Article 100 of the NEC defines the terms “labeled” and “listed,” which are both related with product evaluation.
Labeled or listed indicates the piece of electrical equipment or material has been tested and evaluated for the purpose for which it is intended to be used. Products that are big enough to carry a label are usually labeled. The smaller products are usually listed. Any modification of a piece of electrical equipment in the field may void the label or listing.
In accordance with OSHA Safety Standards, a Nationally Recognized Testing Laboratory (NRTL) must test electrical products for conformity to national codes and standards before they can be listed or labeled. The biggest and best-known testing laboratory is the Underwriters’ Laboratories, identified with the UL logo. The purpose of the Underwriters’ Laboratories is to establish, maintain, and operate laboratories for the investigation of materials, devices, products, equipment, construction, methods, and systems with regard to hazards affecting life and property.
Sources: Electric Motors and Control Systems-Frank D. Petruzella
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