are hollow tubes running from manhole to manhole inside a conduit (see conduits)
in an underground
system. They are of various sizes usually from 2 to 6 inches in diameter.
Electrical cables are run through ducts and the ducts are sized accordingly.
The diameter of a duct should be at least 1/2 to 3/4 inch greater than the
diameter of the cable(s) installed in the duct. They can be made of plastic
(PVC), fiberglass, fiber, tile, concrete, or steel. PVC and fiberglass are
most commonly used.
Figure 1. Duct run within conduit showing drainage in
Figure 2. Conduit on a grade