All underground traps and cleanouts of a building,
except where cleanouts are flush with the floor, and all exterior
underground traps, shall be made accessible by manholes at least 24
inches square inside or 30 inches inside diameter with proper covers
of one-half-inch plate steel. If the trap is below the depth of 24
inches, the size of the manhole shall be increased to three feet by
three feet.
Cellar or basement floor drains shall connect into
a trap so constructed that it can readily be cleaned, and of a size
to serve efficiently the purpose for which it was intended.