National Electrical Installation Standards

Standards as High as Your Own

Friday, October 6, 2017


Re: CQD answer published Friday, September 29, 2017 -Dry Location

Regarding the use of a dry transformer inside a weatherproof in-use cover, I do not find a section prohibiting this. Section 300.38 defines the interior of the conduit as wet, and 300.50(B) defines an underground box as also wet, however, I do not know of a section defining the interior of an exterior above-grade box as wet, much less the space inside the weatherproof in-use cover.

The whole purpose of the special cover is to prohibit the entry of water into the space where the plug is inserted.

Scott Cline



Hey Scott thanks for your comment it's good to hear from you. You are correct there is not a specific rule that prohibits plug in transformers from being installed outdoors under a weatherproof cover. Section 300.38 is in Part II of Article 300 so 300.9 would be more likely applicable to the voltage of a plug in transformer but those sections are for above grade raceways in wet locations.  The plug in transformer would not be inside a box it would be inside a weatherproof cover. Notice the difference between the definitions of "weatherproof" and "water tight" in article 100 - weatherproof does not include preventing the entrance of water. Also as stated in the definition of "outlet box hood" in 406.2 "it reduces the risk of water coming in contact with electrical components".

The original question stated that the plug in transformer was identified for dry location use only and the space inside the weatherproof cover located outdoors could be considered a damp location. As previously mentioned it is somewhat of a judgment call and suggested contacting the manufacturer.

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