100 amp wire size aluminum refers to the specific American Wire Gauge (AWG) cross-sectional area of an aluminum conductor required to safely carry 100 amps of continuous or non-continuous current without exceeding the thermal limits of its insulation and terminations. For a standard residential or commercial 100-amp breaker and panel setup, the correct, code-compliant wire size is 1/0 AWG aluminum. This single sizing decision dictates your required conduit diameter, the physical torque applied to the panel lugs, the overall material cost of the feeder run, and whether your installation passes inspection or creates a localized fire hazard at the breaker terminals. In the field, people most commonly confuse the 90°C ampacity rating of the wire's modern insulation with the 75°C temperature rating of standard breaker terminations, a mistake that leads them to dangerously undersize the wire to 2 AWG.
The 75°C vs 90°C Termination Trap
To understand why 1/0 AWG is the mandatory baseline, you have to look at how the NFPA 70 National Electrical Code (NEC) handles the transition between wire insulation and equipment lugs. Modern aluminum building wire, typically XHHW-2 or THHN/THWN-2, features insulation rated for 90°C. If you look strictly at NEC Table 310.16 under the 90°C column for aluminum, a 2 AWG wire is rated for exactly 100 amps. This leads many DIYers and inexperienced apprentices to buy 2 AWG aluminum for a 100-amp subpanel feeder.
This is a critical error. NEC Section 110.14(C)(1)(a)(4) explicitly states that for equipment rated 100 amps or less, you must size the conductor based on the 60°C or 75°C temperature column, unless the equipment is specifically listed and identified for use with 90°C rated conductors. Standard 100-amp molded-case circuit breakers and subpanel lugs are almost universally rated for 75°C terminations. When you shift over to the 75°C column in Table 310.16, 2 AWG aluminum drops to an ampacity of 90 amps. It is no longer legally permitted to carry a 100-amp load. You must step up to 1/0 AWG aluminum, which carries a 75°C ampacity of 100 amps, perfectly matching the breaker's termination rating.
Worked Numeric Example: 150-Foot Subpanel Feeder
Let's apply this to a real-world installation. You are running a 100-amp feeder to a detached garage subpanel located 150 feet away from the main service. The system is 240V, single-phase. You need two hot conductors, one neutral, and one equipment grounding conductor (EGC).
Conductor Selection: You choose 1/0 AWG XHHW-2 Aluminum for the two hots and the neutral. Per NEC 250.122, for a 100-amp overcurrent device, the minimum EGC size is 6 AWG aluminum or 8 AWG copper. We will use 6 AWG aluminum XHHW-2 to keep the pull uniform.
Voltage Drop Calculation: While the NEC recommends a maximum 3% voltage drop for feeders (NEC 310.15(B) Informational Note), let's verify the math. Using the standard single-phase voltage drop formula: VD = (2 x K x I x D) / CM.
- K (specific resistance for aluminum at 75°C) ≈ 21.2
- I (Current) = 100A
- D (Distance) = 150 feet
- CM (Circular Mils for 1/0 AWG) = 167,800
VD = (2 x 21.2 x 100 x 150) / 167,800 = 636,000 / 167,800 = 3.79 Volts.
Percentage Drop = (3.79V / 240V) x 100 = 1.58%. This is well under the 3% threshold, confirming 1/0 AWG is electrically sound for this distance.
Conduit Fill Sizing: You are pulling four wires (two 1/0 hots, one 1/0 neutral, one 6 AWG ground) through PVC Schedule 40 conduit. The cross-sectional area of 1/0 XHHW-2 is approximately 0.274 square inches per wire, and 6 AWG is 0.073 square inches. Total wire area = (3 x 0.274) + 0.073 = 0.895 square inches. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. Checking Chapter 9, Table 4, a 1.5-inch PVC pipe only offers 0.794 square inches of 40% fill space (too small). A 2-inch PVC Schedule 40 conduit offers 1.147 square inches of 40% fill space. Therefore, 2-inch conduit is the minimum legal size for this pull.
Where You Meet This in Practice
You will encounter 100-amp aluminum feeder sizing most frequently in three scenarios: detached structure subpanels (garages, barns, ADUs), heavy-duty workshop equipment drops, and hardwired Level 2 Electric Vehicle (EV) charging stations that require an 80-amp continuous load (which mandates a 100-amp breaker under NEC 210.20(A) continuous load rules).
