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.
No comments:
Post a Comment