General Order 128
Definitions of Terms as Used in These Rule
22.8     Practicable means capable of being
accomplished by reasonably available, economic, and workable means.
A.     Clearances and depths required by these
rules shall be deemed impracticable only to the extent that existing structures
or natural obstructions require a reduction in such clearances or depths in
the immediate vicinity of such obstruction or in those exceptional cases
where the clearance otherwise required would, in particular and limited circumstances,
create a condition less safe than a reduction of such clearance.