Underground Conduit Systems

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1 Section 32 Underground Conduit Systems Note 1 under the title of Sec. 32 defines how the NESC uses the following terms: Duct Conduit Conduit system The common trade use of these terms is slightly different from the NESC definitions. The terms duct and conduit are sometimes used interchangeably when the conduit consists of only one duct (hole). The use of these terms as they apply to the NESC is shown in Fig Note 2 under the title of Sec. 32 states that a cable installed in a single duct that is not part of a conduit system is covered in Sec. 35. Section 35 provides rules for direct-buried cables. See Fig LOCATION Section 32 applies to the underground structure only, not the cables in the structure. Section 33 applies to supply cables. Section 34 applies to the installation of cables in an underground structure. Rule 320 deals with the location of underground conduit systems, where they can be routed, and how far they must be separated from other underground installations. 320A. Routing. The general requirements for conduit system routing are outlined below: Subject conduit system to the least disturbance practical. When conduit system is parallel to another structure, do not locate directly over or under the other structure if practical. 341

2 342 UNDERGROUND LINES Rule 320A Fig NESC definitions of duct, conduit, and conduit system (Sec. 32). Align conduit without protrusions that would harm the cable. Provide sufficient bending radius to limit cable damage. To many readers amazement, the NESC does not specify depths for conduit burial. Direct-buried supply cables covered in Sec. 35 do have to meet the specified depths (covered in Rule 352 and Table 352-1); however, no such burial requirements exist for cables installed in conduit systems. The only specific burial depth called out in this section is for conduit crossing under railroad tracks. The only other specific distance specified in this section is separation between supply and communications conduits. Figure conveys the message that the NESC does not specify depths for conduit systems. To decide on the proper burial depth for conduit systems, the general rules of Sec. 32 must be applied and Rule 012C, which requires accepted good practice, must be used. One excellent reference for accepted good practice in this case is

3 Rule 320A UNDERGROUND COMDUIT SYSTEMS 343 Fig Application of underground conduit systems and direct-buried cables (Secs. 32 and 35). the National Electrical Code (NEC). The NEC specifies depths for various types of conduits (PVC, steel, etc.) and different voltage levels. The NEC also provides methods for reducing conduit burial depth when rock is encountered. The NESC is not specific on conduit bending radius requirements. Rule 320A simply states that a sufficient bending radius is needed. Rule 012C, which requires accepted good practice, must be used. The National Electrical Code (NEC) provides much more detail than the NESC on conduit bending radius, and the NEC can be used as a reference for accepted good practice. Figure details how conduit-bending radius is typically measured. Typically, field bends require a larger bending radius than factory bends. A bend with too tight a radius will deform or damage the conduit material.

4 344 UNDERGROUND LINES Rule 320A Fig Burial depths for conduit systems and direct-buried supply cables (Rules 320 and 352). Fig Conduit bending radius (Rule 320A1c).

5 Rule 320B UNDERGROUND COMDUIT SYSTEMS 345 In addition to bending radius requirements, the NEC can be referenced for conduit support, conduit fill (i.e., the number of conductors in any one conduit), and various application requirements. The total bending radius of a conduit run can be determined by adding the angles of each bend. To determine how many bends are acceptable in a conduit run, a cable pulling calculation must be done per Rule 341. See Fig Natural hazards due to unstable or corrosive soil should be avoided or proper construction methods should be used to minimize the hazard. If a conduit can be installed outside a roadway, no interference with the road would occur. However, if the conduit is installed longitudinally under the road, using the shoulder or just one lane of traffic will make both installation and maintenance of the conduit less conflicting with the roadway traffic. The NESC does not specify the conduit burial depth under the road for a longitudinal run or for a road crossing. The road department or highway department having jurisdiction may require specific depths or locations to obtain a permit to install utilities under the road. The rules related to conduit routing under highways and streets are outlined in Fig Conduit routing in or on bridges and tunnels should not be damaged by traffic and should be located to provide safe access. Conduits crossing under railroad tracks do have a specific burial requirement. This is the only specific burial requirement provided in Sec. 32. The burial rules for a conduit under a railroad track are outlined in Fig Submarine crossings are discussed in Rule 320A6 and in Sec. 35, Direct Buried Cable, Rule 351C5. Submarine crossings are underwater crossings usually on lake bottoms, river bottoms, or ocean bottoms. A special cable called a submarine cable is commonly used for underwater crossings. Conduit could be used for the entire crossing but it is normally used near the shorelines. An example of a three-phase, 15-kV submarine cable is shown in Fig Higher-voltage submarine cables are also in use and can consist of oil-filled cable with a lead outer jacket. There are no specific conduit requirements in this rule other than protecting the submarine crossing from erosion by tides or currents and locating the crossing away from where ships normally anchor. Submarine crossings frequently use conduit for some portion of the distance into the water and then the submarine cable is exposed and lies on the bottom of the body of water. An example of a submarine crossing is shown in Fig Rule 96C, which requires at least four grounds in each mile, has an exception that states the rule does not apply to underwater crossings if other conditions are met. 320B. Separation from Other Underground Installations. The general requirements for separation between conduit systems and other underground structures are outlined below: When paralleling another structure, provide separation to permit maintenance without damaging the parallel structures. When crossing another structure, provide separation to limit damage to either structure. The parties involved should determine separations.

