The neutral of solidly grounded systems is generally not permitted to be smaller than what percentage of the ungrounded conductor supplied by the system?

In solidly grounded systems, the requirement regarding the size of the neutral conductor is based on ensuring that it can safely carry any unbalanced load. The correct answer, which specifies that the neutral should generally not be smaller than one-third (33 1/3%) of the size of the ungrounded conductors, reflects this principle.

This percentage is established to maintain the balance in the system, especially in three-phase scenarios where load imbalances can occur. A neutral conductor that is too small might not be able to handle the return current during these unbalanced conditions, potentially leading to overheating and greater risk of failure. Thus, specifying a minimum size helps to protect the integrity and safety of the electrical system.

In contrast, the other options represent sizes that are either inadequate or too conservative for standard grounding practices. They do not align with the established industry standards, which prioritize safety and functionality in electrical system design. Therefore, the requirement for the neutral to be no smaller than 33 1/3% of the supplying ungrounded conductor ensures proper operation and reduces the risk of electrical hazards.