Rule 32 of 40 · Chapter V — Water, Weather, and Place
Seal and slope against water
Why this rule exists
Water finds the smallest opening and follows gravity, so the failure mode outdoors is almost always water getting in and pooling where it should not. Conduit runs act like pipes, carrying condensation and rain to the lowest fitting; enclosures without drainage collect it; upward penetrations funnel it to terminals. Left alone, trapped water corrodes connections, fills boxes, and eventually faults. Good installation anticipates this: seal what should stay dry, and where water will enter, give it a low point to leave. You are not keeping water out entirely; you are controlling where it goes.
In practice
Slope conduit so it drains away from enclosures, and add listed drain fittings at low points. Use weatherproof, gasketed connectors and seal cable and conduit entries, preferring entry from the bottom or side over the top. Install drip loops so water runs off the cable before it reaches the connector. Use listed sealant or duct seal on conduit that transitions between temperature zones to stop condensation. Where an enclosure can collect water, provide a weep hole at its lowest point. Check and re-seal these details during maintenance.
When it doesn't apply
Weep and drain holes belong only at the lowest point and must be listed to keep insects and water out; a random drilled hole can defeat the enclosure rating. Some sealed enclosures should not be drilled at all. In classified locations, sealing fittings serve a different, mandatory explosion-containment purpose; follow those rules exactly.