What criteria must be met for fittings used in wiring systems according to NEC Article 300?

Fittings used in wiring systems must provide adequate protection against stress to ensure the safety and reliability of the electrical installation. This requirement is important because electrical systems can be subjected to various mechanical stresses, including tension, compression, and twisting forces, especially in areas where cables may experience movement or vibration.

Adequate protection means that the fittings are designed and constructed to handle these stresses without failure, thereby reducing the risk of damage to the conductors they support and maintaining the integrity of the electrical system. If fittings do not safeguard against mechanical stress, it could lead to insulation damage, cable failure, or even electrical hazards such as short circuits.

The other options present requirements that do not encompass the essential protective aspect against stress. While standardized fittings may be useful in maintaining consistency and safety, they do not inherently ensure protection against physical stressors. Similarly, materials like aluminum have specific applications but are not universally required for all fittings. Lastly, allowing fittings to be made from any material could compromise the safety and performance of the wiring system, as not all materials would adequately protect against the various stresses encountered in electrical installations.