How to Install Underground Electric Conduit Up to Code

Installing underground electric conduit up to code is an important process to ensure the safety and functionality of your electrical system. Properly installed conduit protects wires and cables from damage and provides a pathway for running new circuits in the future. Following local building codes and best practices when installing underground conduit will result in a quality, long-lasting installation.

Planning Your Conduit Installation

Careful planning is crucial before digging trenches or burying conduit. Consider the following factors:

Determine the Scope of Your Project

What areas need power? Calculate expected electrical loads.
How many circuits do you need to run underground?
What type and size of conduit is required?
Will you need to run conduit between buildings? Coordinate trenching.
Get all required permits and call to locate existing utilities.

Choose the Right Conduit Type

PVC – Most common, affordable option. Withstands moisture well.
EMT – Steel tubing that offers good physical protection. Avoid if soil is corrosive.
Rigid metal – Very strong and threaded connections. Costly, difficult to work with.
Fiberglass – Used for long conduit runs. Withstands corrosion well.

Calculate Conduit Fill

National Electrical Code (NEC) limits how many wires can be run through each conduit size.
Measure your wire sizes and quantities to pick appropriate conduit diameter.
Conduit fill should not exceed 40% for easiest wire pulling.

Map Your Conduit Layout

Draw up plans showing all conduit and trench runs.
Indicate conduit sizes, burial depths, approved materials.
Minimize bends and make large radius 90° sweeps when necessary.

Digging the Trench

Digging a proper trench is critical to protect your conduit from damage.

Dig Trench to Correct Depth

Trench depth depends on conduit size and if conductors are over 600 volts.
Minimum depth is 18 inches for conduit under 2 inches in diameter.
For conduit over 2 inches, minimum depth is 24 inches.
Go deeper than minimum if heavy vehicles will pass over the trench area.

Check for Obstructions

Look for existing utilities, pipes, roots, rocks, etc. as you dig.
Adjust trench location to avoid obstructions if possible.
Use hand tools carefully if you must dig around obstructions.

Make the Trench Wide Enough

Trench should be 6 to 12 inches wider than the conduit diameter.
Wider trenches make it easier to lay and join conduit.
Minimum trench width is 6 inches for conduit under 3 inches diameter.

Shore Up the Trench Walls

Use trench box or shoring boards if trench walls slough in.
Preventing trench collapse protects workers from trapped-in hazards.
Scale back trench depth if the soil does not stay vertical on its own.

Call Before You Dig

Always contact local utilities to locate buried pipes and cables.
Allow several days for scheduled marking of utility lines.
Avoid legal issues and damage to existing infrastructure.

Installing Underground Conduit

Follow best practices when assembling and burying conduit for optimal results:

Assemble Conduit Sections Correctly

Use couplers to join conduit sections if not threading in the field.
Stagger conduit joints so they do not all line up.
Use approved solvent cement on PVC joints for watertight seals.
Seal metal conduit joints with listed compounds to prevent leaks.

Arrange Expansion Fittings

Use expansion couplings on long runs to accommodate temperature changes.
Provide flexibility in the conduit to prevent buckling.
Install expansion fittings at building entrance points.

Run Conduit in a Straight Line

Make sweeping bends with large radii where necessary.
Excessive bends make wire pulling difficult.
Use manufactured sweep elbows for large conduit sizes.

Secure Conduit in the Trench

Lay conduit on a layer of sand or fine gravel to avoid sharp rocks.
Use plastic strapping to anchor conduit to prevent shifting.
Concrete encasement provides extra protection for larger conduits.

Backfill Correctly

Backfill in layers, compacting soil as you go to prevent settling.
Avoid large rocks or debris that could damage conduit.
Concrete slurry backfill is recommended for large conduit runs under roads.

Place Warning Tape

Install tracer tape 6-12 inches above conduit in the trench.
Tape warns of buried electric lines during future excavation.
Use detectable underground utility marking tape.

Install Concrete Thrust Blocks

Pour concrete blocks where conduit exits the ground to secure it.
Prevent conduit from shifting on bends and transitions.
Size thrust blocks according to local code based on conduit size.

Conduit Entry and Sealing

Moisture entry into the conduit system can lead to dangerous shorts and corrosion. Use best practices at termination points:

Seal Ends Until Ready

Cap or plug conduit ends to prevent dirt and moisture intrusion when not in use.
Use manufactured seals like duct seal or foam to create a tight barrier.
Avoid makeshift plugs that can fall out or allow moisture ingression.

Mount Closure to Surface

Attach conduit closure fittings flush to the entry surface.
Use gasket seals between the closure and entry barrier.
Closures protect splices and connections from weather and pests.

Slope Conduit Downward

Arrange conduit to slope back towards the ground source.
Prevent water running down into the system.
Use drip loops in wire to drain moisture away from boxes.

Make Waterproof Seals

Apply duct seal or weatherproof silicone at conduit terminations.
Ensure a complete seal around individual cables.
Verify sealants used are compatible with your wiring insulation.

Use Drain Fittings

Include drain fittings at low conduit points to evacuate moisture.
Check drainage fittings periodically to ensure they are not clogged.
Remove any accumulated water if present.

Underground System Grounding

Proper grounding of underground conduit helps ensure safety and prevent damage:

Install Grounding Conductor

Run a grounding electrode conductor through all underground conduit.
Conductor bonds systems together and safely dissipates faults.
Size ground wire according to NEC; typically #4 copper or larger.

