Rule 30 of 40 · Chapter V — Water, Weather, and Place
Use the right enclosure rating outdoors
Why this rule exists
An enclosure protects what is inside only to the degree it is rated. Outdoor and washdown environments throw rain, wind-driven water, dust, and corrosion at equipment, and a box built for a clean dry interior lets all of it reach the live parts. NEMA enclosure types, and the IP system, specify what a box keeps out: spray, hose-down, dust, submersion. Choose one rated below the actual exposure and moisture reaches the terminals, corrodes them, and faults. The enclosure is part of the safety system, not just a cover. Match its rating to the weather.
In practice
Select enclosures by exposure: a NEMA 3R for rain and outdoor general use, 4 or 4X for wind-driven water and washdown, 4X or higher for corrosive coastal and chemical settings. Use gasketed covers, and keep them closed and latched. Bring conduit in from below or use listed fittings and drip loops so water cannot run down into the box. Use listed weatherproof connectors and seal unused openings. Verify gaskets are intact on any enclosure you open, and do not drill new holes that defeat the rating.
When it doesn't apply
Enclosure ratings assume correct installation; a NEMA 4X box with an open knockout or a cracked gasket is no better than an indoor box. Some equipment must breathe and uses vented-but-shielded designs, so do not seal those. In hazardous (classified) locations, enclosure and wiring methods escalate to explosion-proof requirements; that is specialized work, do not approximate it.