6.8 Site Distribution
Exterior distribution systems must be either direct buried conduit or concrete encased conduit systems. Cable selection should be based on all aspects of cable operation and the installation environment, including corrosion, ambient heat, rodent attack, pulling tensions, and potential mechanical abuse and seismic activity.
Direct Buried Conduit. Direct buried PVC, coated intermediate metallic conduit (IMC) or rigid galvanized steel (RGS) is appropriate for the distribution of branch circuits. Direct buried cable should not be used.
Concrete-Encased Duct bank. Concrete-encased duct banks should be used where many circuits follow the same route, for runs under permanent hard pavements and where service reliability is paramount, such as service entrances.
Duct line routes should be selected to balance maximum flexibility with minimum cost and to avoid foundations of other buildings and other structures. Ducts should be provided with a cover of at least 600 mm (24 inches). Duct banks under railroads should be reinforced. Ducts should slope 4 percent toward manholes. Changes in direction should be by sweeps with a radius of 7.5 m (25 feet) or more. Stub-ups into electrical equipment may be installed with manufactured elbows. Duct line routes should be selected to balance maximum flexibility with minimum cost and to avoid foundations of other buildings and other structures.
Where it is necessary to run communication cables alongside power cables, two separate systems must be provided with separate manhole compartments. The same holds true for normal and emergency power cables. Duct banks should be spaced at least 300 mm (1 foot) apart. Site entrance facilities including duct banks and manholes must comply with requirements stated in Federal Information Processing Standard 175: Federal Building Standard for Telecommunication Pathways and Spaces (see also EIA/TIA [Electronic Industrial Association/Telecommunication Industry Association] Standard 568-A and related bulletins)
Electrical and communication ducts should be kept clear of all other underground utilities, especially high temperature water or steam.
Duct Sizes. Ducts should be sized as required for the number and size of cables. Inner ducts must be provided inside communication ducts wherever fiber optic cables will be used. A sufficient number of spare ducts should be included for planned future expansion; in addition, a minimum of 25 percent spare ducts must be provided for unknown future expansion.
Manholes. Manholes should be spaced no farther than 150 m (500 feet) apart for straight runs. The distance between the service entrance and the first manhole should not exceed 30 m (100 feet). Double manholes should be used where electric power and communication lines follow the same route. Separate manholes should be provided for low and medium voltage systems. Manholes should have clear interior dimensions of no less than 1800 mm (6 feet) in depth, 1800 mm (6 feet) in length, and 1800 mm (6 feet) in width with an access opening at the top of not less than 750 mm (30 inches) in diameter. Manholes must have a minimum wall space of 1800 mm (6 feet) on all sides where splices are to be racked.
Stubs. Minimum of two spare stubs should be provided (to maintain a square or rectangular duct bank) so that the manhole wall will not need to be disturbed when a future extension is made. Stubs for communications manholes must be coordinated with GSA Federal Technology Service.
Smaller manholes may be used for low voltage feeders (600V and below), branch circuits or communications circuits. They should be not less than 1200 mm (4 feet) in depth, 1200 mm (4 feet) in length, and 1200 mm (4 feet) in width with a standard manhole cover and sump of the same type provided for manholes. Generally, at least four racks should be installed.Where more than two splices occur (600V feeders only), a 6 feet by 6 feet by 6 feet manhole may be more appropriate.