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During O&M we often find combiner boxes and disconnect switches with water inside them, despite the box being NEMA 4 and fully gasketed, no top or side penetrations, and weather tight connectors properly installed.
Where is the water coming from?
The water enters the enclosure as moisture in the air, which later condenses into water. During the day, the bus bars heat up and warm the air in the enclosure. When air warms, the amount of moisture it can hold increases. At night, the air in the conduit or enclosure cools, and the water vapor in the air condenses. Each daily cycle only generates a tiny drop of water, but after several months this begins to add up.
Water may only travel downwards due to gravity, but moist warm air can enter and travel through a conduit and enter the enclosure from any direction. This is how water enters an enclosure even when there is no apparent way for water to run down and into the cabinet.
The solution is to seal the conduits where they enter the enclosure to prevent fresh air from getting inside the box. Traditional duct seal is effective and readily available. Expanding foam sealants area also available and very effective, however they must be approved for direct contact with the electrical cables.
Here are two examples of enclosures with only bottom entry conduits, but ground water entered the underground conduit, evaporated up the conduit into the enclosure, then condensed.
