A well-designed underground distribution systems must provide for anticipated load growth that can be accommodated economically. This means that provisions must be made to furnish electrical service to new as well as existing customers.
Both overhead and underground distribution systems have existed in large metropolitan areas for many years, but underground distribution was rarely used in suburban residential areas, small towns, or rural areas because of the high cost of these installations. Overhead distribution was almost universally used in those locations.
However, within the past 20 years low-cost solid dielectric cables suitable for direct burial have been introduced and pad-mounted distribution transformers and accessories have been mass-produced, reducing their cost. These developments, along with the introduction of trenching machines for the mechanizing the burial of cable, have made underground residential distribution (URD) more affordable
Despite these reductions, the cost of a typical URD system for a new residential subdivision can be as much as 50 percent greater than that for an overhead system in locations where the soil conditions are unfavorable for cable burial. On the other hand, where the land is dry, relatively treeless, and with few or no underground rocks, the cost gap between the two technologies has narrowed. It is believed that this cost gap between URD and overhead service will shrink even further because of the falling prices of the equipment due to increasing demand.
Studies have shown that both builders and homeowners will accept a reasonable cost differential because of the recognized improvements made in residential landscaping as a result of eliminating overhead wires and poles. It has been estimated that 70 percent of new residences are being served by URDs. Moreover, some states have passed legislation making underground distribution mandatory for new residential subdivisions because of the improvements in the appearance of the new developments.
Most of the widely used underground distribution systems in densely populated parts of North American cities are secondary systems with cables installed in conduit or ducts beneath the streets and sidewalks. The conductors for primary voltages of 5 to 35 kV are typically paper-insulated, lead-covered (PILC) cable. Single-conductor secondary cables for both primary and secondary circuits typically have rubber insulation and neoprene jackets for conducting up to 34 kV, but single-conductor, polyethylene-insulated cable is gaining in popularity.
While copper predominated as the conductor of choice in the past, aluminum as a conductor in cable is gaining in popularity for new installations. However, because of its larger diameter than cable with copper wires, cable with aluminum wires is not usually used if it is anticipated that it must share the limited internal space of a conduit with other cables in the future.
Underground Distribution Cables
In suburban areas, underground distribution systems serve shopping centers and commercial and industrial parks. The conductors for this service are typically direct-buried cables, but they are usually run in conduits or ducts when passing under streets, sidewalks, or other paved surfaces. If the cable must be removed later for replacement or repair, it would be costly and labor intensive, to say nothing of inconvenient, if it had to be dug out by breaking up concrete slabs or pavement. Aluminum conductors predominate in these newer installations. For primary cables, the insulation most widely used is solid dielectric cross-linked polyethylene and ethylene-propylene rubber (EPR), also known as ethylene-propylene monomer (EPM).
Concentric neutral wires have become common. Secondary cables in these applications are constructed of similar material, and aluminum conductors with cross-linked polyethylene insulation are most common. The neutral in this secondary cable is typically an insulated conductor, but bare copper neutrals are also being used.
In distribution systems for 5 kV or higher, the cables are usually shielded with conducting or semiconducting materials that cover the cable insulation to confine the electric field to the insulation. This shielding can be outside the cable insulation, directly over the main conductor, or both.
Outside shielding, typically done with wound metal tape, a metal sheath, or concentric wires, must be grounded. This shielding provides a return path for short-circuit current if the cable fails, while also protecting installers from shock. Underground cables can be separately insulated single- as well as two-, three-, and four-conductor, enclosed by a single sheath or jacket
NEC UNDERGROUND INSTALLATION REQUIREMENTS
NEC 2002 Sections 300.5, “Underground Installations,” and 310.7, “Direct Burial Conductors,” cover the requirements for various aspects of underground installations. Section 300.5, for example, discusses minimum cover requirements, grounding,underground cables under buildings, and protection (of conductors) from damage. Section 310.7 gives the requirements for direct-burial conductors. It states that “Cables rated above 2000 V shall be shielded,” but it also gives some exceptions. This section also states that “The metallic shield, sheath, or armor shall be grounded through an effective grounding path meeting the requirements of NEC 2002 250.4(A)(5) or 250.4(B)(4).”
