For a 100-amp main service entrance, 2 AWG aluminum wire (XHHW-2) on a 100-amp breaker is code-compliant via the NEC 83% rule. However, for a 100-amp subpanel feeder, 2 AWG aluminum maxes out at 90 amps, requiring an upsize to 1 AWG aluminum for a full 100A breaker.
- Material: Aluminum (XHHW-2, THWN-2, or USE-2 insulation)
- Temperature Column: 75°C (Standard for modern residential breakers and panel lugs)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit/Raceway: Maximum of 3 current-carrying conductors bundled together (no derating applied)
- Voltage: 240V single-phase residential
The Critical Difference: Main Service vs. Subpanel Feeder
The most common mistake DIYers and junior apprentices make is treating a main service entrance and a subpanel feeder as electrically identical. They are not, and the National Electrical Code (NEC) treats them very differently.
When you buy a '100-Amp Service Entrance Kit' at a big-box hardware store, it typically includes 2 AWG aluminum wire (often configured as 2-2-2-4 SER or USE-2). This wire is perfectly legal for a 100-amp main service because of NEC Article 310.12 (formerly 310.15(B)(7)). This article contains the '83% Rule,' which recognizes that a 100-amp residential service rarely sees a continuous 100-amp load. Therefore, the code allows service entrance conductors to be sized at 83% of the service rating. For a 100A service, 100 x 0.83 = 83 amps. Since 2 AWG aluminum is rated for 90 amps in the 75°C column, it easily clears the 83-amp hurdle.
However, the 83% rule does not apply to subpanel feeders. If you are running power to a detached garage, a workshop, or a large addition, you are installing a feeder, not a service entrance. For a 100-amp subpanel feeder, the wire must be rated for the full 100 amps. Because 2 AWG aluminum is only rated for 90 amps, you must either downgrade the breaker to 90 amps (if your panel accepts a 90A breaker) or upsize the wire to 1 AWG aluminum, which is rated for exactly 100 amps at 75°C.
| Wire Size | 75°C Ampacity | Max Subpanel Breaker | Max Main Service Breaker (NEC 310.12) |
|---|---|---|---|
| 4 AWG Al | 65A | 60A | N/A (Too small for 100A svc) |
| 3 AWG Al | 75A | 70A | N/A (Too small for 100A svc) |
| 2 AWG Al | 90A | 90A | 100A (via 83% rule) |
| 1 AWG Al | 100A | 100A | 120A (via 83% rule) |
| 1/0 AWG Al | 120A | 120A | 150A (via 83% rule) |
Why 2 AWG and Not One Size Smaller?
You might wonder if 3 AWG aluminum could work for a 100A main service. It cannot. 3 AWG aluminum is rated for 75 amps at 75°C. Because 75A is less than the 83A minimum required by the 83% rule for a 100-amp service, 3 AWG is a code violation. 2 AWG (90A) is the absolute minimum size permitted for a 100-amp residential service entrance.
Voltage Drop Check: When 100 Feet Changes the Math
Ampacity tables assume the wire can handle the heat generated by the current. They do not account for the voltage lost over distance. NEC Article 215 recommends a maximum voltage drop of 3% for feeders and 5% overall for branch circuits and feeders combined.
Let's run a voltage drop calculation for our 2 AWG aluminum wire on a 100-amp main service, assuming a realistic continuous load of 80 amps over a 100-foot run from the utility pole to the main panel.
- Formula: VD = (2 × Length × Current × Resistance) / 1000
- Resistance (R): ~0.319 ohms per 1,000 feet for 2 AWG uncoated aluminum (AC resistance)
- Calculation: (2 × 100 ft × 80A × 0.319) / 1000 = 5.1 Volts
- Percentage: 5.1V / 240V = 2.12%
At 100 feet, 2 AWG aluminum performs beautifully, sitting well under the 3% threshold. But what if your service drop is 200 feet away from the transformer? The voltage drop doubles to 4.24%. While still under the 5% total system limit, it is high enough that sensitive electronics might experience brownouts during heavy appliance startups. For runs exceeding 150 feet, it is highly recommended to upsize to 1/0 AWG aluminum to maintain optimal power quality, regardless of what the ampacity tables say.
