Refer to the Drawing M3 in the Mechanical Drawings Booklet. What is the minimum acceptable size of a square return grille if the face velocity at the return grille is limited to 400 FPM:

Refer to the Drawing M3 in the Mechanical Drawings Booklet. What is the minimum acceptable size of a square return grille if the face velocity at the return grille is limited to 400 FPM: