Complete guide to running PVC underground

Running PVC underground looks simple until the inspector pulls a tape measure or the trench caves in on a 90. The work moves fast when the layout is right and the depths are dialed before the first stick goes in the ground. This is the field guide: burial depths, fittings, expansion, and the small stuff that fails inspections.

Burial depth and Table 300.5

Every underground PVC run starts with NEC Table 300.5(A). Rigid nonmetallic conduit gets 18 inches minimum cover under most conditions. Drop to 12 inches if the circuit is 120V or less, GFCI protected, and 20A or smaller. Under a building slab, you only need the conduit in the concrete with 2 inches of cover, no trench depth required.

Cover means the distance from the top of the conduit to finished grade, not the bottom of the trench. Measure from the crown of the pipe up. Under a driveway or parking lot subject to vehicle traffic, you need 24 inches regardless of voltage. Under a residential driveway, 18 inches is the rule per 300.5(A).

• Direct buried PVC, general: 18 in.
• Residential branch circuit, 120V GFCI, 20A: 12 in.
• Under 4 in. concrete slab outdoors: 18 in. (residential driveway exception applies)
• Under street, highway, road, alley, driveway, parking lot: 24 in.
• Irrigation, landscape lighting (low voltage): 6 in.

Schedule 40 vs Schedule 80

Schedule 40 is the default for buried runs. Schedule 80 has thicker walls and is required where the conduit is exposed to physical damage per NEC 352.10(F). The classic call is the riser coming up out of the ground at a service or a pedestal: that exposed section gets Schedule 80, even if the buried portion is Schedule 40.

Transition with a coupling at the elbow if you want to save material, but most crews just run 80 from the LB or weatherhead down past the sweep into the ground. Cleaner, fewer joints, fewer leak paths for water to wick into the run.

"If a lawnmower, weed whip, or kid on a bike could hit it, it's Schedule 80. Inspectors call this one a lot on hot tub feeds and pool pump risers."

Sweeps, bends, and the 360 rule

NEC 352.26 limits you to 360 degrees of total bend between pull points. Four 90s and you are done, pull a box. Long sweeping radius bends pull easier than tight factory 90s, especially for larger conductors. For service entrance work, most utilities want a 36 inch or 48 inch radius sweep at the riser. Check the local utility spec sheet before you order.

Use a heat blanket or hot box for field bends, never a torch. A torch scorches the inside, kinks the wall, and makes the conduit brittle at the bend. Plug both ends with a bend plug or rags before heating to keep the round shape, then form against a template or chalk line on the trench bottom.

1. Mark the bend center and start point on the conduit.
2. Heat evenly, rotating the stick, until the PVC is uniformly soft.
3. Form against the template, hold until cool to the touch.
4. Check the radius with a tape, no flat spots, no kinks at the throat.

Expansion fittings and thermal movement

PVC moves. NEC 352.44 requires expansion fittings where the length change due to temperature exceeds 1/4 inch. Use Table 352.44 to find the expansion in inches per 100 ft for your temperature swing. In most of the country, any exposed run over 25 to 30 feet needs a fitting.

Buried conduit usually does not need expansion fittings because soil temperature is stable, but the transition from buried to exposed at a riser does. Set the piston in the fitting to the middle of its travel at install temperature, not all the way in or all the way out. Mark the piston with a Sharpie before you glue so the inspector can see it was set, not bottomed out.

Solvent welding that holds

Cold joints leak, then conduits fill with water, then conductors corrode at the splice. Cut square, deburr inside and out, wipe the socket and spigot clean. Apply primer (purple, so the inspector sees it) to both surfaces, then cement to both, push together with a quarter turn, and hold for 30 seconds. Working time drops fast in hot weather.

Do not glue wet pipe. If the trench is muddy, dry the joint with a rag and work fast. In freezing temps, use low-temp cement rated for the conditions. A bad joint will not show until the pull, when the conduit pops apart 4 feet underground.

"Glue every joint like the locate crew is going to drive a backhoe over it tomorrow, because eventually somebody will."

Warning tape, bedding, and backfill

NEC 300.5(D)(3) requires a warning ribbon 12 inches above underground service conductors not encased in concrete. Most jurisdictions extend this practice to all buried conduit as good practice. Bed the conduit in 3 to 4 inches of clean sand or screened fill, no rocks against the pipe. Sharp rock under load will eventually puncture Schedule 40.

Backfill in lifts and compact. Do not dump 4 feet of fill on top of an empty conduit and drive over it, the run will deflect and the pull will be miserable. Pull a mule tape during the backfill if the run is long, it costs nothing and saves a fish tape session later.

• 3 to 4 in. sand bedding under and around conduit
• Warning tape 12 in. above the conduit
• Compact in 6 to 8 in. lifts
• Pull a mule tape before backfill on any run over 50 ft