What Changes the Answer? (Derating and Edge Cases)
The ampacity table above represents 'ideal' conditions. Real-world jobsites rarely cooperate. Here is what forces you to upsize from 2 AWG aluminum.
1. Ambient Temperature (The Attic Trap)
NEC Table 310.16 is based on an ambient temperature of 30°C (86°F). If you are routing your service entrance conductors through an unconditioned attic in the American South or Southwest, summer ambient temperatures can easily exceed 110°F (43°C). According to the temperature correction factors at the bottom of Table 310.16, operating at 41-45°C requires a derating factor of 0.82 for 75°C rated wire. Your 90A 2 AWG wire suddenly drops to an ampacity of 73.8A—far below the 83A minimum for a 100A service. In hot attics, you must upsize to 1 AWG or 1/0 AWG, or reroute the wire through conditioned space.
2. Bundling and Conduit Fill
If you are pulling your service conductors through a conduit alongside other current-carrying circuits (like a solar PV inverter feed or a generator transfer switch), you trigger NEC Table 310.15(C)(1). If you have 4 to 6 current-carrying conductors in the same raceway, you must derate the ampacity to 80%. Again, this drops 2 AWG aluminum below the legal threshold for a 100A service, forcing an upsize.
3. Aluminum Termination Prep
Unlike copper, aluminum rapidly forms a non-conductive oxide layer when exposed to air. If you simply strip 2 AWG aluminum and shove it into a breaker lug, the connection will eventually overheat, expand, loosen, and potentially cause a fire. Proper installation requires three steps:
- Wire Brushing: Use a stainless-steel wire brush to remove the factory oxide layer from the exposed strands immediately before termination.
- Oxide Inhibitor: Coat the freshly brushed strands with an approved antioxidant compound (commonly known by the brand name Noalox). This seals out oxygen and prevents future oxidation.
- Precise Torque: NEC 110.14(D) strictly requires terminations to be torqued to the manufacturer's specifications. You cannot 'tighten it until it feels right.' Use a calibrated torque wrench or torque screwdriver. A typical 100A main breaker lug requires between 40 and 50 inch-pounds of torque, but always verify the exact value printed on the panel's wiring diagram label.
When to Pull in an Engineer or the AHJ
While the NEC provides a clear framework for standard residential construction, certain scenarios require a stamp from a Professional Engineer (PE) or a direct consultation with your local Authority Having Jurisdiction (AHJ).
- Continuous Heavy Loads: If the 100A panel will supply a continuous load (defined as operating for 3 hours or more) such as a commercial-grade server rack, a massive aquarium heating system, or dual Level 2 EV chargers. Continuous loads require conductors sized at 125% of the load, which invalidates the 83% service rule.
- Utility Transformer Limitations: If the local utility company limits the secondary voltage drop to 2% rather than the NEC's 3% recommendation, you will need an engineer to calculate the exact voltage drop at the transformer taps and size the wire accordingly.
- Mixed Material Lugs: If you are terminating 2 AWG aluminum into an older panel where the lugs are not explicitly marked 'AL' or 'AL/CU'. Modern panels are rated for aluminum, but legacy panels from the 1970s or earlier may only be rated for copper, requiring you to use copper wire or install a bimetallic lug adapter.
Sizing wire is never just about matching a breaker size to a chart. Understanding why 2 AWG aluminum works for a main service but fails for a subpanel feeder is the difference between a safe, code-compliant installation and a failed inspection. Always verify your local amendments to the National Electrical Code, as some municipalities strictly prohibit the 83% rule or mandate copper for all service entrances.
