The 100 amp aluminum wire size refers to the minimum American Wire Gauge (AWG) cross-sectional area of an aluminum conductor required to safely carry 100 amperes of current without exceeding its thermal limits at the termination points. For a standard 100-amp feeder circuit, the minimum size is 1 AWG aluminum (based on the 75°C column of NEC Table 310.16). However, if you are wiring a 100-amp residential service entrance, NEC Article 310.12 allows you to use 2 AWG aluminum due to the 83% derating rule for whole-home loads.
What this wire size changes in a real installation is your physical conduit fill capacity, the specific terminal lugs you must buy, and your voltage drop over distance. What people most commonly confuse it with is assuming the 90°C ampacity rating printed on modern wire jackets (like XHHW-2) dictates the breaker size. It does not, because standard panel lugs are only rated for 75°C, meaning the 75°C ampacity column always governs your final wire size.
The Core Rules: 1 AWG vs. 2 AWG vs. 1/0 AWG
Choosing the right aluminum wire for a 100-amp circuit hinges entirely on whether you are wiring a service entrance or a feeder/branch circuit. Failing to distinguish between these two is the number one reason DIYers and junior apprentices fail electrical inspections.
Scenario A: The 100-Amp Feeder (Subpanels, EV Chargers, Workshops)
For any circuit feeding a subpanel, a hardwired appliance, or an outbuilding, the standard ampacity tables apply without exception. According to NFPA 70 (NEC) Table 310.16, 1 AWG aluminum at 75°C is rated for exactly 100 amps. Therefore, 1 AWG is your absolute minimum. You cannot use 2 AWG aluminum (rated 90A at 75°C) for a 100A feeder, even if the load is non-continuous.
Scenario B: The 100-Amp Residential Service Entrance
If you are running the main feeders from the utility meter to your home's main service panel, NEC Article 310.12(A) applies. This article recognizes that a 100-amp main breaker rarely sees a true continuous 100-amp load across all phases simultaneously. It applies an 83% multiplier (100A x 0.83 = 83A). Because 2 AWG aluminum is rated for 90 amps at 75°C, 2 AWG aluminum is legally permitted for a 100A residential service.
Scenario C: The 1/0 AWG Upsize (Voltage Drop & Continuous Loads)
While 1 AWG is code-compliant for ampacity, many professional electricians default to 1/0 AWG aluminum (rated 120A at 75°C) for 100-amp feeders. This is done to mitigate voltage drop on longer runs, to handle continuous loads (which require a 125% multiplier, pushing a 100A continuous load to 125A), or simply because 1/0 AWG is often more readily available at electrical supply houses than 1 AWG.
Worked Example: Voltage Drop and the 1/0 AWG Upsize
Ampacity tells you the wire won't melt; voltage drop tells you your equipment will actually run correctly. The NEC recommends a maximum 3% voltage drop on feeders. Let us run the math on a real-world scenario using data from the Southwire Voltage Drop Calculator and standard AC resistance tables.
The Scenario: You are feeding a 100-amp detached garage subpanel located 150 feet from the main panel. The supply is 240V, single-phase. You are pulling aluminum XHHW-2 wire.
| Wire Size (Al) | Resistance (Ohms/kFT at 75°C) | Calculated Voltage Drop | Drop Percentage (of 240V) | Verdict |
|---|---|---|---|---|
| 1 AWG | 0.253 | 7.59V | 3.16% | Fails 3% recommendation |
| 1/0 AWG | 0.201 | 6.03V | 2.51% | Passes (Optimal) |
The Math for 1 AWG:
Voltage Drop = (2 x Length x Resistance x Current) / 1000
VD = (2 x 150 x 0.253 x 100) / 1000 = 7.59V
At 3.16%, the 1 AWG wire technically meets the NEC's mandatory safety thresholds (the 3% rule is a recommendation, not a strict code violation for feeders in all cases), but your 240V tools and EV chargers will only see ~232V under heavy load. By stepping up to 1/0 AWG, the drop falls to 2.51%, ensuring clean power delivery to the subpanel.
Where You Meet This in Practice
Sizing the wire is only half the battle. When you are physically pulling 100-amp aluminum wire, you will run into specific mechanical and chemical realities on the jobsite.
- Conduit Fill and Bending Radius: Three current-carrying 1 AWG conductors plus a 6 AWG ground will max out a 1.5-inch PVC Schedule 40 conduit. Because aluminum is stiffer than copper and has a larger physical diameter for the same ampacity, sweeping bends are mandatory. Use a 2-inch conduit if the run has more than one 90-degree sweep to avoid damaging the insulation during the pull.
- Terminal Lug Compatibility: Standard 100-amp breakers and subpanel lugs are typically rated to accept up to 1/0 AWG. If you buy a cheap, off-brand 100A disconnect switch, verify the lug depth. Some older or imported lugs physically cannot accept the thicker 1/0 AWG strand bundle without aggressive trimming, which violates code.
- Oxidation and Creep: Aluminum oxidizes rapidly when exposed to air, creating a resistive layer that generates heat. Furthermore, aluminum undergoes 'creep'—it expands and contracts under thermal cycling more than copper, which can loosen terminal screws over time. You must coat stripped aluminum ends with an anti-oxidant compound (like Noalox) and torque the lugs to the manufacturer's exact specification using a calibrated torque screwdriver, as mandated by NEC 110.14(D).
Frequently Asked Questions
Can I use 2 AWG aluminum for a 100 amp subpanel feeder?
No. 2 AWG aluminum is rated for 90 amps in the 75°C column. The 83% rule (NEC 310.12) that allows 2 AWG for a 100-amp service only applies to the main service entrance feeding the entire dwelling. For a subpanel feeder, an EV charger circuit, or a hot tub disconnect, you must use a minimum of 1 AWG aluminum to meet the 100-amp requirement.
Does the 90°C rating on XHHW-2 aluminum wire let me use a smaller gauge?
No. While modern XHHW-2 and THHN-2 aluminum wires are manufactured with 90°C insulation, NEC 110.14(C) requires you to size the wire based on the lowest temperature rating of any connected component. Since almost all residential breakers, busbars, and lugs are rated for 75°C, you must use the 75°C ampacity column. The 90°C column is only used as a starting point for applying ambient temperature derating factors.
Do I need anti-oxidant paste on 100 amp aluminum wire terminations?
Yes, it is highly recommended and often required by local inspectors and manufacturer instructions. Aluminum oxide is an electrical insulator. Applying a conductive anti-oxidant paste (such as Ideal Noalox) to the stripped wire before inserting it into the lug breaks down the oxide layer and prevents future corrosion, ensuring a low-resistance connection that won't overheat under a 100-amp load.
What size breaker and conduit do I need for 1 AWG aluminum?
For a standard 100A feeder using 1 AWG aluminum, you will use a 100-amp 2-pole breaker. For conduit, three 1 AWG XHHW-2 conductors and one 6 AWG bare copper ground will fit inside a 1.5-inch PVC Schedule 40 or 80 conduit. However, if the run exceeds 50 feet or includes multiple bends, stepping up to a 2-inch conduit will drastically reduce pulling tension and prevent insulation scoring.
