UNDERGROUND SERVICES AND UNDERGROUND WIRING

ADVANTAGES OF UNDERGROUND WIRING
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.

UNDERGROUND WIRING METHODS
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).

PARTS OF ELECTRIC SERVICE ENTRANCE BASICS

How to install service entrance? You will only know it if you know its parts. The details of how the service lateral or triplex cable provided by the electrical utility is fastened and spliced to the customer’s service point are illustrated in Fig. 5-3.

The bare neutral/grounding cable that supports the triplex is fastened at one end to the utility pole and the other end is fastened at the service point with a combination insulator and anchor bolt crimped onto the cable. This attachment method leaves the end of the neutral/grounding cable and the bare ends of the two insulated “hot” conductors available for splicing.

Electrical utility employees splice these three ends to corresponding ends of the customer’s service entrance (SE) cable, which is installed by the electrical contractor. The three ends of the customer’s SE cable are pulled through the bushings in a protective metal hood called the weatherhead or service head with enough slack to permit an adequate drip loop to be formed when the three conductors are spliced.

The drip loop, which must be at least 36 in. long, prevents water from entering the weatherhead. Without a drip loop, water could drain down the conduit to the cable connections inside the meter base to the bus bars that power the watthour meter, corroding them and causing a short circuit.

The details of two different aerial feed service entrances are illustrated in Fig. 5-4. A 200-A service entrance made with three-wire service entrance (SE) cable is shown in Fig. 5-4a. The cable from the weatherhead is brought down to the meter base, where the busbar connections to the meter are made. Another length of SE cable goes from the meter bus bars to the loadcenter.

A second version of a 200-A service entrance has the SE cable protected by metal or nonmetallic conduit between the weatherhead and the meter base, as shown in Fig. 5-4b. The service entrances for 175- and 100-A service are identical except that the SE cable has either a 175- or a 100-A rating. The limits of the meter height dimensions above grade level are approved by NEC 2002.

A 200-A service with the triplex cable terminating on a metal conduit mast is shown in Fig. 5-5. The mast projects high enough to comply with NEC 2002 for the minimum distances of the triplex termination above the ground. The hollow pipe functions as both a mast and a conduit for SE cable from the weatherhead to the meter base. As in Fig. 5-4, the service entrances for 100- to 175-A service are identical except for the lower SE cable ratings. All of the dimensional limits shown are those approved by NEC 2002.