Condensation
in electrical enclosures
Updated:
Why water forms in an electrical enclosure, the temperature at which it becomes critical and which measures reliably prevent condensation.
The dew point: why water forms in the enclosure
Air contains water vapour. How much it can hold depends on the temperature: warm air carries more moisture than cold air. If the temperature drops to the dew point, the air is saturated — every further degree causes water to condense on cold surfaces: on doors, roof, mounting plate and, unfortunately, also on terminals and circuit boards.
| Air temperature | Relative humidity | Dew point |
|---|---|---|
| 15 °C | 90 % | 13.4 °C |
| 20 °C | 50 % | 9.3 °C |
| 20 °C | 70 % | 14.4 °C |
| 25 °C | 60 % | 16.7 °C |
| 30 °C | 70 % | 23.9 °C |
| 35 °C | 80 % | 31.0 °C |
Calculated using the Magnus formula. In humid air, the dew point is only a few degrees below the air temperature — even a small drop in temperature is enough for condensation.
The four most common causes
- Day/night temperature changes: during the day the air in the enclosure absorbs moisture; at night walls and roof cool down — especially with enclosures outdoors.
- Cooling unit set too cold: if the setpoint is below the dew point of the enclosure air, water forms on the evaporator and on cold surfaces.
- Humid air from below: open cable entries draw humid air from cable ducts and the ground into the enclosure.
- Standstill: if the equipment is not running, its waste heat is missing — the enclosure cools down to ambient temperature.
What really helps
- Heater with thermostat or hygrostat: it keeps the internal temperature a few degrees above the dew point. Sizing and units are shown on the enclosure heaters page.
- Seal cable entries: cable glands, seals and closed floor plates keep moisture from the cable duct out.
- Check the cooling unit setpoint: as cool as necessary, not as cool as possible. Good cooling units evaporate their condensate themselves — with the Cosmotec Protherm units from CVE11 upwards.
- Closed-loop cooling instead of outside air: where the ambient air is humid, an air/air heat exchanger is better than a filter fan, because no outside air enters the enclosure.
- Double wall outdoors: the air layer of a double-walled outdoor enclosure dampens temperature changes and so reduces the risk of condensation (→ single-walled or double-walled?).
At ZPAS: the outdoor enclosures of the SZD series are double-walled as standard. We fit heaters, thermostats or hygrostats and cooling at the factory — sized for the site and heat load.
Frequently asked questions about condensation
Why does condensation form in an electrical enclosure?
If the air in the enclosure cools below its dew point, the moisture it contains condenses on the coldest surfaces. Typical triggers are sudden drops in temperature at night, cooling units with a setpoint that is too low, and humid air from cable ducts.
What helps against condensation in an electrical enclosure?
A heater with thermostat or hygrostat keeps the internal temperature above the dew point. In addition: sealed cable entries, a cooling unit that is not set too low and — outdoors — a double wall that dampens temperature changes.
Thermostat or hygrostat?
A thermostat switches according to temperature, a hygrostat according to relative humidity. Against condensation, the hygrostat is the more direct solution because it heats exactly when humidity rises. Details on the enclosure heaters page.
Condensation in your enclosure?
Describe the installation site and equipment — we will recommend heating, sealing and cooling that work together.
Request a quote Calculate heating and cooling capacity