What Type of Conduit to Use Underground? | Material Guide for Every Job

One wrong conduit choice underground means digging it back up in a few years. The right one runs for decades with zero maintenance. The three factors that decide what type of conduit to use underground are burial location, soil conditions, and how much physical protection the wiring needs. This guide walks through each approved conduit type, the NEC burial depths that apply, and the installation steps that keep your run code-compliant and trouble-free.

Which Conduit Types Are Approved for Underground Use?

Five conduit types are rated for underground burial under the National Electrical Code: fiberglass (RTRC), Schedule 40 PVC, Schedule 80 PVC, rigid metal conduit (RMC), and intermediate metal conduit (IMC). Each one serves a different balance of cost, durability, and corrosion resistance. HDPE and PVC-coated steel are also accepted for specific below-grade applications.

Fiberglass RTRC must meet UL 94 HB for belowground use and comply with NEC Article 355. Utility companies like Rocky Mountain Power often specify it as ZG 033 for their service requirements. Schedule 40 PVC is the go-to for most homeowners because it’s lightweight, easy to cut and glue, and costs less than any metal or fiberglass alternative. Champion Fiberglass’s guide to underground conduit types provides the full specification breakdown for all approved materials.

Electrical Metallic Tubing (EMT) is generally prohibited for underground burial. It corrodes rapidly in direct contact with soil unless fully encased in concrete per the 2023 NEC. Flexible conduit also fails underground — it won’t lay straight, creates kinks, and makes wire pulling nearly impossible.

How Deep Do You Need to Bury Each Conduit Type?

The National Electrical Code (NEC Table 300.5) sets minimum cover depths measured from the top of the conduit to finished grade. Rigid metal conduit needs only 6 inches, PVC requires 18 inches, and direct-bury UF cable needs 24 inches. Local frost lines may require deeper trenches regardless of the NEC minimums.

Conduit must also be buried below the local frost line or incorporate freeze protection. When running multiple circuits, low-voltage wires (12–24V) cannot share the same conduit as 120/240V lines but can share the same trench in separate conduits. Always check with your local building department for amendments to the NEC depths.

Step-by-Step Underground Conduit Installation

Following a consistent install sequence prevents the most common callbacks. Start by confirming utility locates and marking safe dig zones. Excavate to the required depth plus a bedding allowance, sloping the trench at least 1/8 inch per foot for drainage.

1. Prepare the bedding. Install and lightly compact a layer of sand or fine gravel so the conduit has uniform support.
2. Lay the conduit. Set the conduit on the bedding with support spacing no more than 5 feet for PVC. Use sweeping bends rather than tight 90s when possible — they make wire pulling easier.
3. Solvent-weld all joints. PVC connections must be glued with approved solvent cement. Factory-made elbows are available for 90° and 45° turns; field cuts require clean deburring and proper cement application.
4. Install expansion fittings. Place them where the conduit enters a building and every 200–250 feet on long runs to accommodate thermal movement.
5. Lay warning tape. Bury utility caution tape 12 inches above the conduit so future diggers see it before they reach the line.
6. Photograph the installation. Take clear pictures of the open trench showing conduit placement, joint locations, and depth markings before backfilling.
7. Backfill in lifts. Add 6-inch layers of appropriate material, compacting each lift to at least 85% density to prevent settling.
8. Seal the ends. Apply exterior-grade acrylic-latex caulk around conduit outlet bodies and any above-ground transition points to keep water out.

Use THWN wire inside PVC conduit. If you’re running a direct-bury feeder without conduit, UF-B cable is rated for that purpose and must be buried at the depths shown in the table above.

Common Mistakes That Ruin Underground Conduit Jobs

The most frequent error is assuming one burial depth fits all conduit types. Rigid metal needs only 6 inches while PVC needs 18 — defaulting to 24 inches for everything is overkill on metal and still right for PVC, but mixing up the minimums wastes time or fails inspection.

Burying EMT is another fast track to failure. The thin steel corrodes within a few years in most soils unless it’s fully encased in concrete. And skipping warning tape is a liability call — without it, someone digging years later has no clue there’s a line 12 inches down.

Flexible conduit kinks instead of laying straight, making wire insertion impossible. PVC joints that aren’t solvent-welded will separate under ground movement. Each of these mistakes means digging the trench again, which costs more than doing it right the first time.

Choosing the Right Conduit for Your Project

For most residential runs to a shed, garage, or landscape lighting system, Schedule 40 PVC delivers the best balance of cost and durability. It’s easy to work with, meets NEC requirements at 18 inches deep, and handles standard soil conditions without issues. If your trench passes through areas with heavy traffic or you need shallower burial, step up to rigid metal conduit at 6 inches of cover.

For corrosive soil, high-temperature environments, or utility-grade installations, fiberglass RTRC is worth the higher upfront cost — it outlasts PVC in those conditions and won’t corrode like metal. HDPE is the right call when you’re directional boring under a driveway or across a lawn.

Before you buy materials, check local code requirements and compare current pricing and availability. Our guide to the best underground conduit breaks down the top-rated options with specs and prices.