In article 100 of the 2005 National Electrical Code (NEC) the definition of rainproof and rain tight sound very similar. However in the NEC they have different applications on how they are applied.
The definition of rainproof is; constructed, protected or treated so as to prevent rain from interfering with the successful operation of the apparatus or result in wetting of live parts and wiring within the enclosure under specified test conditions. A rainproof device may be used outside in rainy conditions. A rainproof device or apparatus may allow rain water to enter the enclosure, but such rain will not interfere with its operation. Rainproof may also be used indoors where dripping water is a problem.
The definition of the term rain tight in the 2005 NEC states; constructed or protected so that exposure to a beating rain will not result in the entrance of water into the enclosure under specified test conditions. Rain tight devices or enclosures are designed to keep the rain out of the enclosure.
The only similarity these terms may have is in the scope of the work that is at hand. In most cases the only thing in common is that they are outside and under constant bombardment to the elements such as rain, sleet and snow.
In Article 430, table 430.91 in the NEC for specification on enclosures the term rainproof is typically used in conjunction with enclosure type 3R, 3RK. The term rain tight is typically used in conjunction with enclosure types 3.3s, 3.sx, 3x, 4, 4x, 6, 6P.
Types of rainproof devices used are conduit bodies such as LB’s, LR’s and LL’s with gaskets and covers in residential as well as commercial and industrial. I have installed many electrical enclosures of type 3 outdoor enclosures in the industrial end at motor control boxes. In cement plants (LaFarge) and concrete mixing facilities (Cadman). Most of these places are outside and are in constant exposure to the elements.
Rain tight enclosures are frequent in both residential...