For a 100-amp feeder or subpanel circuit, 1/0 AWG aluminum wire paired with a 100-amp breaker is the correct size. This assumes XHHW-2 or THHN insulation, 75°C rated terminations, and a 30°C ambient temperature. While 2 AWG aluminum is allowed for dwelling service entrances, 1/0 is required for general 100A feeders.
Baseline Sizing Assumptions
- Material: Aluminum (AL)
- Insulation Type: XHHW-2 or THHN/THWN-2
- Temperature Column Used: 75°C (per NEC 110.14(C) termination limits)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Maximum of 3 current-carrying conductors in a single raceway
- Voltage: 240V split-phase (standard residential feeder)
The NEC Ampacity Table and the 75°C Termination Rule
When sizing conductors, the most common mistake DIYers and junior apprentices make is looking at the 90°C column of NEC Table 310.16. If you look at the 90°C column, 1/0 aluminum shows an ampacity of 115A. However, you cannot use this number for your final breaker sizing.
NEC 110.14(C) dictates that you must size your wire based on the lowest temperature rating of any connected component. Almost all standard residential breakers, panel lugs, and disconnects are rated for 75°C. Therefore, you must use the 75°C column. In the 75°C column, 1/0 AWG aluminum is rated for exactly 100 amps.
Aluminum and copper are not interchangeable. If you were using copper wire for this same 100-amp circuit, you would only need 3 AWG copper (rated 100A at 75°C). 1/0 copper is rated for 150A at 75°C. Never substitute wire materials without recalculating the ampacity and adjusting your conduit fill, as aluminum requires a larger wire diameter for the same current capacity.
Why this size and not one smaller? You might wonder why 2 AWG aluminum (rated 90A at 75°C) isn't sufficient, especially since NEC 310.12(A) allows 2 AWG aluminum for a 100-amp dwelling service entrance. That specific code article is an exception exclusively for the main service conductors feeding a single-family home's main panel. It does not apply to subpanel feeders, detached garage circuits, or commercial applications. For any standard 100A feeder, 1/0 aluminum is the absolute minimum to safely carry the load without overheating the breaker lugs.
Voltage Drop Verification at 100 Feet
Ampacity tells you if the wire will melt; voltage drop tells you if your appliances will actually work. The NEC recommends a maximum 3% voltage drop for feeders (Informational Note to NEC 310.15(B)). Let's run the math for a 100-foot run of 1/0 aluminum carrying a full 100A load on a 240V circuit.
We use the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM
- K (Resistivity for Aluminum at 75°C): 21.2
- I (Current): 100 Amps
- D (One-way Distance): 100 Feet
- CM (Circular Mils for 1/0 AWG): 105,600
VD = (2 × 21.2 × 100 × 100) / 105,600 = 424,000 / 105,600 = 4.01 Volts
On a 240V feeder, a 4.01V drop represents a 1.67% voltage drop. This is well under the 3% NEC recommendation, meaning 1/0 aluminum is perfectly adequate for a 100-foot run. However, if this were a 120V dedicated circuit, that same 4.01V drop would equal 3.34%, pushing you slightly over the 3% threshold and requiring an upsize to 2/0 AWG or a switch to copper.
Derating and Edge Cases: What Changes the Answer?
The 100A rating for 1/0 aluminum assumes ideal conditions. Real-world jobsites rarely offer ideal conditions. When you alter the installation environment, you must apply NEC correction and adjustment factors.
Crucially, derating for bundling is calculated using the 90°C column, even though your final termination sizing uses the 75°C column. The 90°C ampacity for 1/0 AL is 115A.
| Installation Scenario | NEC Rule | Math & Result | Action Required |
|---|---|---|---|
| Standard (Max 3 conductors, 30°C ambient) | None | 100A (75°C col) | Use 1/0 AL |
| 4 to 6 current-carrying conductors in one conduit | NEC 310.15(C)(1) - 80% adjustment | 115A (90°C col) × 0.80 = 92A | Upsize to 2/0 AL (130A × 0.8 = 104A) |
| Conduit routed through a 40°C (104°F) attic | NEC 310.15(B)(1) - 0.82 correction | 115A (90°C col) × 0.82 = 94.3A | Upsize to 2/0 AL |
| Continuous Load (Operating 3+ hours) | NEC 210.20(A) / 215.3 - 125% rule | 100A × 1.25 = 125A required | Upsize to 2/0 AL and use 125A breaker |
If you are pulling two separate 240V circuits (4 current-carrying conductors) through the same piece of PVC to a detached garage, 1/0 aluminum will fail inspection. The derated ampacity drops to 92A, which is insufficient for a 100A breaker.
When to Defer to an Engineer or the AHJ
While the math above covers 95% of residential and light commercial feeder applications, there are specific scenarios where you must step back and consult a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ):
- Utility Service Entrance Upgrades: If you are upgrading the main service mast or lateral from the utility transformer, the utility company owns the specifications. Many utilities have their own 'Green Books' that supersede standard NEC feeder rules for service drops.
- Runs Exceeding 250 Feet: At extreme distances, voltage drop calculations become non-linear due to AC reactance and power factor. An engineer should calculate the exact impedance to prevent motor burnout at the subpanel.
- High Fault Current Availability: If your utility provides exceptionally high available fault current (e.g., over 22,000 Amps), standard 100A breakers may not have a high enough Amps Interrupting Capacity (AIC). An engineer must verify the breaker's let-through current and the wire's bracing.
Frequently Asked Questions
Can I use 2 AWG aluminum wire for a 100 amp subpanel?
No. 2 AWG aluminum is rated for 90 amps in the 75°C column. While NEC 310.12 allows 2 AWG aluminum for a 100-amp main dwelling service entrance, this exception explicitly does not apply to subpanel feeders. If you are feeding a detached garage, a workshop, or an addition subpanel with a 100A breaker, you must use 1/0 AWG aluminum.
Do I need to use anti-oxidant paste on 1/0 aluminum wire?
Yes, it is highly recommended and often mandated by local inspectors. Aluminum oxidizes rapidly when exposed to air, creating a resistive layer that causes heat buildup at the termination lug. Apply a UL-listed anti-oxidant compound (like Noalox or Ideal No-Ox-Id) to the stripped aluminum conductor before inserting it into the breaker lug. Note that modern XHHW-2 insulation and CO/ALR rated lugs are more forgiving, but paste remains the industry best practice for preventing thermal runaway.
What size conduit do I need for three 1/0 aluminum wires?
For three 1/0 AWG XHHW-2 conductors (plus a ground), NEC Chapter 9 Tables 1 and 4 dictate a minimum of 1.25-inch (1 1/4") EMT or Schedule 40 PVC. However, pulling 1/0 aluminum through 1.25-inch conduit with multiple bends is physically exhausting and risks damaging the insulation. I strongly recommend upsizing to 1.5-inch (1 1/2") conduit to maintain a comfortable pull and accommodate future upgrades.
Why does my 1/0 aluminum wire feel warm at the breaker lug?
If the wire is warm to the touch at the termination point, you likely have a loose connection caused by 'aluminum creep.' Aluminum expands and contracts at a different rate than the steel or brass lugs it connects to, which can cause the screw to loosen over time. You must torque the lug to the manufacturer's exact specification—typically between 40 and 50 inch-pounds for 1/0 wire—using a calibrated torque screwdriver. Never guess the tightness by hand.
