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The Reliance Controls, PB30 power inlet box is rated as a NEMA 3R enclosure and will be suitable as an outdoor connection.
The NEC standard you are referring to covers outdoor, �live� receptacles which is why you will notice that many outdoor receptacles feature gasket material. In the case of a power inlet box, the device itself is not considered live until hooked up to a generator and the transfer mechanism switched. Under any other circumstances, the outlet would not be live as there should be no closed circuits leading to it.
For an added degree of protection, we could recommend one of the "rain tight" power inlet boxes. These will feature the connection point on the bottom of the enclosure to further minimize the risk of water infiltration.
Our recommendation would be the PBN30.
30 Amp Power Inlet Boxes