Final answer:
For an NPS 6 pipe, current engineering recommendations for dirt pocket design exceed the options provided, suggesting a minimum length of about 48 inches and a maximum diameter of approximately 90 inches for efficient operation and safety.
Step-by-step explanation:
When considering the design of a dirt pocket for an NPS 6 pipe that drops to an appliance, referencing typical engineering practices can be beneficial. A traditional rule of thumb for the size of a dirt pocket might be around five times the pipe diameter side-to-side and seven to eight times the pipe diameter in depth. However, more contemporary recommendations suggest that this might be enlarged to as much as eight diameters side-to-side and fifteen diameters deep for the dirt pocket.
Given that the pipe in question is an NPS 6, which equates to a nominal pipe size of 6 inches in diameter, applying these guidelines would suggest a minimum length (side-to-side dimension) of around 48 inches (8 x 6 inches) and a recommended depth (maximum diameter) of approximately 90 inches (15 x 6 inches). While these figures exceed any of the options presented, they show the evolving nature of engineering recommendations over time, which account for better efficacy and safety in pipe design.
The options provided in the question do not precisely match current recommendations or the older rule of thumb. Therefore, it is essential to consult specific codes or standards related to the piping system you are working with to determine the appropriate dimensions for your dirt pocket.