keeps in the moisture that was in the air when the door was closed, plus whatever gets in later. The standards say so plainly. The scope of NEMA 250 excludes protection of the enclosed equipment against conditions such as condensation that occur within the enclosure. IEC 60529 excludes moisture produced by condensation, and its water tests note that dew forming inside during the test is not to be mistaken for water ingress. A NEMA 4X or IP66 rating is therefore not a promise of a dry interior.
Dew point: the number that matters
The US National Weather Service publishes the formulas behind its weather calculator. The saturation vapour pressure is es = 6.11 x 10^(7.5T / (237.3 + T)) hPa, with T in °C; multiplying by the relative humidity gives the actual vapour pressure, and solving the same equation backwards gives the dew point. Values from those formulas, at sea-level pressure:
| Air | Relative humidity | Dew point |
|---|---|---|
| 20 °C (68 °F) | 50% | 9.3 °C (48.7 °F) |
| 25 °C (77 °F) | 60% | 16.7 °C (62.1 °F) |
| 30 °C (86 °F) | 60% | 21.4 °C (70.5 °F) |
| 30 °C (86 °F) | 80% | 26.2 °C (79.1 °F) |
| 35 °C (95 °F) | 70% | 28.7 °C (83.7 °F) |
| 40 °C (104 °F) | 90% | 38.0 °C (100.5 °F) |
Read it this way: if an enclosure is closed on a humid 30 °C afternoon at 80% relative humidity, any surface inside that drops below about 26 °C will collect water. A clear night outdoors, a cold-water rinse or an air-conditioned room will do that easily. At the last row's conditions almost any cooler surface condenses.
Breathing: temperature cycles pump moist air in
The air trapped in one enclosure holds little water. Using the NWS mixing-ratio formula, air at 30 °C and 80% carries about 21.6 g of water per kg of dry air; saturated air at 20 °C holds about 14.7 g/kg. For the inside volume of a 24 x 20 x 8 in enclosure (0.607 x 0.505 x 0.200 m = 0.061 m³) at an air density of 1.2 kg/m³, cooling from the first state to the second releases 0.061 x 1.2 x 6.9 = about 0.5 g of water. That is a few drops.
The trouble is repetition. IEC 60529 recognises enclosures in which "the normal working cycle of the equipment causes reductions in air pressure within the enclosure below that of the surrounding air, for example, due to thermal cycling effects", and its water tests require a pressure balance if the water is more than 5 K colder than the enclosure. Cooling from 30 °C to 20 °C shrinks the trapped air by about 3.3% (293 K / 303 K), so each cycle can pull outside air in through a worn gasket, a loose gland or a conduit. One drain maker describes these temperature-induced vacuums as pulling water and moisture into the enclosure. Each charge of humid air leaves its water behind, and over weeks the drops become a puddle.
Conduit makes it worse: NEMA 250's scope also excludes conditions that enter via conduit or unsealed openings, and a conduit run from a warmer or wetter space carries moist air straight in. Seal conduit entries and use glands that match the enclosure rating; see cutouts and glands.
Heaters: keep the walls above the dew point
The most direct fix is to keep the inside surfaces warmer than the dew point of the air inside. An enclosure heater does that, and one enclosure maker's heater guide notes a less obvious case for one: a system can run all day, then cool quickly when it shuts down at night, and that drop can cause condensation and corrosion.
Two published sizing methods use the same relation as heat dissipation:
Heater output = ΔT x k x A - heat from components
ΔT = how far above ambient you want to hold the enclosure (K); k = heat transfer coefficient (about 3.7 W/m²K for stainless, 5.5 W/m²K for painted steel); A = surface area (m²). For an enclosure outdoors in moving air, both methods double the ΔT x k x A term.
Worked example. The same 24 x 20 x 8 in stainless enclosure on a wall exposes 0.764 m². For this example we choose to hold it 5 K above ambient, and we assume the equipment inside is switched off at night, so it contributes no heat:
- Indoors: 5 x 3.7 x 0.764 = 14.1 W.
- Outdoors, doubled: 2 x 14.1 = 28.3 W.
- Holding 10 K above ambient outdoors would need 2 x 10 x 3.7 x 0.764 = 56.5 W.
When the equipment runs, subtract its heat. Pick a heater above the result, and control it with a thermostat so it does not add heat on hot days; the heat calculation guide shows how quickly a sealed box overheats.
Where to mount it: low. A climate-control catalogue's airflow study found that the areas beneath a heater are only minimally warmed and called mounting the heater in the base area of the enclosure essential for an even temperature. Keep it clear of cables and of anything heat-sensitive.
Drains and breathers rated to the enclosure type
A vent drain mounted in the bottom of the enclosure lets condensed water run out by gravity and equalizes pressure, which reduces the vacuum that pulls moist air in. Listed vent drains exist with Type 4 and 4X ratings, stated to maintain the rating when properly installed on a listed Type 4 or 4X enclosure. On a manufacturer-rated enclosure such as ours, the rating with a drain fitted is the maker's own assessment.
- Put it at the lowest point, where the water collects.
