How deep must I dig a trench for Schedule 40 PVC conduit?

Per NEC Table 300.5, rigid nonmetallic conduit (Schedule 40 or 80 PVC) requires a minimum burial depth of 18 inches to the top of the conduit in residential installations.

Can I pull NM-B Romex wire through underground conduit?

No. Under NEC 300.5(B), any conduit installed underground is classified as a wet location. NM-B Romex has paper wrapping that absorbs moisture and deteriorates, creating a severe fire and ground-fault hazard. You must use individual wet-rated conductors like THWN-2.

What size conduit should I run for a 50A or 60A EV charger?

While 3/4-inch PVC can technically fit four 6 AWG THWN wires within 40% fill limits, you should always run at least 1-inch or 1-1/4-inch Schedule 40 PVC over long trench runs to eliminate wire binding and simplify pulling through 90-degree sweeps.

Technically Audited & Authored ByElena Vance, PELead Electrical Systems Engineer & Master Electrician

Running Conduit to a Detached Garage for an EV Charger

Adding an EV charger to a detached garage is a fantastic upgrade, but getting the power from your main electrical panel to the outbuilding requires careful planning. Trenching and running underground conduit must strictly follow the National Electrical Code (NEC) to ensure the safety of your property and the longevity of the installation.

Quick Answer:
To run electrical wire to a detached garage, you must dig a trench. For standard Schedule 40 PVC conduit, the NEC requires a minimum burial depth of 18 inches. If you use Rigid Metal Conduit (RMC), you only need to go 6 inches deep. Most importantly, you cannot run standard indoor Romex (NM-B) wire underground; you must use individual THWN-2 water-resistant wires.

Table of Contents

1. NEC Trench Depths: How Deep Must You Dig?
2. The Golden Rule: No Romex Underground
3. Sizing Your Conduit for EV Charging
4. Pro Tips for a Smooth Trenching Project

1. NEC Trench Depths: How Deep Must You Dig?

Safety First: Call Before You Dig
Always call 811 (or your local utility location service) a few days before you start trenching. Striking a buried gas or high-voltage utility line can be fatal.

The NEC (Table 300.5) dictates how deep your trench must be depending on the type of conduit you choose. The measurement is from the top of the finished grade to the top of the conduit.

Schedule 40 PVC (Most Common): Minimum depth of 18 inches. This is the standard for DIYers and most residential applications.
Schedule 80 PVC: Minimum depth of 18 inches. Schedule 80 is thicker and is often required where the conduit emerges from the ground to protect against physical damage (like being hit by a lawnmower).
Rigid Metal Conduit (RMC): Minimum depth of 6 inches. While digging is much easier, the metal pipe is incredibly expensive and difficult to cut and thread.
Direct Burial Cable (UF-B): Minimum depth of 24 inches. We do not recommend this for EV chargers, as replacing the cable requires digging up the entire yard again.

2. The Golden Rule: No Romex Underground

This is the #1 mistake homeowners make. You cannot run standard yellow or orange indoor NM-B cable (commonly known as Romex) inside an underground conduit.

Comparison showing approved THWN-2 individual wires versus unapproved NM-B Romex inside an underground PVC conduit.

Why? Because the NEC defines the inside of an underground conduit as a “wet location.” No matter how well you glue the PVC joints, condensation will eventually form, and the conduit will fill with water. Standard NM-B wire has a paper wrapper inside that acts like a sponge, eventually rotting the insulation and causing a short circuit.

The Solution: You must pull individual stranded wires rated for wet locations, primarily THWN-2. When sizing these wires for an EV charger, make sure you understand the 125% continuous load rule and reference our 6 AWG vs 4 AWG sizing guide to ensure you buy the correct gauge copper.

3. Sizing Your Conduit for EV Charging

Running a hardwired Level 2 charger typically requires three conductors (two hots and one ground). If you are running a 60-Amp circuit to your detached garage using 6 AWG THWN-2 copper wires, you might think a small ¾-inch conduit is fine.

While it might technically meet NEC fill capacity limits, pulling thick 6 AWG wire through ¾-inch pipe over a long distance is a nightmare. Always oversize your conduit. We strongly recommend using at least 1-inch to 1 ¼-inch PVC. The minor increase in pipe cost will save you hours of physical labor and frustration when pulling the wires.

4. Pro Tips for a Smooth Trenching Project

Avoid Sharp 90° Elbows: Underground, try to use sweeping bends (large radius elbows) instead of standard sharp 90-degree fittings. Every sharp turn adds massive friction when pulling heavy EV cables.
Install Warning Tape: Once the conduit is laid, backfill about 6 inches of dirt, then lay a strip of red “CAUTION: BURIED ELECTRICAL LINE” tape along the entire trench before filling it to the top. This protects future homeowners who might dig there.
Leave a Pull String: Always use a shop-vac to suck a lightweight string (often called a “mouse”) through the empty pipe, then use that to pull a heavy nylon pulling rope, which is then used to pull your wires.
Keep it Clean: Before pulling wire, make sure the inside of the conduit is clean of dirt and PVC glue residue, and use plenty of electrical wire pulling lubricant.

Final Verdict: Digging the trench is hard work, but getting it right the first time guarantees a safe, code-compliant path to power the EV charger in your detached garage for decades to come.