DIRECT -BURIAL CONDUCTORS
Type USE cable (underground service entrance) is approved for direct burial in the earth because its moisture-resistant jacket does not require any additional protective cover. Single-conductor USE cable as well as parallel or cabled conductor assemblies can include bare copper conductors. These assemblies do not require outer overall jackets. USE cable can be used for underground services, feeders, subfeeders, and branch circuits.
Type UF cable(underground feeder and branch circuit) is also approved for direct burial in the earth. It includes copper conductors sized from No. 14 AWG through No. 4/0. The jackets of type UF cable are flame-retardant and moisture-, fungus-, and corrosionresistant. This cable is approved for direct earth burial as feeders or branch circuits if it meets the rated ampacity overcurrent protection requirements of the NEC.
Type MC and type MI cable are approved for direct burial under certain conditions.
CABLE BURIAL INSTRUCTIONS
Where single-conductor cables are to be installed, all cables of the feeder circuit, subfeeder circuit, or branch circuit (including the neutral and equipment grounding conductors,
if any) must be run together in the same trench or raceway.
Nonmetallic-armored cable can also be used in underground installations. Its interlocking armor consists of a single strip of interlocking tape that extends the length of the cable. The round surface of the cable allows it to resist inadvertent blows from trenching tools better than flat-bend armored cable. The cable must have an outer covering that will not corrode or rot. A covering of asphalt-jute can be used as cable covering if the cable is to be exposed to particularly corrosive chemicals such as gasoline.
Cables approved for direct burial range from single-conductor insulated wires for low voltage applications to multiconductor cables for conducting electrical energy, communications, or alarm signals. In direct-burial methods, the conductors are buried in the ground by placing them at the bottom of an excavated trench, which is later backfilled. Where soil conditions and circuit configurations are favorable, the cable can be buried directly with a special machine called a cable plow. It breaks the earth ahead of the cables, guides them into the furrow, and immediately backfills the furrow over the cable.
DIGGING THE CABLE TRENCH
The methods for installing direct-burial cable vary according to the length and the size of the cable being installed and the soil conditions. For short runs such as those from a residential basement to a garage located about 20 ft away, the excavation can be made manually; but for longer runs, trenching machines or backhoes are usually hired to perform the work faster and more economically.
After digging the trench to a depth at least 3 to 4 in. beyond the minimum depth specified by NEC 2002 Table 300.5, “Minimum Cover Requirements, 0 to 600 Volts,” all sharp rocks, roots, and extraneous solid objects should be removed from the trench to prevent them from damaging the direct-burial cable.
MINIMUM COVER REQUIREMENTS
According to NEC 2002, Article 300.5, all underground installations must be grounded and bonded in accordance with Article 250, and underground cable installed under a building must be in a raceway that extends beyond the outside walls of the building. The article further states that direct-buried conductor and cable shall be protected by enclosures or raceways (including conduit) under the following conditions:
- Emerging from grade from the minimum cover distance to a point at least 8 ft above finished grade. The protection required need not exceed 18 in. below finished grade
- Conductors entering buildings must be protected to the point of entrance.
- Underground service conductors not encased in concrete that are buried at least 18 in. below grade must have their location identified by a warning ribbon that is placed in the trench at least 12 in. above the conductors.
- Conductors and cables subject to damage must be installed in approved metal or nonmetallic conduit.
