Which classification is defined as an accessory that performs a mechanical function in a wiring system?

The classification that is defined as an accessory performing a mechanical function in a wiring system is a fitting. Fittings are critical components in electrical installations as they help hold, secure, and connect various elements of the wiring system. They are used to guide wires, connect different sections, or manage transitions between different types of conduits or wiring methods, thereby playing a vital role in ensuring the integrity and organization of the electrical system.

Connectors, although important, primarily serve to join two or more electrical circuits and do not typically function as mechanical support. Support components are generally designed to hold or support conduits or cables but do not interact directly with the wiring connection itself. Insulators serve to provide electrical isolation and protect from current leakage rather than perform a mechanical function in the setup. Thus, fittings are the most appropriate answer for this question as they directly relate to the mechanical support and management of wiring systems.