How to Size 100 Amp Underground Wire for Subpanels and Outbuildings

100 amp underground wire refers to the specific gauge, material, and insulation rating of electrical conductors buried in a trench to safely deliver a 100-ampere load from a main service panel to a detached structure without exceeding thermal limits or allowable voltage drop. Selecting the correct wire changes your physical trench depth, conduit diameter, and pulling lubrication requirements compared to standard indoor NM-B (Romex) runs. Most DIYers confuse the breaker rating (100A) with the continuous load limit (80A), and mistakenly buy direct-burial UF-B cable when they actually need individual THWN-2 or XHHW-2 conductors pulled through a conduit.

Sizing 100 Amp Underground Wire: The Core Rules

When sizing conductors for a 100-ampere feeder, you must reference the 75°C column of the NEC ampacity tables (Table 310.16), as most modern breakers and subpanel lugs are rated for 75°C terminations. You also need a 4-wire feed: two ungrounded (hot) conductors, one grounded (neutral) conductor, and one equipment grounding conductor (EGC).

Always size the equipment grounding conductor (EGC) per NEC 250.122 based on the breaker size, not the ungrounded conductor size. For a 100A breaker, that means a minimum #8 AWG copper or #6 AWG aluminum ground wire.

While copper is the traditional standard, aluminum building wire has become the default for 100 amp underground wire runs due to severe copper price volatility. Modern AA-8000 series aluminum alloy is perfectly safe for subpanel feeders, provided you use anti-oxidant paste (like Noalox) on the terminations and torque the lugs to the manufacturer's exact inch-pound specifications.

Direct Burial vs. Conduit: What Changes in the Trench

You have two physical methods for burying your feeder: direct burial cable or individual conductors in a raceway. This choice dictates your trench depth and future-proofing.

Direct Burial (USE-2 or UF-B)

Direct burial cables are jacketed to withstand soil acidity and moisture. Under NEC Table 300.5, direct burial cables for a 100A residential feeder must be buried at least 24 inches deep. While this saves you the cost of PVC conduit, it is a permanent installation. If you later decide to upgrade to a 200A subpanel for a heavy-duty welder, you will be digging a brand new trench.

Conduit (Schedule 40/80 PVC with THWN-2)

Pulling individual THWN-2 or XHHW-2 wires through 1.5-inch or 2-inch PVC conduit allows you to bury the line at just 18 inches deep (or 12 inches if you encase it in 2 inches of concrete).

Where You Meet This in Practice (and the 80% Trap)

You will typically spec 100 amp underground wire for detached garages, backyard workshops, barns, or driveway EV charging pedestals. In these scenarios, the most common point of failure is ignoring the continuous load rule.

Under NEC Article 210.20, if a load is expected to run for 3 hours or more (like an EV charger, a space heater, or a commercial compressor), it is considered a continuous load. You must derate the wire and breaker by 125% (or multiply the continuous load by 1.25).

Worked Scenario: The 250-Foot Garage Run That Failed

To understand why ampacity tables aren't the whole story, let's look at a real-world failure involving voltage drop.

• Setup: A homeowner ran power to a detached workshop 250 feet away to run a 60A plasma cutter and 20A of continuous lighting/heating. They installed a 100A breaker and pulled #3 AWG Copper THWN-2 (rated exactly 100A at 75°C).
• Numbers: The total continuous load was 80A. The one-way distance was 250 feet. Using the voltage drop formula VD = (2 × K × I × D) / CM (where K=12.9 for copper, I=80A, D=250ft, and CM=52,620 for #3 AWG), the calculated voltage drop is 9.8 Volts.
• Outcome: On a 240V circuit, a 9.8V drop represents a 4.08% voltage drop. When the plasma cutter fired up, the voltage at the receptacle sagged to 218V. The cutter struggled to strike an arc, and the 100A breaker ran uncomfortably hot to the touch on 90°F summer days.
• What Went Wrong: The installer sized the wire strictly for thermal ampacity (NEC 310.16) and ignored voltage drop recommendations (NEC 310.15(B) Informational Note), which suggest a max 3% drop on feeders. Furthermore, they didn't account for thermal derating; a shallow trench baking in the sun raises the ambient soil temperature above the 30°C baseline used in the ampacity tables.
• The Fix: For a 250-foot run with an 80A continuous load, the wire should have been upsized to #1 AWG Copper or #2/0 AWG Aluminum to keep the voltage drop under 3% and provide a thermal buffer for the trench environment.

Step-by-Step Trench and Pull Guide

Once you have calculated your wire size and purchased your materials, follow this sequence to ensure a code-compliant installation. Always call 811 before digging to have utility lines marked.

1. Dig and Bed the Trench: Dig to the required depth (18 inches for PVC conduit). Lay down a 2-inch bed of sifted sand or soft dirt to protect the conduit from sharp rocks.
2. Lay the Conduit: Assemble your Schedule 40 PVC using PVC primer and cement. Use sweeping 90-degree bends (LB or LL fittings) at the ends rather than sharp elbows to reduce pulling friction. Glue all joints to make them watertight.
3. Install a Warning Ribbon: Lay a bright red or yellow "Caution: Buried Electric Line Below" plastic ribbon about 12 inches above the conduit. This warns anyone digging in the future before their shovel hits the PVC.
4. Pull the Conductors: Feed a fiberglass fish tape or pull string through the conduit. Attach your 4 wires to the pull string using a pulling eye or electrical tape, and coat the wires generously with a wire-pulling lubricant like Polywater. Pull steadily; do not yank.
5. Terminate and Torque: Strip the wires and land them in the main panel and subpanel lugs. If using aluminum, brush the exposed conductor with anti-oxidant compound. Use a calibrated torque screwdriver or torque wrench to tighten the lugs to the exact inch-pound value printed on the panel's wiring diagram.
6. Verify and Test: Before energizing, use a multimeter to check for continuity and ensure there are no short circuits between the hots, neutral, and ground. Energize the breaker and measure the voltage at the subpanel lugs to confirm your voltage drop calculations match reality.