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Many National Electric Code requirements refer to “listed” or “labeled” devices. This means that products or components must meet the design, manufacture, test or inspection, and marking requirements of the listing agency.
In the United States, OSHA uses the term “Nationally Recognized Testing Laboratory” (NRTL) to refer to private sector organizations that perform product certification.
The most prominent NRTL is Underwriters Laboratories, often abbreviated as “UL”.
OSHA currently recognizes 18 NRTLs, 10 of which list products under the UL48 Standard for Electric Signs:
