Conduit, Trenching and Underground Infrastructure
Trench and conduit rules
Underground signal infrastructure includes trench excavation, bedding, conduit, pull boxes, warning ribbon, locate wire, backfill, and restoration. The contract usually sets cover depth, but NEC Table 300.5 sets minimum cover for wiring methods; use the stricter governing requirement. Bed conduit in specified material, keep bell ends toward pull points, and cap open ends during construction. Backfill in lifts with required compaction and place warning tape and locate wire above the conduit.
Pull points and fill
Conduit runs must not exceed 360 degrees of bends between pull points. Size pull boxes per NEC 314.28 for conductors 4 AWG and larger: a straight pull needs at least eight times the trade size of the largest raceway. Conduit fill follows NEC Chapter 9; for three or more conductors, the common limit is 40 percent. Leave pull string and service slack.
Worked example: A run uses 2-inch PVC and terminates in a straight pull box. The minimum box length is 8 × 2 in = 16 in. If the largest raceway were 3 in, the minimum would be 24 in. The reasoning is clearance for the pulling grip and conductor bend radius, not aesthetics. Verify the box depth and locknuts too, because a box that is long enough but too shallow still fails the pull.
Traps: exceeding 360 degrees; no locate wire; sharp bends; backfilling before inspection; mixing conduit types without listed fittings; failing to slope a run to a drain point.