A 100 amp underground service wire is the buried conductor assembly that delivers up to 100 amperes of continuous electrical current from the utility transformer or meter to a building's main disconnect panel, specifically rated for direct burial or wet conduit environments. When you run power underground, the surrounding soil acts as a thermal blanket, which fundamentally changes how the wire dissipates heat compared to open-air runs, forcing you to select specific insulation types and often upsizing the gauge to prevent voltage drop over long trench runs.
Sizing 100 Amp Underground Service Wire: The Core Parameters
Sizing conductors for a 100-amp underground service entrance requires navigating two primary constraints: thermal ampacity and voltage drop. According to NEC Article 310, the minimum wire size for a 100A service is dictated by the 75°C column of the ampacity tables, assuming your panel terminals are rated for 75°C (which virtually all modern 100A main breaker panels are).
For a standard 240V split-phase residential service, you need two ungrounded (hot) conductors and one grounded (neutral) conductor. While #3 AWG Copper or #1 AWG Aluminum meets the bare minimum 100A ampacity requirement, underground installations rarely use the bare minimum. The thermal resistivity of soil and the physical distance of the trench almost always force an upsizing to maintain power quality.
| Conductor Material | AWG Size | Base Ampacity (75°C) | Resistance (Ohms/1000ft) | Best Application Scenario |
|---|---|---|---|---|
| Copper (THWN-2 in conduit) | #3 AWG | 100A | 0.245 | Short runs (<40 ft), high budget, strict space limits |
| Copper (THWN-2 in conduit) | #2 AWG | 115A | 0.194 | Medium runs (40-80 ft) to mitigate voltage drop |
| Aluminum (USE-2 Direct Burial) | #1 AWG | 100A | 0.201 | Standard direct burial, highly cost-effective baseline |
| Aluminum (USE-2 Direct Burial) | #1/0 AWG | 120A | 0.159 | Long trench runs (>100 ft) to limit voltage drop to <3% |
Cost Note: As of early 2026, #1/0 AWG Aluminum USE-2 cable costs roughly $1.80 to $2.20 per foot, whereas #2 AWG Copper THWN-2 can exceed $4.50 per foot. This massive price delta is why aluminum dominates underground service installations.
The Physics of the Trench: Voltage Drop and Thermal Constraints
What changes in a real circuit when you move from an overhead drop to an underground trench? The primary variable is thermal dissipation. Overhead wires are cooled by ambient air convection. Underground wires are encased in soil (or trapped inside a PVC conduit filled with stagnant air and moisture), which retains heat. Furthermore, underground runs are inherently longer because they must travel down the utility pole, trench horizontally across the property, and climb up into the meter base.
This extra length introduces voltage drop. The NEC recommends a maximum 3% voltage drop on service conductors to ensure appliances receive adequate voltage. Let us run a worked numeric example to see why the bare minimum #1 AWG Aluminum often needs to be upsized.
Worked Example: Voltage Drop on a 150-Foot Underground Run
Scenario: You are running a 100A underground service to a new workshop. The trench is 150 feet long. You plan to use #1 AWG Aluminum USE-2 wire on a 240V system.
- Max Continuous Load (I): 80A (A 100A breaker allows 80A of continuous load).
- One-Way Distance (D): 150 feet.
- Wire Resistance (R): 0.201 ohms per 1000 ft (for #1 AWG Al at 75°C).
Calculation:
Voltage Drop (VD) = 2 × I × R × (D / 1000)
VD = 2 × 80A × 0.201Ω × (150 / 1000)
VD = 160 × 0.201 × 0.15 = 4.82 Volts
Percentage Drop: (4.82V / 240V) × 100 = 2.01%
Verdict: At 2.01%, this is well under the 3% NEC recommendation. #1 AWG Aluminum is perfectly acceptable here. However, if the trench were 250 feet long, the drop would hit 3.35%, forcing you to upsize to #1/0 AWG Aluminum to maintain power quality for heavy inductive loads like welders or air compressors.
Where You Meet This in Practice (and Common Confusions)
You will typically encounter 100 amp underground service wire when connecting a new residential build to a pad-mounted transformer, running power to a detached garage with its own 100A main disconnect, or upgrading an older 60A overhead farm service to a modern underground loop. In these scenarios, the physical installation details are just as critical as the wire gauge.
When working at the workbench or in the trench, DIYers and junior apprentices frequently fall into three specific traps:
1. Confusing USE-2 with THWN-2 Insulation
This is the most common and dangerous mistake. USE-2 (Underground Service Entrance) cable has a thick, moisture-resistant, sunlight-resistant cross-linked polyethylene (XLPE) jacket designed to be direct buried in the dirt without conduit. THWN-2 is rated for wet locations, but it lacks the physical abrasion resistance and chemical protection to survive direct burial. If you buy THWN-2, it must be pulled through a continuous PVC conduit (typically Schedule 40 underground, transitioning to Schedule 80 where it emerges above grade to protect against weed whackers and physical damage).
2. Service Conductors vs. Subpanel Feeders
People frequently confuse a service with a feeder. A 100A underground service runs from the utility transformer to the first point of disconnect (the main panel). It only requires three wires: two hots and a neutral. The neutral is bonded to ground at the main disconnect, and a local ground rod handles the earth reference. However, if you are running a 100A feeder from an existing main panel to a detached garage subpanel, NEC Article 250 requires a 4-wire setup: two hots, a neutral, and a separate Equipment Grounding Conductor (EGC). You cannot use the neutral as a ground for a subpanel.
3. The 60°C vs. 75°C Ampacity Column
Older electrical equipment (pre-1990s) often has terminals rated only for 60°C. If you are tying into legacy gear, you must use the 60°C column, which would require #1 AWG Copper or #1/0 AWG Aluminum for 100A. Modern 100A main breaker panels are universally rated for 75°C, allowing you to use the smaller, more economical sizes listed in the table above. Always verify the terminal temperature rating printed on the panel label.
FAQ: Underground Service Entrance Nuances
How deep does 100 amp underground service wire need to be buried?
According to NEC Table 300.5, the minimum cover requirement for direct-buried USE-2 cable (rated 0-600V) is 24 inches. If you are running the wire inside a PVC conduit, the minimum depth drops to 18 inches. If the trench is under a concrete slab or a residential driveway, the depth requirements can be reduced to 18 inches for direct burial and 12 inches for rigid metal conduit, but local Authority Having Jurisdiction (AHJ) rules always supersede baseline NEC minimums.
Do I need to use anti-oxidant paste on aluminum service wire?
Yes. Aluminum naturally forms a non-conductive oxide layer when exposed to air, which increases resistance and causes terminal lugs to overheat. Whenever you terminate #1 AWG or #1/0 AWG aluminum wire into a panel's main breaker or meter lugs, you must wire-brush the conductor, coat it with an anti-oxidant compound (like Noalox or Penetrox), and torque the lug to the manufacturer's exact inch-pound specification using a calibrated torque screwdriver or wrench.
Can I parallel two smaller wires to get 100 amps?
No. NEC Section 310.10(G) strictly prohibits paralleling conductors smaller than 1/0 AWG. You cannot run two sets of #4 AWG wire in parallel to achieve 100A capacity. Paralleling is only permitted for 1/0 AWG and larger, and it requires identical lengths, materials, and routing methods for every conductor in the parallel set. For a 100A service, simply pull the single #1 AWG or #1/0 AWG conductor.