- Check the drain's own rating. NEMA's rule is that a field-installed assembly's type rating is limited by the least-rated component, and fittings must be tested to a type rating equal to or better than the enclosure's.
- An open hole is not a drain on a 4X enclosure: it lets hose water in, and the enclosure no longer meets Type 4X.
Water that gets in has to get out, but a drain does not remove the moisture source. Combine it with good gaskets, sealed entries and, where needed, a heater.
Desiccant for enclosures that stay shut
Desiccant absorbs water vapour until it is saturated, so it suits enclosures that are rarely opened. Its capacity goes quickly in open air. One instrument maker's application note cites desiccant-maker testing at 60% relative humidity and 30 °C in which about 25% of a bag's capacity was used up after one hour of exposure, and it tells users to install new desiccant each time the housing is opened and the electronics exposed. The same applies to an enclosure: keep spare packs sealed, replace them whenever the door has been open for long, and do not expect desiccant to cope with a box that breathes through a leak.
Air conditioners and washdown: two special cases
Enclosure air conditioners remove water from the inside air as a side effect: it condenses on the cold evaporator coil and must be carried out of the enclosure. A climate-control catalogue works an example: with the door closed, dehumidifying a 0.6 m³ enclosure releases 0.6 x 1.2 x 11 = 7.92 g, about 8 ml. With poorly sealed entries or damaged gaskets the air keeps being replaced, and at a leakage of 5 m³/h up to 80 ml of condensate can form every hour. Its advice: run cooling units only with the door closed, seal the enclosure on all sides, fit a door switch, set the inside temperature only as low as needed, and do not point cold air directly at electronic components.
Washdown creates both conditions at once. A warm enclosure hit with cold rinse water cools fast and draws in air that is close to saturated. Where possible, rinse with water near the enclosure's temperature, keep gaskets and glands in good condition, and fit a rated vent drain. See washdown enclosures for food and beverage, and for outdoor sites, outdoor electrical enclosures.
Custom builds. NEMA4 Enclosure builds custom enclosures in 304 or 316L stainless, powder-coated carbon steel, aluminum, polycarbonate and fiberglass (the non-metallics in standard molded sizes cut to your layout), from one piece. Tell us the drain, breather, heater wiring entry or gland you will fit and where, and we cut the openings for it; the assembled rating is limited by the rating of each fitting. Our wall-mount enclosures in 304 and 316L stainless are manufacturer-rated NEMA 4X-type / IP66 - not UL listed.
Frequently asked questions
Why is there water in my NEMA 4X enclosure?
Most often it is condensation. Humid air inside the enclosure condenses on walls that cool below its dew point, and temperature cycles draw more moist air in through gaskets, glands and conduit. A rating covers water from outside, not condensation inside.
How do I stop condensation in an electrical enclosure?
Keep the inside surfaces above the dew point with a thermostat-controlled heater, let water out with a vent drain rated to the enclosure type, seal conduit and cable entries, and use desiccant in enclosures that are rarely opened.
What size heater do I need to prevent condensation?
Multiply the temperature rise you want above ambient by the heat transfer coefficient and the surface area, double it outdoors, and subtract heat from equipment that is running. Holding a 0.764 m² stainless enclosure 5 K above ambient outdoors takes about 28 W.
Does a breather or drain void the NEMA 4X rating?
A drain rated Type 4X and installed as its maker instructs is designed not to; on a listed enclosure its maker states that the rating is kept, and on a manufacturer-rated enclosure the enclosure maker assesses it. An unrated drain or an open hole limits the whole enclosure to the rating of that opening.
Can an enclosure air conditioner cause condensation?
It removes water from the air on its cold coil, and that water must be drained out. If the door is open or the seals leak, fresh humid air keeps coming in and the condensate keeps forming, so run it only with the enclosure closed and sealed.
Sources
- US National Weather Service (El Paso), "Vapor Pressure" formula sheet
- US National Weather Service (El Paso), "Wet-bulb Temperature and Dewpoint Temperature from Air Temperature and Relative Humidity" (dew point formula)
- US National Weather Service (El Paso), "Mixing Ratio" formula sheet
- ANSI/NEMA 250-2020, contents and scope (condensation within the enclosure, conduit and unsealed openings not covered)
- IEC 60529 Ed. 2.2 (scope; Category 1 enclosures and thermal cycling; water temperature within 5 K and pressure balance; dew during tests)
- NEMA, "NEMA FAQs: Enclosures" (Q15, fittings and the weakest-link rule)
- Hammond Manufacturing, heating products selection (night-time condensation, heater formula, outdoor factor)
- Rittal, Technical System Catalogue Climate Control, enclosure heaters (output formula, exterior siting, heater in the base area)
- Rittal, cooling units (condensate calculation, leakage example, door closed, door switch)
- Rittal, cooling units selection (cold air not pointed at components, to prevent condensation)
- nVent HOFFMAN, vent drains product sheet (gravity drain, pressure equalizing, temperature-induced vacuums, Type 4/4X)
- Sea-Bird Electronics, Application Note 71, "Desiccant Use and Regeneration" (Jan 2014)