What is a key requirement when installing a 240/120-V three-wire feeder underground to a detached garage?

When installing a 240/120-V three-wire feeder underground to a detached garage, one of the key requirements is to include an equipment grounding conductor. This is essential for safety reasons, as the equipment grounding conductor provides a path for fault current to flow back to the source, which helps to prevent electrical shock and ensures that overcurrent protective devices (like circuit breakers) will operate properly in the event of a fault.

Grounding is crucial in any electrical system, and when it comes to installations in detached structures, it promotes the safety and stability of electrical installations. An equipment grounding conductor helps mitigate the risks of electrical faults by helping establish a grounding point for the electrical system, thus enhancing overall safety for anyone using the electrical appliances or systems within the garage.

As for the other options, while certain conditions might apply to underground installations, they do not encompass the fundamental safety aspect represented by having an equipment grounding conductor. This grounding element is pivotal and often specifically mandated by electrical codes and standards for such installations.