Except for a residential installation, if a fixture opening is more than ______ feet from the main soil or waste system, the vent shall be continued full size.

In plumbing systems, the distance from a fixture to the main soil or waste system is critical for ensuring adequate drainage and preventing siphoning of traps, which can lead to plumbing problems like sewer gas entering the home. The regulation states that if a fixture opening is positioned beyond 25 feet from the main soil or waste system, the vent line must be maintained at full size throughout this distance.

Maintaining the vent pipe at full size helps ensure that there is sufficient air flow to equalize pressure within the drainage system, thus preventing the traps from being drawn dry and allowing the proper functioning of the drainage system. If the vent were to reduce in size, it might not handle the necessary air flow effectively, potentially leading to issues with drainage and venting in the system.

The other distances provided — 15, 20, and 30 feet — do not align with the specific requirement that dictates maintaining the vent size beyond 25 feet. Thus, focusing on the requirement of 25 feet as the threshold to continue the vent line full size accurately reflects the plumbing codes applicable in this context.