What is the minimum permissible size for the neutral of solidly grounded systems compared to the ungrounded conductor supplied by the system?

In grounded electrical systems, the neutral conductor plays a crucial role as it carries the return current and ensures proper operation of protective devices. The minimum permissible size for the neutral conductor in a solidly grounded system in relation to the ungrounded conductor is guided by safety standards and design considerations.

The choice indicating 15% reflects the principle that the neutral must be adequately sized to handle the unbalanced loads effectively. When considering that ungrounded conductors may see varying loads due to single-phase usages or other imbalances, a neutral sized at 15% of the ungrounded conductor's size ensures that it can safely carry return current without overheating or risking failure.

In practical terms, size ratios help maintain system stability and mitigate the risks of faults or overload conditions. A neutral that is undersized could lead to excessive voltage drops and inefficient system performance, or potentially hazardous conditions, while also ensuring compliance with the electrical code or standards governing the installation.

The other choices represent higher percentages, which would not be economically sensible or necessary under typical circumstances. A neutral exceeding the required size could lead to increased costs and unnecessary material usage without providing additional safety benefits in most standard applications.