6 346 UNDERGROUND LINES Rule 320B Fig Example of determining the total bending radius of a conduit run (Rule 320A1c).

7 Rule 320B UNDERGROUND CONDUIT SYSTEMS 347 See Photo Fig Conduit routing under highways and streets (Rule 320A3). Exception: Conduits may be supported from roofs of manholes, vaults, and subway tunnels with concurrence of the parties. The term structure in this rule is used loosely to apply to many types of buried structures including building foundations, other conduit systems, and other utility lines such as sewer lines, water lines, gas and other fuel lines, steam lines, etc. No specific dimensions are provided. Enough separation for safe maintenance and avoiding damage are the primary concerns. The Code is very specific for separations between supply and communications conduit systems as shown in Fig One of the main reasons that supply and communication conduit systems need to be separated is that a fault on the supply cable in the supply conduit could damage the supply conduit and the communications conduit and cable next to it. Another reason is to provide adequate space between the conduits for maintenance. These two issues do not present many problems in residential subdivisions. With proper fuse protection and use of concentric neutral underground residential distribution (URD) cable, it is not common for the URD cable to fault and damage the conduit. Quite often the supply and communication utilities will apply the exception to Rule 320B2 and use no separation between the supply and communications conduits in well-tamped earth. Typical residential construction may consist of a single trench with supply and communication conduits in random separation (i.e., placed in the

8 348 UNDERGROUND LINES Rule 320B Fig Conduit crossings under railroad tracks (Rule 320A5). trench next to each other without any intentional separation). This type of installation should not be confused with random separation of direct-buried supply and communication cables. The random separation requirements for direct-buried cables are discussed in Rule 354. The requirement for conduits crossing and paralleling sanitary and storm sewers, water lines, gas and other fuel lines, and steam lines are very general common sense requirements. No specific dimensions are provided. The Code is primarily concerned with providing enough separation for maintenance and avoiding damage to the lines involved. See Fig The conduit system locations discussed in Rule 320A indicate that vertical placement of parallel underground lines is not preferred. The conduit system

9 Rule 320B UNDERGROUND CONDUIT SYSTEMS 349 kv kv Fig Example of a three-phase, 15-kV, submarine cable (Rule 320A6). Fig Example of a submarine crossing (Rule 320A6).

10 350 UNDERGROUND LINES Rule 320B Fig Separation between supply and communication conduit systems (Rule 320B2). locations discussed in Rule 320A indicate the vertical placement of parallel underground lines is not preferred. The conduit system separations discussed in Rule 320B1 simply require adequate separation between paralleling lines without any reference or preference to the horizontal or vertical placement. For separation between direct-buried supply and communication cables and other underground structures, see Rule 353.

11 Rule 323A UNDERGROUND CONDUIT SYSTEMS 351 Fig Separation from sewer, water, gas, fuel, and steam lines (Rules 320B3 through 320B6) EXCAVATION AND BACKFILL The trench and backfill rules for conduit systems are outlined in Fig For trench and backfill requirements related to direct-buried supply and communications cables, see Rule 352A DUCTS AND JOINTS 322A. General. The general rules for ducts and joints are outlined in Fig B. Installation. The installation rules for ducts and joints are outlined in Fig MANHOLES, HANDHOLES, AND VAULTS 323A. Strength. Manholes, handholes, and vaults must be designed to withstand a variety of loads as outlined in Fig In general, dead loads are the weight of the structure and permanent attachments to the structure. In general, live loads are loads that are not permanent, like