- Listing: Cables and insulated conductors installed in enclosures or raceways in underground installations must be approved for wet locations.
below Figure is a cross-sectional view of a trench with two direct-burial cables installed. NEC2002, Table 300.5, defines five different types of buried cable and wiring situations:
1 Direct-burial cables or conductors
2 Rigid or intermediate metal conduit
3 Nonmetallic raceways for direct burial without concrete encasement
4 Residential branch circuits rated 120 V or less with GFCI and overcurrent protection
5 Low-voltage circuits for irrigation or lighting
The table also gives seven different locations of wiring or circuits, of which five are :
1 In a trench under a 2-in. concrete slab
2 Under a building
3 Under a concrete slab at least 4 in. thick
4 Under streets, highways, roads, alleys, driveways, and parking lots
5 Under residential driveway
In general, burial depths range from a 4 in. for all categories under 4-in.-thick concrete slabs to 24 in. for direct-burial cable in unspecified locations. For example, direct-burial cables and conductors must be buried at least 18 in. deep under residential driveways, but GFCI- and overcurrent-protected 120-V or less residential branch circuits must be buried at least 12 in.
after the trench is dug deeper than the minimum depth requirements in underground distribution systems , a 3- to 4-in. bed of sand should be placed at the bottom of the trench to protect the cable from sharp stones that might still be at the bottom of the trench. The cable or cables should then be placed in the trench without crossovers and slightly “snaked,” to allow enough slack for earth settlement, movement, or heaving due to frost action. Single conductor cables should be kept uniformly apart by about 6 in. along the length of the trench.
After the cable is laid in the trench, another 3-in. layer of sand or sifted backfill should cover the cables. Then a treated wooden plank or concrete slab, wide enough to cover the cables, should be placed over them to protect the cables from any future excavations. The trench is then backfilled another 12 in. (if depth permits). A colored plastic ribbon should then be placed in the trench before completing the backfilling, to warn future excavators that electrical conductors are buried underneath
MANHOLES
A cross-sectional view of a typical electrical manhole in underground distribution systems is shown in below Figure . . The base ring or square is positioned at the bottom of the excavation, and the conical or pyramidal throat is placed on top of the base with its opening at ground or finished-grade level.
Manholes are sized to provide enough room for installers and maintenance personnel to splice cables or mount equipment as well as carry out routine inspections. There are rules for dimension of access openings and other features of electrical manholes.
- Round access: If the manhole is round and it contains only power cables, the diameter must not be less than 26 in.
- Round access: If the manhole is round and it contains a fixed ladder that does not obstruct the opening or it contains optical fiber cables, fire alarm circuits, or remote control or signaling circuits, the diameter must not be less than 24 in.
- Rectangular access:If the opening in the manhole is rectangular, it must not be less than 26 X 22 in.
- Slope:The slope of the sidewalls must be sufficient to provide protection for cable splices or other installed equipment so they will not be directly under the opening.
- Drain:The drain must be located centrally under the opening.
- Traps:Where water drainage will go into sewers, suitable traps or other means must be provided to prevent the entry of sewer gas into the manhole.
Manholes for underground electrical distribution are separated by distances in an underground duct system that are short enough to permit pulling conductors or cables between them during initial construction. They also permit access to the conductors or cables for testing, cable replacement, and maintenance. Access to manholes is gained through openings or throats extending from the manhole cavity to the surface (ground level or finished grade). At ground level a heavy, durable manhole cover is used to close the manhole securely.
Underground cable runs normally terminate inside a manhole, where there is sufficient room for them to be spliced to another length of cable. Manholes can be constructed manually from bricks and mortar, but today most of them are prefabricated from reinforced concrete in two parts: a base ring or base square section and a conical or pyramidal throat.
There are three basic designs for electrical/communications manholes: two-way, three-way, and four-way in underground distribution systems.
- Two-way manhole: Ducts and cables enter this manhole from one side and leave from the other side, 180° away.
- Three-way manhole: Same as for a two-way manhole, except that a third duct for cables leaves the manhole 90° away the other two ducts.
- Four-way manhole: It has two entry ducts and two leaving ducts, all 90° apart.
A cross-sectional view of a typical underground duct system linking two two-way manholes is shown in below Figure. The ductwork should be arranged so that it slopes toward the manhole, so that water cannot accumulate in the ducts. Underground duct systems typically include manholes, handholes, transformer vaults, and risers.
