Direct burial wire for a 100 amp service is an underground-rated cable assembly—typically USE-2, MHF, or UF-B—designed to carry 100 amperes of current directly through soil without the protection of a raceway or conduit. Choosing the correct direct burial cable changes your installation by eliminating the material cost and pull-friction of PVC conduit, but it mandates strict adherence to trench depth and specific insulation ratings to resist soil moisture and chemical degradation. The most common mistake DIYers and junior electricians make is confusing UF-B (Underground Feeder) with USE-2 (Underground Service Entrance) or MHF (Mobile Home Feeder). They mistakenly buy UF-B for a 100A main service, not realizing UF-B is legally limited to the 60°C ampacity column per NEC 339.3 and is rarely manufactured in the large gauges needed for 100A without severe voltage drop or physical stiffness.
The Core Specs: AWG, Material, and Insulation Type
When sizing conductors for a 100-amp service or subpanel feeder, you must follow NEC 110.14(C), which requires you to use the 75°C column of NEC Table 310.16 for terminations rated at 75°C (which covers virtually all modern breakers and panel lugs). While the wire insulation itself might be rated for 90°C (like XHHW-2 or USE-2), the ampacity is bottlenecked by the terminal temperature rating.
For a standard 100-amp non-continuous load, the absolute minimum wire size is 3 AWG Copper or 1 AWG Aluminum. However, aluminum is the industry standard for direct burial service entrance and feeder cables due to its lower cost and lighter weight, despite requiring a larger gauge.
| Material | Minimum AWG (75°C Col) | Ampacity at 75°C | Approx. Cost per 100ft (2026) | Best Use Case |
|---|---|---|---|---|
| Copper (THHN/UF-B) | 3 AWG | 100A | $350 - $450 | Short runs, high corrosion areas |
| Aluminum (USE-2/MHF) | 1 AWG | 120A | $110 - $150 | Standard residential 100A feeders |
| Aluminum (USE-2/MHF) | 2/0 AWG | 150A | $180 - $240 | Long runs requiring voltage drop mitigation |
Worked Numeric Example: Voltage Drop at 200 Feet
Ampacity tables only tell you what size wire will prevent a fire. They do not account for voltage drop over distance. For a 100-amp subpanel located 200 feet from the main service, pushing 100A through the minimum 1 AWG Aluminum wire will result in unacceptable voltage drop, causing lights to dim and motors to overheat.
Let us calculate the single-phase voltage drop using the standard formula: VD = (2 × L × I × R) / 1000, where L is one-way length in feet, I is current in amps, and R is the AC resistance per 1,000 feet from NEC Chapter 9, Table 8.
- Scenario A: 1 AWG Aluminum (Minimum Code Size)
Resistance (R) for 1 AWG Al = 0.247 Ω/kft.
VD = (2 × 200 × 100 × 0.247) / 1000 = 9.88V.
Percentage Drop = (9.88 / 240) × 100 = 4.11%.
Result: Exceeds the NEC recommended 3% maximum for branch circuits/feeders. - Scenario B: 2/0 AWG Aluminum (Upsized for Distance)
Resistance (R) for 2/0 AWG Al = 0.194 Ω/kft.
VD = (2 × 200 × 100 × 0.194) / 1000 = 7.76V.
Percentage Drop = (7.76 / 240) × 100 = 3.23%.
Result: Marginally over 3%, but perfectly acceptable for a combined feeder + branch circuit allowance of 5%. - Scenario C: 4/0 AWG Aluminum (Strict 3% Compliance)
Resistance (R) for 4/0 AWG Al = 0.122 Ω/kft.
VD = (2 × 200 × 100 × 0.122) / 1000 = 4.88V.
Percentage Drop = (4.88 / 240) × 100 = 2.03%.
Result: Ideal for sensitive electronics or if the subpanel will feed long secondary branch circuits.
For a 200-foot run, stepping up from the minimum 1 AWG to 2/0 AWG Aluminum USE-2 or MHF is the most cost-effective sweet spot, balancing material cost with reliable voltage delivery.
Where You Meet This in Practice
On the jobsite, direct burial wire changes your workflow from pulling and gluing conduit to trenching and backfilling. According to NEC Article 300.5 and local utility requirements, the physical installation dictates the longevity of the cable.
- Trench Depth: For standard residential 100A service entrance or feeder cables (USE-2 or MHF) without a metal sleeve or concrete encasement, the minimum cover requirement is 24 inches. If you are using UF-B (which is rare for 100A), the minimum depth is also 24 inches for residential branch circuits and feeders unless GFCI protected, but service entrance rules dictate 24 inches regardless.
- Bedding: Never lay direct burial wire directly on rocky soil. Dig the trench 2 inches deeper than your required cover and fill the bottom with a bed of clean, fine sand. This prevents sharp rocks from piercing the XLP insulation over time as the soil settles and freezes.
- Warning Tape: Lay a continuous strip of bright yellow or red "Caution: Underground Electrical Line" warning tape exactly 12 inches above the buried cable. If someone digs in the yard five years from now, the shovel will hit the tape before it hits the 100A live conductors.
- Backfill: Backfill the trench with the native soil, but pick out any large rocks or debris. Tamp the soil down in 6-inch lifts to prevent future trench settling, which can create a trip hazard or pull the wire taut against the panel lugs.
Frequently Asked Questions
How deep does direct burial wire need to be for a 100 amp service?
For a 100-amp underground service entrance or feeder using USE-2, MHF, or UF-B cable, the NEC requires a minimum burial depth of 24 inches (measured from the top of the cable to the final grade). If the cable passes under a concrete slab or driveway, the depth can be reduced to 18 inches, provided the cable is protected by a metal sleeve or rigid metal conduit (RMC) where it emerges from the ground up to the termination point (minimum 8 feet above grade or to the point it enters the building).
Can I use UF-B cable for a 100 amp subpanel?
Technically yes, but it is highly discouraged and often impractical. UF-B (Underground Feeder and Branch-Circuit Cable) is restricted by NEC 339.3 to the 60°C ampacity column, regardless of its 90°C insulation rating. To carry 100A at 60°C, you would need massive 1 AWG Copper or 1/0 AWG Copper (aluminum UF-B is virtually non-existent in these sizes). This cable is incredibly expensive, stiff, and difficult to strip. For a 100A subpanel, individual USE-2 conductors or a bundled MHF (Mobile Home Feeder) cable rated for the 75°C column is the correct, cost-effective choice.
What is the difference between MHF and USE-2 direct burial wire?
Both are rated for direct burial and use similar XLP (cross-linked polyethylene) insulation, but their physical construction differs. USE-2 (Underground Service Entrance) consists of individual, single-conductor cables that you must lay out and tape or zip-tie together in the trench. MHF (Mobile Home Feeder) is a pre-assembled, bundled cable containing two hot wires, a neutral, and a ground, twisted or bound together in a single jacket-like assembly. MHF is much faster to pull and lay in a trench for a 100A subpanel because it acts as a single unit, whereas USE-2 requires you to manage four separate heavy wires in the dirt. According to major wire manufacturers like Southwire, MHF is specifically engineered for the convenience of subpanel and mobile home feeders, making it the preferred choice for DIYers and electricians alike.
