Is the definition of Standard and 15-meter classes in the U.S.A.
different from the FAI definition as it pertains to motor gliders? As I
understand the current U.S. National FAI Class Rules, motor gliders are
not allowed in the Standard and 15-meter Nationals unless an engine is
disabled.

I did not find any reference in the FAI sporting code that excludes
motor gliders from these classes. In fact the point 7.6.2 of FAI
sporting code section 3 makes it clear that motor gliders are
integrated into the other classes (except the World Class).

- Wiktor Kozlik