One of the main applications of underground circuits is for underground residential distribution (URD), underground branches or loops supplying residential neighborhoods.

Utilities also use underground construction for substation exits and drops to padmounted transformers serving industrial or commercial customers. Other uses are crossings: river crossings, highway crossings, or transmission line crossings.

All-underground construction — widely used for decades in cities — now appears in more places. Underground construction is expensive, and costs vary widely.

Table 3.1 shows extracts from one survey of costs done by the CEA; the two utilities highlighted differ by a factor of ten.

The main factors that influence underground costs are:

•Degree of development —
Roads, driveways, sidewalks, and water pipes, these and other obstacles slow construction and increase costs.

•Soil condition —
Rocks and frozen ground increase overtime pay for cable crews.

•Urban, suburban, or rural —
Urban construction is more difficult not only because of concrete, but also because of traffic. Rural construction is generally the least expensive per length, but lengths are long.

•Conduit —
Concrete-encased ducts cost more than direct-buried conduits,
which cost more than preassembled flexible conduit, which
cost more than directly buried cable with no conduits.

•Cable size and materials —
The actual cable cost is a relatively small part of many underground applications. A 1/0 aluminum full-neutral 220-mil TR-XLPE cable costs just under $2 per ft; with a 500- kcmil conductor and a one-third neutral, the cable costs just under $4 per ft.

•Installation equipment —
Bigger machines and machines more appropriate for the surface and soil conditions ease installations.

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