When working with aluminum in these environments, physical handling differs from copper. Aluminum is lighter and more flexible, making long conduit pulls significantly easier on your arms. However, aluminum is prone to galvanic corrosion and thermal expansion. Modern AA-8000 series aluminum alloy wire is highly stable, but you must still apply an anti-oxidant compound (like Noalox) to the stripped ends of stranded aluminum conductors before terminating them, unless the manufacturer explicitly states the wire is pre-treated. Furthermore, because aluminum expands and contracts more than copper under thermal loading, NEC 110.14(D) strictly requires the use of a calibrated torque screwdriver or torque wrench to tighten the lugs to the manufacturer's specified inch-pound rating. Hand-tightening aluminum feeders is a guaranteed path to a high-resistance connection, localized heating, and eventual lug failure.
Decision Path: Sizing Your 100A Aluminum Feeder
Use this decision matrix to finalize your material list before heading to the electrical supply house. This path accounts for standard equipment and edge cases.
| Condition / Scenario | Required Action | Final Wire Size Pick |
|---|---|---|
| Standard 100A breaker (75°C rated terminations) | Use NEC Table 310.16, 75°C column. | 1/0 AWG Aluminum |
| 100A breaker explicitly listed/marked for 90°C | Use 90°C column (rare in residential). Verify with AHJ. | 2 AWG Aluminum |
| Feeder run exceeds 200 feet at full 100A load | Calculate voltage drop; 1/0 AWG will likely exceed 3%. | 2/0 AWG Aluminum |
| Ambient temperature in conduit exceeds 86°F (30°C) | Apply NEC Table 310.15(B)(1) derating factors to 90°C column, then verify 75°C termination limit. | 1/0 AWG or 2/0 AWG (depends on exact temp) |
The Concrete Pick: For 99% of residential and light-commercial 100-amp installations using standard breakers and normal ambient temperatures, buy 1/0 AWG XHHW-2 Aluminum for your current-carrying conductors. It satisfies the 75°C termination rule, handles standard voltage drop distances up to roughly 180 feet, and fits cleanly inside 2-inch conduit.
FAQ: Aluminum Feeder Nuances
Is aluminum wire safe for subpanels?
Yes. The modern AA-8000 series aluminum alloy used for building wire is completely safe, highly conductive, and explicitly permitted by the NEC for feeder and service entrance applications. The historical issues with aluminum wire were isolated to pre-1970s AA-1350 alloy used for 15A and 20A branch circuits, which suffered from creep and oxidation at small screw terminals. Modern 1/0 AWG feeder wire does not suffer from these legacy issues when properly torqued.
How does the cost of 1/0 aluminum compare to copper?
The cost difference is substantial. To carry 100 amps at 75°C, copper requires 3 AWG wire. As of current market pricing, 1/0 AWG aluminum XHHW-2 typically costs between $1.50 and $2.50 per foot, whereas 3 AWG copper THHN costs between $4.50 and $6.50 per foot. For a 150-foot run requiring three current-carrying conductors, aluminum saves you roughly $900 to $1,200 in wire costs alone, which easily offsets the slightly larger conduit and anti-oxidant paste requirements.
Can I mix copper and aluminum in the same panel?
You can land aluminum feeder wires on the main lugs of a subpanel that has copper branch circuit wires attached to its bus bars. The bus bar itself is typically tin-plated aluminum or copper, designed to accept both. However, you must never splice copper and aluminum wires together directly using standard wire nuts; if a transition is required mid-run, you must use a dual-rated (ALR/CU) mechanical split bolt or a Polaris insulated connector, filled with anti-oxidant paste.
When sizing a 100 amp aluminum feeder, ignore the internet forums that tell you 2 AWG is sufficient based on the 90°C column. Trust the physical limitations of your breaker's lugs, adhere strictly to NEC 110.14(C), and pull 1/0 AWG XHHW-2. It is the only choice that guarantees code compliance, thermal safety, and a clean inspection sign-off. Always verify your specific local amendments with your Authority Having Jurisdiction (AHJ) before purchasing materials, as local inspectors have the final say on all installations.
