Which of the following is classified as a fitting in a wiring system?

The classification of fittings within a wiring system is essential for understanding how different components interact within that system. A locknut is designed to secure electrical conduit connections or fixtures, preventing them from loosening due to vibration or other mechanical stresses. This function is critical in maintaining the integrity of the electrical system and ensuring safety. By securing connections, locknuts help to keep components aligned and can facilitate proper grounding, which is essential for minimizing the risk of electrical faults.

In contrast, other components listed serve different primary functions. While a wire connector connects wires to each other, it is typically categorized as a connector rather than a fitting. An electrical box houses and protects electrical connections but is not primarily a fitting itself. Similarly, a circuit breaker is a protective device designed to interrupt current flow in case of an overload or short circuit, rather than serving as a fitting within the system.