100 amp underground wire refers to the specific gauge, material, and wet-location-rated insulation (like XHHW-2 or THWN-2) required to safely carry a 100-ampere load through a buried conduit or direct-burial trench without exceeding thermal limits or suffering excessive voltage drop. This single specification changes everything about your installation: it dictates your trench depth, conduit diameter, breaker sizing, and total material cost. Most DIYers and junior electricians confuse basic indoor wire (THHN) with wet-rated underground wire, or they size strictly for the breaker's ampacity while completely ignoring voltage drop over long trench runs, leading to tripped breakers and damaged equipment.

The Core Theory: Ampacity vs. Voltage Drop in Buried Feeders

To size a feeder correctly, you must satisfy two separate physics constraints: thermal limits (ampacity) and resistance over distance (voltage drop). Ampacity is the maximum current a wire can carry before its insulation degrades, governed by NEC Article 310. Voltage drop is the loss of electrical pressure due to conductor resistance over the length of the run.

The Garden Hose Analogy: Think of electricity like water in a long garden hose. The pump (your 100-amp breaker) can push 100 gallons a minute, but friction in a 200-foot hose drops the pressure at the nozzle. To maintain pressure (voltage) at the far end without changing the pump, you must widen the hose diameter (upsized wire gauge).

A Worked Numeric Example: The 150-Foot Trap

Let us calculate the voltage drop for a 100A load at 240V over a 150-foot one-way underground run using 3 AWG Copper wire. According to the Cerrowire Ampacity Charts, 3 AWG copper is rated for exactly 100A at 75°C. It meets the thermal requirement, but does it meet the voltage drop requirement?

  • Formula: VD = (2 × K × I × D) / CM
  • Variables: K (Copper) = 12.9, I = 100A, D = 150ft, CM (3 AWG) = 52,620 circular mils.
  • Calculation: (2 × 12.9 × 100 × 150) / 52,620 = 7.35 Volts dropped.
  • Percentage: 7.35V / 240V = 3.06%.

The NEC recommends a maximum 3% voltage drop for feeders (NEC 210.19(A) Informational Note). At 150 feet, 3 AWG copper pushes you to 3.06%, technically exceeding the recommendation. To stay strictly under 3%, you must upsize to 2 AWG Copper, which drops the loss to 2.4%. This is where blindly trusting a basic ampacity chart leads to undersized, inefficient installations.

Insulation Types: Why "Wet Location" Dictates Your Wire

Underground conduits flood. Even if your PVC joints are perfectly glued, temperature fluctuations cause condensation to build up inside the pipe. Under NEC Article 100, the interior of an underground raceway is classified as a wet location.

Critical Mistake: Standard THHN wire is only rated for dry and damp locations. If you pull THHN into a buried conduit that inevitably collects water, the insulation will absorb moisture, break down, and cause a ground fault or short circuit. You must use THWN-2 or XHHW-2, both of which carry a 90°C wet-location rating.

While direct-burial cables like UF-B exist, they are rarely used for 100A feeders. UF-B maxes out at smaller gauges, requires a wider trench, and cannot be easily upgraded later. The industry standard for a 100A underground run is individual THWN-2 or XHHW-2 conductors pulled through Schedule 40 or 80 PVC conduit.

Where You Meet This in Practice

You will encounter the 100A underground wire requirement in specific high-load residential and light-commercial scenarios:

  • Detached Garages with EV Chargers: A Level 2 EV charger pulls 40A to 50A continuously. Add garage lighting, a refrigerator, and power tools, and a 60A subpanel will constantly trip. A 100A feeder is the modern baseline.
  • Workshops and Barns: Running a 240V MIG welder or a large air compressor requires the headroom a 100A panel provides.
  • Hot Tub Subpanels: While many hot tubs only require 50A or 60A, long trench runs to a backyard pad often necessitate a 100A feeder to offset voltage drop, feeding a local 60A GFCI breaker.

Real-World Failure Mode: A homeowner uses 2 AWG Aluminum UF-B direct burial cable to save on conduit costs. Because UF-B cannot be easily spliced or pulled through bends, when they later decide to add a 50A RV receptacle at the same location, they have to dig a completely new trench because the direct burial cable cannot be rerouted or extended through a junction box easily. Conduit prevents this.

Decision Tree: Sizing Your 100 Amp Underground Wire

Use this decision path to select your exact wire gauge and material based on your trench length. This assumes a standard 120/240V single-phase split system with a maximum 3% voltage drop target.

Trench Length (One-Way) Copper Wire Size (THWN-2) Aluminum Wire Size (XHHW-2) Minimum PVC Conduit Size
Under 75 feet 3 AWG 1 AWG 1.25 inch
75 to 120 feet 2 AWG 1/0 AWG 1.5 inch
120 to 180 feet 1 AWG 2/0 AWG 1.5 inch
Over 180 feet 1/0 AWG 3/0 AWG 2 inch
The Default Pick: For 90% of residential runs under 120 feet, buy 1/0 AWG Aluminum XHHW-2 and pull it through 1.5-inch Schedule 40 PVC. Aluminum is roughly 40% of the cost of copper per foot, XHHW-2 is highly water-resistant and slicker for pulling, and 1/0 AWG provides 120A of thermal headroom while keeping voltage drop well under 3%.

Installation Caveats and Grounding Rules

Sizing the current-carrying conductors is only half the battle. The NEC has strict rules for the neutral and grounding conductors in a 100A underground feeder.

Neutral Sizing (NEC 220.61)

Your neutral (white or gray wire) must be sized to carry the maximum unbalanced load. In a standard detached garage with mostly 240V loads (EV charger, welder), the neutral carries very little current. However, for a standard 100A feeder, inspectors typically require the neutral to be the same size as the hot legs, or at minimum 2 AWG Aluminum, to handle 120V lighting and receptacle loads safely.

Equipment Grounding Conductor (EGC)

Under NEC 250.122, a 100A breaker requires a minimum 8 AWG Copper or 6 AWG Aluminum equipment grounding conductor. Do not rely on the conduit as your ground; pull a dedicated insulated green wire. Furthermore, if you are feeding a detached structure, NEC 250.32 requires you to install a local grounding electrode system (typically two ground rods spaced 6 feet apart) at the subpanel, and the neutral and ground bars in the subpanel must remain strictly isolated.

Frequently Asked Questions

Can I use 2 AWG Aluminum for a 100 amp breaker?

Yes, but it is oversized for the ampacity. 2 AWG Aluminum is rated for 90A at 75°C, which means it cannot be used on a 100A breaker unless the load calculation proves the actual continuous load is under 90A. To safely use a 100A breaker, you must step up to 1 AWG Aluminum (100A rating) or 1/0 AWG Aluminum (120A rating).

Do I need to bury the conduit 18 or 24 inches deep?

NEC Table 300.5 dictates burial depths. For rigid metal conduit (RMC) or intermediate metal conduit (IMC), the minimum cover is 6 inches. For PVC conduit (the most common DIY choice), the minimum cover is 18 inches for residential branch circuits and feeders. If you use direct-burial UF-B cable without conduit, it must be buried at least 24 inches deep.

Can I mix copper and aluminum in the underground run?

You can, but only if you use proper transition lugs rated for both metals (marked CU/AL). Never splice copper and aluminum inside a buried conduit using standard wire nuts, as galvanic corrosion will cause a high-resistance fault. It is vastly easier to pull aluminum all the way from the main panel to the subpanel and use aluminum-rated lugs on both ends.