For a standard 100-amp feeder, use 1 AWG aluminum wire with a 100A breaker. However, if this is the main service entrance to a single-family home, NEC 310.12 permits 2 AWG aluminum.
Baseline Sizing and the 310.12 Dwelling Exception
To size wire correctly, we must first define the exact application. The National Electrical Code (NEC) treats main residential services differently than subpanels or commercial feeders due to load diversity (the statistical reality that a home never runs every appliance at peak draw simultaneously).
- Material: Aluminum (specifically not copper; they are not interchangeable)
- Temperature Column: 75°C (Standard for almost all modern breakers and lugs rated 100A and below)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Type: Single circuit in PVC, EMT, or standard NM-B cable
- Insulation Type: THWN-2, XHHW-2, or THHN
The General Feeder Rule (NEC Table 310.16)
If you are running a 100-amp feeder to a detached garage, a workshop subpanel, or a commercial space, you must use Table 310.16. In the 75°C column for aluminum, 1 AWG is rated for exactly 100 amps. You cannot use 2 AWG aluminum here, because its 75°C ampacity is only 90 amps. Pushing 100A through a 90A-rated wire will cause the insulation to degrade over time, creating a fire hazard long before the 100A breaker trips.
| Wire Size (AWG/kcmil) | 75°C Ampacity | Application for 100A Circuit |
|---|---|---|
| 2 AWG | 90A | Undersized for general feeders; Allowed ONLY for main dwelling services |
| 1 AWG | 100A | Minimum size for 100A subpanels and general feeders |
| 1/0 AWG | 120A | Used for voltage drop mitigation or high-ambient derating |
The Main Service Exception (NEC 310.12)
If this 100-amp wire is the main service entrance feeding the primary panel of a single-family dwelling, NEC 310.12(A) applies. This rule explicitly allows 2 AWG aluminum for a 100-amp residential service. This exception exists because residential loads are highly diversified; a home with a 100A main breaker rarely pulls a continuous 100A load. If your project is a main service upgrade, 2 AWG is legal and standard. If it is a subpanel, you must step up to 1 AWG.
Voltage Drop Check: The 100-Foot Benchmark
Ampacity tables tell you what size wire prevents a fire. They do not guarantee your equipment will actually work. The NEC recommends (in Informational Note 210.19) that branch circuits and feeders be sized to limit voltage drop to 3%.
Let's run the math for 1 AWG aluminum on a 240V circuit pulling a full 100A load over 100 feet:
- Formula: VD = (2 × K × I × D) / Circular Mils
- K (Aluminum): 21.2 ohms
- I (Current): 100A
- D (Distance): 100 ft
- Circular Mils (1 AWG): 83,690
Calculation: (2 × 21.2 × 100 × 100) / 83,690 = 5.06 Volts.
At 240V, a 5.06V drop is 2.1%. This is well under the 3% recommendation. However, if your run is 200 feet, the drop doubles to 10.12V (4.2%). At that distance, you must upsize to 1/0 AWG aluminum to keep the drop under 3%. You can verify these figures using the Southwire Voltage Drop Calculator or similar engineering tools.
Decision Tree: What Changes the Answer?
Real-world jobsites rarely match textbook assumptions. Use this decision tree to adjust your wire size when conditions change.
| Condition / Variable | Impact on 1 AWG Aluminum | Required Action |
|---|---|---|
| Run exceeds 150 feet | Voltage drop exceeds 3% at full load | Upsize to 1/0 AWG Aluminum |
| Ambient temp is 110°F (43°C) | THHN derates to 87A (too low) | Use XHHW-2 (90°C column for derating) or upsize to 1/0 AWG |
| 4 current-carrying conductors in one conduit | Ampacity derates by 80% (100A × 0.8 = 80A) | Upsize to 1/0 AWG Aluminum (120A × 0.8 = 96A, still tight; consider 2/0 AWG) |
| Terminating at a 60°C rated lug | 1 AWG Al drops to 75A capacity | Upsize to 1/0 AWG (rated 100A at 60°C) or replace the lug |
Why not just use copper? Copper and aluminum are not interchangeable. If you wanted to use copper for a standard 100A subpanel feeder, you would use 3 AWG copper (rated 100A at 75°C). However, aluminum is significantly cheaper and lighter, which is why it dominates service entrances and heavy feeder runs. Never assume a wire size chart applies to both metals.
Termination, Conduit, and AHJ Confirmation
Conduit Sizing
If you are pulling three 1 AWG aluminum THHN wires (two hots, one neutral) plus a ground through PVC conduit, you must respect NEC Chapter 9, Table 1 (40% conduit fill). Three 1 AWG THHN wires require a minimum of 1-inch PVC Schedule 40 conduit. Do not try to jam them into 3/4-inch conduit; the friction will tear the insulation during the pull, and the AHJ will fail your rough-in inspection.
The Anti-Oxidant Requirement
Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that causes terminal lugs to overheat. You must apply a UL-listed anti-oxidant compound (like Noalox or Ideal No-Ox) to the stripped aluminum conductor before torquing it into the breaker or busbar lug. Furthermore, use a calibrated torque screwdriver or wrench to tighten the lug to the exact inch-pound specification printed on the breaker label. Hand-tightening aluminum is a leading cause of panel fires.
When to Call an Engineer or the AHJ
You must consult your local inspector or a licensed electrical engineer if:
- You are upgrading a main service entrance (utilities often require specific meter socket and service mast clearances).
- Your conduit run passes through areas with extreme ambient heat (e.g., above an unventilated attic space exceeding 110°F).
- You are mixing older 60°C rated equipment with modern 75°C wire.
Frequently Asked Questions
Can I use 2 AWG aluminum for a 100-amp subpanel?
No. While NEC 310.12 allows 2 AWG aluminum for the main service entrance of a dwelling, it does not apply to subpanels. For a 100-amp subpanel feeder, Table 310.16 dictates that 2 AWG aluminum is only rated for 90 amps at 75°C. You must use 1 AWG aluminum to safely carry 100 amps to a subpanel without violating the breaker's protection curve.
What happens if I use copper instead of aluminum for 100 amps?
If you switch to copper, you can use a smaller wire: 3 AWG copper is rated for 100A at 75°C. However, copper is roughly three to four times more expensive per foot than aluminum for these large gauges. If your breaker lugs are rated for copper only (marked 'CU' and not 'AL/CU'), you must use copper. Never use aluminum on lugs not explicitly rated for it.
Do I need anti-oxidant paste on aluminum wire connections?
Yes, absolutely. Aluminum oxide is an electrical insulator. If you terminate bare aluminum into a panel lug, the oxide layer will form over time, increasing resistance and generating dangerous heat. Apply a conductive anti-oxidant paste (like Noalox) to the stripped wire strands immediately before inserting them into the lug and torquing to spec.
What size ground wire do I need for a 100-amp aluminum feeder?
According to NEC Table 250.122, the minimum equipment grounding conductor for a 100-amp overcurrent device is 8 AWG copper or 6 AWG aluminum. If you upsize your current-carrying aluminum conductors to 1/0 AWG to compensate for voltage drop over a long distance, NEC 250.122(B) requires you to proportionally upsize your ground wire as well (e.g., stepping up to 4 AWG aluminum ground).
