Section 125.31 - Construction of Duct and Cable Systems(1)Material, Size, and Finish of Ducts. Ducts shall be of such material, size, mechanical strength, and finish as to facilitate the installation and maintenance of conductors or cables. Ducts shall be freed from burrs before laying and shall have clear bores.(2)Grading of Ducts. Where it is necessary to drain ducts, the grade of the ducts shall be such as to permit proper and adequate drainage.(3)Settling. Ducts should be suitably reinforced or be laid on suitable foundations of sufficient mechanical strength where necessary to protect them from settling.(4)Clearances.(a)General. The clearance between duct or cable systems and other underground structures paralleling them, shall be as great as practicable. The distances between the top covering of the system and the pavement surface, or other surface under which the system is constructed, shall be sufficient to protect the system from injury by traffic.(b)Below Base of Rail. The top of all duct and cable system structures, except as hereinafter specified, shall generally be located at a depth of not less than 30 inches in the case of street railways, and not less than 42 inches in the case of diesel and electric railroads, below the base of rail. Where unusual conditions exist or where proposed construction would interfere with existing construction, a greater depth than specified above may be required. Exception: Where this is impracticable, or for other reasons, this clearance may be reduced by agreement between the parties concerned. In no case, however, shall the top of the conduit protection extend higher than the bottom of the ballast section which is subject to working or cleaning.
(c)Metallic Conduit. Where metallic pipe is used as a conduit for underground cables or conductors, it shall not be laid in contact with water, gas or steam metallic pipe systems. Where the clearance is less than 12 inches for gas mains or gas pipelines and less than two inches for other metallic structures, the metalconduit shall be adequately separated from other metallic pipe systems by a barrier of suitable materials, or they shall be electrically bonded together at the point of least separation. Where metallic electrical conduit is known to be located with in four feet of other buried metallic structures, the owner of said structures should be notified relative to mutual protection.(5)Duct Entrances into Manholes. Iron pipe conduit terminating in manholes, hand holes or other permanent openings of underground systems, shall be provided with an effective shield, bushing, fair leader, or other smooth outlet.(6)Duct Arrangement for Dissipation of Heat. Duct systems intended to carry transmission cables of large current capacity should be arranged, where practicable, so that ducts carrying such cables will not dissipate their heat solely through other ducts.