For a standard 100-amp residential service or subpanel feeder, you need 1 AWG aluminum wire (specifically XHHW-2 or THHN) protected by a 100A breaker. This assumes 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway. Never use 2 AWG aluminum; it is only rated for 90 amps at 75°C.
- Material: Aluminum (AA-8000 series building wire, per NEC 310.14)
- Temperature Column: 75°C (Standard for modern panelboard and breaker lugs)
- Ambient Temperature: 30°C (86°F)
- Raceway: PVC Schedule 80 or EMT conduit, maximum 3 current-carrying conductors
- System Voltage: 240V single-phase split service
The Ampacity Table: Why 1 AWG Aluminum?
To understand why 1 AWG is the mandatory minimum for a 100-amp aluminum feeder, we have to look at the intersection of wire insulation ratings and equipment termination limits. Modern aluminum building wire (like XHHW-2 or THHN) is manufactured with 90°C rated insulation. However, NFPA 70 (NEC) Section 110.14(C) dictates that you must size the wire based on the temperature rating of the terminations (the lugs), not the wire itself. Almost all residential and light commercial breakers and panel lugs are rated for 75°C.
Below is the critical ampacity data for aluminum conductors. Notice how the 75°C column dictates the actual allowable ampacity for your installation.
| AWG Size (Aluminum) | 60°C Column (Amps) | 75°C Column (Amps) | 90°C Column (Amps) | Standard Breaker Match |
|---|---|---|---|---|
| 3 AWG | 65A | 75A | 85A | 70A or 80A |
| 2 AWG | 75A | 90A | 100A | 90A |
| 1 AWG | 100A | 120A | 130A | 100A or 110A |
| 1/0 AWG | 120A | 135A | 150A | 125A |
Why Not 2 AWG Aluminum?
A common and dangerous mistake is looking at the 90°C column, seeing that 2 AWG aluminum is rated for 100A, and pulling it through the conduit. This violates NEC 110.14(C). At the 75°C termination limit, 2 AWG aluminum is only good for 90 amps. While NEC 240.4(B) allows you to round up to the next standard breaker size if a wire's ampacity falls between standard sizes, 90A is already a standard breaker size (per NEC 240.6). Therefore, you cannot round up to 100A. You must use a 90A breaker for 2 AWG, or step up to 1 AWG aluminum for a 100A breaker.
Voltage Drop and Distance Adjustments
Ampacity tables only tell you what size wire will prevent the insulation from melting and the breaker from tripping. They do not account for voltage drop over distance. The NEC recommends a maximum 3% voltage drop for branch circuits and feeders to ensure equipment operates efficiently and motors don't overheat.
Let's run a voltage drop check for a 150-foot underground feeder from a main panel to a detached garage subpanel, carrying a full 100A load at 240V.
- Formula: VD = (2 × K × I × L) / CM
- K (AC resistance for Aluminum): ~21.2
- I (Current): 100A
- L (One-way Length): 150 ft
- CM (Circular Mils for 1 AWG): 83,690
Calculation: (2 × 21.2 × 100 × 150) / 83,690 = 7.6 Volts.
Percentage: 7.6V / 240V = 3.16%.
Derating, Bundling, and Termination Limits
The baseline 1 AWG answer assumes ideal conditions. Real-world jobsite variables will force you to adjust your wire size. Aluminum and copper are not interchangeable; while 3 AWG copper achieves 100A at 75°C, aluminum requires 1 AWG due to lower conductivity. Never substitute one for the other without recalculating conduit fill and verifying lug compatibility.
Bundling and Conduit Fill (NEC 310.15(C)(1))
If you pull more than three current-carrying conductors (CCCs) through a single raceway, the heat generated by the bundled wires cannot dissipate. You must apply a derating factor to the 90°C column of the wire. If you run four CCCs in a conduit, you must derate to 80%. For 1 AWG aluminum, the 90°C rating is 130A. 130A × 0.80 = 104A. Because 104A is still above your 100A breaker, 1 AWG survives this bundling scenario. However, if you bundle 5 or 6 CCCs (derating to 70%), 130A × 0.70 = 91A. You would be forced to upsize to 1/0 AWG.
Aluminum-Specific Termination Prep
Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that causes heat buildup and potential fires at the lug. When terminating your 1 AWG aluminum feeder:
- Verify the Alloy: Ensure the wire is stamped AA-8000. Older AA-1350 aluminum (used in the 1970s) is brittle and illegal for modern branch and feeder wiring.
- Use Anti-Oxidant Paste: Apply a UL-listed compound like Ideal Noalox or PenetroxEJ to the stripped conductor before inserting it into the lug. This seals out oxygen and improves conductivity.
- Torque to Spec: Do not guess the tightness. Use a calibrated torque screwdriver or wrench. A typical 100A panel lug requires between 40 and 50 in-lbs of torque, but you must read the manufacturer's label inside the panel door to be certain. Loose aluminum connections are the number one cause of subpanel fires.
When an Engineer or AHJ Must Confirm
While 1 AWG aluminum is the definitive answer for standard residential and light commercial 100A feeders, certain edge cases require stamped engineering drawings or explicit approval from your local Authority Having Jurisdiction (AHJ). You must consult a professional when:
- Running Parallel Conductors: NEC 310.10(G) generally prohibits paralleling conductors smaller than 1/0 AWG. If your voltage drop calculation demands massive wire, an engineer must design a parallel run using multiple 1/0 or larger feeders.
- High Ambient Temperatures: If the conduit runs across a rooftop or through a boiler room where ambient temperatures exceed 40°C (104°F), severe derating applies. An engineer must calculate the exact thermal envelope.
- Available Fault Current: If the utility transformer can deliver massive short-circuit current (e.g., >10,000 AIC), the 100A breaker's interrupting rating and the wire's bracing must be verified to ensure the conductors don't violently repel each other during a fault.
- Continuous Loads: If the 100A load is considered 'continuous' (running at maximum capacity for 3 hours or more, like a commercial EV charger or server room HVAC), NEC 210.20 requires the breaker to be sized at 125% of the load (125A), which fundamentally changes your wire size requirement to 1/0 AWG aluminum minimum.
For standard home subpanels, detached garages, and residential service drops, sticking to 1 AWG XHHW-2 aluminum (or 1/0 AWG for long runs) with proper anti-oxidant prep and torque will yield a safe, code-compliant installation that passes inspection on the first try. Always verify final sizing with your local inspector, as regional amendments to the NEC and local utility standards can occasionally supersede general guidelines.
