100 Amp Direct Burial Wire Size: Copper vs Aluminum & NEC Rules

The correct 100 amp direct burial wire size is 3 AWG copper or 1 AWG aluminum for a standard subpanel feeder, or 4 AWG copper / 2 AWG aluminum if it is the main residential service drop, assuming 75°C terminations. Direct burial wire refers to underground-rated cable assemblies (like USE-2 or UF-B) engineered with moisture-resistant, sunlight-resistant insulation that can be laid directly into a trench without a protective conduit. In a real installation, choosing the right direct burial size dictates not just whether the breaker trips, but how much voltage is lost over distance and how much heat the surrounding soil can safely dissipate. The most common mistake DIYers and junior electricians make is confusing main service entrance conductors (which get a sizing discount under the NEC) with general feeder conductors (which do not).

The Core Theory: Ampacity, Insulation, and the Earth

When you bury a cable, the earth acts as a thermal blanket. Soil has a specific thermal resistivity, meaning it traps heat generated by the current flowing through the conductor. Because of this, the National Electrical Code (NEC) requires us to look closely at the insulation type of the direct burial cable, as it directly dictates the ampacity column we are allowed to use.

There are two primary types of direct burial cable you will encounter for a 100A run: USE-2 (Underground Service Entrance) and UF-B (Underground Feeder and Branch Circuit). They are not interchangeable when it comes to sizing.

• USE-2 Cable: This insulation is rated for 90°C in wet locations. However, because standard panel lugs and breaker terminals are rated for 75°C, NEC 110.14(C) forces us to use the 75°C ampacity column. USE-2 100A requires 3 AWG Cu / 1 AWG Al.
• UF-B Cable: Per NEC Article 339.5, the ampacity of UF-B cable is limited to the 60°C column of Table 310.16, regardless of its actual thermal rating. This severely restricts its current-carrying capacity. UF-B 100A requires 2 AWG Cu / 1/0 AWG Al.

Worked Numeric Example: Sizing a 100A Garage Subpanel

Ampacity gets you out of the panel, but voltage drop gets you to the load. Let's run a real-world calculation for a 100A subpanel feeding a detached garage workshop. The trench is 150 feet long, and we are using a 240V split-phase system.

While the panel is rated for 100A, a realistic maximum continuous load for a workshop (welder, compressor, lighting) is around 80A. We will size for an 80A continuous load to keep voltage drop under the NEC-recommended 3% for feeders.

The Setup:

• Wire: 1 AWG Aluminum USE-2 (Direct Burial)
• Circular Mils (CM): 83,690 (from NEC Chapter 9, Table 8)
• K-Factor (Aluminum): 21.2
• Distance (D): 150 feet (one way)
• Current (I): 80 Amps

A 2.53% drop is well under the 3% recommended limit for a feeder. If we had tried to use 2 AWG Aluminum (CM = 66,360), the drop would have been 3.19%, pushing us over the recommended threshold and potentially causing arc welders in the garage to underperform. For a 150-foot run, 1 AWG Aluminum USE-2 is the perfect choice. You can verify these figures using the Southwire Voltage Drop Calculator, which is an industry-standard reference tool.

Where You Meet This in Practice

Sizing the wire is only half the battle; getting it into the ground safely requires strict adherence to trenching and transition rules. When you are on the jobsite with a shovel or a trencher, these are the physical constraints you will meet.

Trenching Depths (NEC Table 300.5):

• Direct Burial USE-2: Requires a minimum of 24 inches of cover (measured from the top of the wire to the final grade).
• Conduit (PVC) Underground: If you pull individual THWN-2 wires through Schedule 40 PVC, the minimum cover drops to 18 inches.
• Warning Tape: You must lay a bright yellow or red 'Caution: Underground Electrical Line' warning tape exactly 12 inches above the cable. This ensures anyone digging in the future hits the tape before their shovel hits your 1 AWG aluminum.

The Transition Points:
Direct burial wire cannot simply emerge from the dirt and run up the side of your garage. It must be protected from physical damage. You must install a sweep elbow and a vertical piece of Schedule 80 PVC (or rigid metal conduit) from 18 inches below grade all the way up to the bottom of the subpanel enclosure. The USE-2 cable feeds through this conduit. Do not use tape or duct seal at the bottom of the conduit to 'keep water out'—underground conduit will fill with water, and your wire insulation is rated for wet locations.

Common Confusions: Service vs. Feeder Sizing

The most frequent argument on electrical forums revolves around NEC Article 310.12 (Single-Phase Dwelling Services and Feeders). This article provides a 'sizing discount' for residential service entrance conductors. Under 310.12, a 100A residential service drop only requires 4 AWG copper or 2 AWG aluminum.

Here is where people get it wrong: A subpanel feeder to a detached garage is not a service entrance, nor does it supply the entire load of the dwelling. Therefore, the 310.12 discount does not apply. You must use the standard 75°C ampacity table (310.16), which demands 3 AWG copper or 1 AWG aluminum. If an inspector sees 2 AWG aluminum feeding a 100A garage subpanel, they will fail the inspection and make you dig it up. Always default to the 310.16 values for outbuilding feeders. For a complete breakdown of NEC articles governing underground installations, refer to the National Fire Protection Association's NEC resources.