The advantages of underground electric service are attractiveness (lack of physical and visual clutter overhead), service reliability, and long life. The principal disadvantage is high cost. To overcome this, utilities frequently use direct burial techniques that, by eliminating a raceway, reduce costs considerably.

Because direct buried cable cannot be pulled out if it faults, as is the case with raceway-installed cable, restoration of service after a cable fault is time-consuming. It is recommended that the decision on which technique will be used be based upon the consideration of these factors:

• The cost premium for underground raceway installation, including handholes if required.
• The history of outages for direct burial installation by this installer, in the immediate area
• Cost and availability of repair service (utilities frequently will repair customer-owned underground service laterals for a fee)
• Impact of electric service outage in terms of time delays, inconvenience, necessity of digging up lawns and paved areas, and cost impact in the case of a commercial facility.

The methods available for underground wiring are:
• Direct burial
• Installation in Type I, concrete-encased duct
• Installation in Type II, direct burial duct

The first alternative offers low cost and ease of installation, with the disadvantage regarding repairs stated previously. The second offers high strength and permanence, but at the highest price of the three. The last offers median cost but little strength. It is applicable only for installations on undisturbed earth and/or under light paving.

Nonmetallic duct (conduit) intended for underground electrical use is commercially available in two wall thicknesses. NEMA (National Electrical Manufacturers Association) Type II with a heavy wall provides the physical protection required for direct burial installation with no concrete encasement.

Type I is manufactured with a thinner wall and is intended for encasement in a minimum of 2 in. (50 mm) of concrete. Common trade names for asbestos-cement and fiber ducts are Transite and Orangeburg. Plastic conduit is referred to as PVC or simply as plastic.

Nonmetallic conduit is most frequently used without concrete encasement for low-voltage and signal wiring and with encasement for high-voltage wiring. It offers several advantages over steel conduit for underground work, such as lower cost and freedom from corrosion.

When underground electric wiring is ductinstalled and the run extends over several hundred feet (meters) (the exact distance depending upon the pulling tension), a pulling handhole or manhole is necessary. Handholes are used for low-voltage power and signal cables and for runs with a small number of cables. Manholes are used for high-voltage cables and where large duct banks must be accommodated.

Precast handholes and manholes are readily available in many standard sizes and are usually cheaper than field-formed and poured units.

Cable used in underground wiring must be specially manufactured and approved for that purpose. Type SE is the basic service entrance cable, constructed with a moisture- and flameresistant covering. When it is provided with moistureproofing for underground use, the designation is SE type U, or simply USE.

Underground cable for other than service runs is classified as type UF (underground feeder).


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