Use Moisture-Sealed Connections

Use listed fittings to join ground wires that seal out moisture.
Watertight cold shrink splices or irreversible compression connectors are good choices.
Avoid relying solely on regular wire nuts underground.

Bond Conduit Ends

Connect grounding bushings to each end of metal conduit runs.
Electrically join conduit ends using grounding jumper wires.
PVC conduit runs may still require grounding – check with inspector.

Drive Ground Rods

Drive copper-clad steel ground rods 8 feet into the earth at various locations.
Join rods to main grounding system using conductors and proper clamps.
Follow NEC on proper rod sizing, quantity, and maximum resistance.

Use Water Pipe Bonding

Bond to underground metal water pipe and building steel if available.
Provides additional paths to dissipate electrical faults.
Ensure bonding connections are accessible and maintain metal-to-metal contact.

Conduit Labeling and Documentation

Proper records make it easier to locate conduit runs and identify circuits:

Label Both Ends

Use durable labels at each conduit termination indicating destination.
Helps identify the other end point in the future.
Include origin and destination on long conduit runs with access points along the way.

Create Conduit Schematics

Draw up as-built drawings showing conduit location and routing after installation.
Indicate dimensions from permanent objects to locate conduit for repairs.
Include conduit contents, size, and type on schematics.

Document Underground Utilities

Register underground conduit installation with local utilities.
Provides records of your system if future excavation occurs.
Consider private utility locating service to map conduits.

Note Maintenance Needs

Log any observations during installation that could affect conduit integrity.
Document conduit backfill compaction efforts or any difficult pull points.
Flag needs to routinely inspect or retest underground systems.

Mark Access Points

Place markers to indicate conduit system access locations.
Helps locate box covers that may get buried over time.
List contents and next access points on labels.

Troubleshooting Underground Conduit

Detect and resolve common conduit installation issues to maintain integrity:

Check for Backfill Settlement

Look for depressions in soil along conduit path which indicate settling.
Re-excavate to verify conduit integrity if significant subsidence found.
Backfill and compact properly before repaving to prevent future sinking.

Test for Moisture Intrusion

Pull accessible wires and check for corrosion every 3-5 years.
Use a fiber optic borescope to inspect for water in difficult to access conduit.
Measure resistance between conduit and grounding – lower indicates moisture.

Confirm Proper Sealing

Perform visual inspection along the conduit path to check for gaps at joints and penetrations.
Use sealant to close any openings that could allow water ingress.
Verify closure seals are still tight where conduits enter boxes.

Inspect for Damage

Carefully excavate areas showing electrical malfunctions or reduced capacity.
Check for cracked, deformed, or separated conduit sections.
Replace any damaged conduit runs as necessary.

Avoid Blockages When Pulling

Ensure wire pulls do not exceed maximum NEC bend radii.
Try a smaller mandrel first if significant friction is encountered.
Check for blockages like rocks or collapsed conduit if wire will not pass.

FAQs

How deep should underground conduit be buried?

The minimum burial depth is 18 inches for conduit under 2 inches. For conduit over 2 inches, minimum depth is 24 inches. Go deeper than minimums beneath roads or heavy vehicle paths.

What size conduit do I need for a 60 amp subpanel?

Use 1.5″ PVC conduit for a 60 amp, 240 volt subpanel, per NEC guidelines. This allows room for the feeder conductors, ground wire, and some spare space.

Can you bury Romex underground?

No. Romex and other NM cable is not rated for wet locations and can short out when buried. Individual THHN/THWN-2 conductors in conduit must be used underground instead.

How far can you run conduit before pulling wires?

NEC allows conduits over 2 inches to run up to 200 feet and under 2 inch conduit up to 100 feet before pulling conductors. Follow manufacturer guidance for maximum wire pulling distances.

Should underground conduit be installed below or above ground?

Below ground. Conduit exposed above ground risks physical damage. Buried conduit avoids hazards like digging, vehicles, freezing, and provides cleaner aesthetics.

What is the best type of conduit to use underground?

Schedule 40 PVC is preferred by many for underground installation. EMT metal conduit also works well, but avoid galvanized coating that may corrode over time when buried.

How do you prevent conduit from filling with water?

Use water-tight fittings at terminations, slope conduit back to source, install drip loops in wires, and place drain fittings at low points to evacuate moisture. Keep ends capped until ready for use.

Can you bury electrical wires without conduit?

In general, no. Individual conductors buried without conduit risk damage and ground faults. Direct burial cables like UF can substitute conduit in some applications. Check local codes.

How should I backfill a trench around underground conduit?

Backfill in layers using sand or screenings around conduit to avoid damage. Compact backfill material frequently as you go using hand or machine tamping tools for stability.

Where should I install pull points and junction boxes?

Install junction boxes where multiple conduit runs converge. Allow pull points every 100 feet on straight runs for easier conductor installation. Use pull boxes at major direction changes.

Conclusion

Installing underground conduit correctly is important for safety and performance of electrical systems. Planning your conduit layout, trenching appropriately, assembling and burying conduit with care, sealing entries, grounding effectively, labeling, and documentation will result in quality installation that conforms to code. Pay close attention to NEC requirements and manufacturer specifications for your products. Take time to troubleshoot issues like settling, moisture ingress, and damage to maintain the system over the long-term. Investing the proper effort into underground conduit installation will provide reliable infrastructure for your project.