Hello Cristian, Section 352.6 provides guidance on this topic, requiring PVC conduit, factory elbows and associated fittings to be listed. Article 100 defines Listed as: Equipment, materials, or services included in a list published by an organization that is acceptable to the authority having jurisdiction and concerned with evaluation of products or services, that maintains periodic inspection of production of listed equipment or materials or periodic evaluation of services, and whose listing states that either the equipment, material, or service meets appropriate designated standards or has been tested and found suitable for a specified purpose.
NEMA TC-6 & 8 is a NEMA published Standard on Polyvinyl Chloride (PVC) Plastic Utilities Duct for Underground Installations.
This Standard provide product design specifications for PVC but is not a product listing. The listing is generally provided by a Nationally Recognized Testing Lab (NRTL) These testing labs may use standards like NEMA TC-6 & 8 in determining product design suitability, but each testing lab will provide additional marking on the product to prove it has been evaluated and listed. A complete and current list of NRTL’s registered with OSHA, can be found here along with their listing mark.