When can the bottom clearance of a vertical outside oil tank be reduced and what is the minimum clearance when reduced?

The correct answer highlights the specific condition under which the bottom clearance of a vertical outside oil tank can be reduced. When the oil tank is installed in a listed secondary containment system, it is permitted to have a bottom clearance reduced to a minimum of 1 inch.

This provision is in place because a secondary containment system is designed to prevent leaks and spills from the tank, thus minimizing the risk of environmental contamination. The requirement for reduced clearance acknowledges that, even with a containment system, there's still a need for some minimal clearance for safety and maintenance purposes.

The other scenarios outlined in the options do not meet the specific conditions established in the Maine Fuel Code regarding oil tank installations. For example, while options mentioning compliance with building codes or using approved materials hold merit, they do not specifically enable a reduction in clearance to the same extent as when a listed secondary containment system is in place.