Underground electrical services are not subject to as much mechanical damage from equipment, personnel, and weather as overhead systems. Cable can be buried in the ground or pulled through conduit systems. To avoid a drop in voltage, underground systems usually run no more than 360 feet after the last transformer in the system. The underground service uses sizes of No. 8 copper or No. 6 aluminum conductors. This lesson will teach you how to describe an underground electrical service.
The ground-cover requirements for underground conductors or conduit vary according to the site. In nontraffic areas, SE cable should be buried at least 18 inches and covered with a 2-inch-thick concrete pad (Figure 32). If rigid or intermediate conduit is installed, the depth can be reduced to 6 inches.
Figure 32. Buried SE cable (non-traffic area).
When SE cable is subject to vehicle traffic, at least 24-inces of ground cover is required (Figure 33). The 24-inch-depth requirement will allow the service to withstand the weight of vehicles passing over it. The depth requirement is the same when conduit is used.
Figure 33. Buired SE cable (traffic area).
SE cable buried in solid-rock areas can be installed down to the surface of the rock if the cable is covered with at least 2 inches of concrete that extends to the rock surface (Figure 34). The same requirement exists for conduit.
Figure 34. Buried cable in solid rock area.
Conductors coming out of the ground should be protected by enclosures, raceways, or conduit from below grade to 8 feet above the ground (Figure 35).
Figure 35. Conductor protection.
When backfilling a trench where there are cables or conductors, do not backfill with-
Placement of SEs. Underground services can be installed using either underground service entrance (USE) cable, direct-burial conductors, or conduit (Figure 36). Conductors should not be spliced unless they are placed in a properly installed splice box.
Figure 36. Types of installation.
When direct-burial conductors or USE cables are above grade they must be protected in the plastic or metallic conduit at both ends (Figure 37).
Figure 37. Protection at ends of SE cable.
The installation of a USE to the power panel varies according to the site and the equipment. (Figure 38) shows a typical setup that is subject to change to fit the installation.
|NOTE: A ¼-inch air space is required when mounting a power panel to a damp wall or surface.|
Figure 38. Typical USE power-panel setup.