A. From Foreign Substructures (Except Communication Systems)Rule 31.4-A1 (1) Independently Installed: Duct systems for supply cables, when independently installed, shall be separated, where practicable from gas, water, oil, or other pipe systems or other foreign substructures, by a clearance of at least 12 inches when paralleling and by at least 6 inches when crossing. (2) Concurrently Installed: