API 2201 is the recommended practice for hot tapping. ASME B31 essentially has nothing to do with hot tapping as it's a new construction code.
The piping (such as a saddle-on connection) or the special hot tap equipment (such as a split tee) might be designed to B31.x but API 2201 would be consulted for the hot tap procedure. API 2201 also covers the risks involved, required flow rates, preheat, welding minimums/maximums, drilling equipment, etc.
-InspEngr