100 amp aluminum wire is an electrical conductor made from modern AA-8000 series aluminum alloy, sized to safely carry 100 amperes of continuous current by utilizing a physically larger gauge than copper to compensate for its lower conductivity. When you choose aluminum over copper for a 100A feeder, you change three physical realities in your installation: the conduit fill volume, the required termination hardware (AL/CU rated lugs), and the mandatory use of anti-oxidant paste. The most dangerous confusion in home electrical work is equating modern AA-8000 series aluminum with the brittle, fire-prone AA-1350 solid branch-circuit aluminum used in the 1960s—they are entirely different metallurgical beasts, and treating them the same is how panels burn down.
The Physics of Aluminum vs. Copper at 100 Amps
To understand why we size aluminum differently, you have to look at the material science. Aluminum has roughly 61% the conductivity of copper by volume. To push 100 amps through aluminum without exceeding the thermal limits of the insulation, you simply need more cross-sectional area. This is why the National Electrical Code (NEC) requires you to step up two wire sizes when switching from copper to aluminum for the same ampacity.
But conductivity is only half the story. The real danger with aluminum lies in its coefficient of thermal expansion and its oxidation rate.
- Thermal Creep: Aluminum expands and contracts about 30% more than copper when it heats up under load and cools down. Over hundreds of heating cycles, this can cause the wire to physically deform (creep) and loosen under a standard setscrew lug.
- Instant Oxidation: When you strip copper, it oxidizes slowly, and copper oxide is still somewhat conductive. When you strip aluminum, aluminum oxide forms on the surface within minutes. Aluminum oxide is a highly effective electrical insulator. If you don't break this oxide layer during termination, the connection will run hot.
Sizing 100 Amp Aluminum Wire: A Worked Numeric Example
Let's run the numbers for a typical 100-amp subpanel feed to a detached garage, located 150 feet from the main panel. We will use XHHW-2 insulation, which is rated for 90°C in conduit but must be terminated at the 75°C column per NEC 110.14(C).
Step 1: Ampacity Sizing (NEC Table 310.16)
Looking at the 75°C column for a 100A requirement:
- Copper: 3 AWG (Rated 100A)
- Aluminum: 1 AWG (Rated 100A)
If we stopped here, we'd buy 1 AWG aluminum. But we must check voltage drop.
Step 2: Voltage Drop Calculation
The NEC recommends a maximum 3% voltage drop on feeders. Assuming a realistic continuous load of 80A on this 100A circuit at 240V:
- Formula: VD = (2 × K × I × L) / CM
- K (Aluminum): 21.2
- I (Current): 80A
- L (Length): 150 ft
- CM (Circular Mils for 1 AWG): 83,690
VD = (2 × 21.2 × 80 × 150) / 83,690 = 6.08 Volts.
Percentage drop: (6.08 / 240) × 100 = 2.53%. Because 2.53% is under the 3% threshold, 1 AWG XHHW-2 Aluminum is the correct, code-compliant, and electrically sound choice for this run. According to NFPA 70 guidelines, this satisfies both ampacity and performance requirements.
Where You Meet This in Practice
You won't use 100A aluminum wire for standard 15A or 20A branch circuits (where copper is king). You will encounter 100 amp aluminum wire in three specific high-current scenarios:
- Subpanel Feeders: Feeding a 100A subpanel in a detached workshop, barn, or addition. Aluminum's cost savings over copper become massive at these gauges and distances.
- Service Entrance Conductors: The main feeder from the utility meter to your main service panel is frequently 100A or 200A aluminum (often SEU or USE-2 cable).
- Heavy HVAC Disconnects: Large commercial or high-end residential heat pumps and electric furnaces that require dedicated 100A fused disconnects.
Real-World Scenario: The Melted Subpanel Lug
Theory is clean; the jobsite is not. Here is a scenario that illustrates exactly what happens when aluminum's physical properties are ignored.
The Setup: A DIY homeowner fed a detached garage subpanel using 1 AWG aluminum THHN in PVC conduit. The run was 60 feet. He stripped the wire, shoved it into the 100A main breaker lugs of the subpanel, and tightened the setscrews with a standard screwdriver until they felt 'tight'. He did not use anti-oxidant paste.
The Numbers: 100A breaker, 1 AWG Aluminum, 60-foot run, standard brass lugs (not explicitly AL/CU rated), zero anti-oxidant compound, uncalibrated torque.
The Outcome: Eight months later, the homeowner was running a table saw and a space heater simultaneously (pulling about 75A). The subpanel main breaker tripped. When he reset it, he smelled burning plastic. The plastic housing around the neutral and Phase A lugs had melted, and the wire insulation was charred back two inches.
What Went Wrong: Three compounding failures. First, the lack of anti-oxidant paste allowed aluminum oxide to form between the wire strands and the lug, increasing resistance. Second, without a torque screwdriver, the lug was under-torqued. Third, the thermal expansion of the aluminum under the 75A load caused the wire to expand, deform slightly under the setscrew (creep), and contract when the load stopped. This thermal cycling physically loosened the connection. A loose connection equals high resistance, which equals intense localized heat (I²R losses). The heat melted the lug and tripped the breaker's thermal protection just before a fire started.
Installation Rules That Keep Aluminum Safe
To prevent the scenario above, modern electrical practice mandates a strict protocol for terminating aluminum. As detailed by major wire manufacturers like Southwire, following these steps ensures a connection that will outlast the building.
- Wire Brush the Strands: After stripping the insulation, use a dedicated stainless steel wire brush to lightly score the outer strands of the aluminum. This breaks the initial oxide layer.
- Apply Anti-Oxidant Paste: Immediately coat the brushed strands with a zinc-dusted anti-oxidant compound (commonly known by the brand name Noalox). The zinc dust helps bite through any remaining microscopic oxide, while the paste seals out oxygen to prevent future oxidation.
- Verify Lug Ratings: Ensure the breaker or terminal block is explicitly stamped 'AL' or 'AL/CU'. Modern 100A breakers almost universally accept aluminum, but always verify.
- Torque to Spec: This is non-negotiable. Use a calibrated torque screwdriver or torque wrench set to the exact inch-pound value printed on the breaker's wiring diagram (typically between 40 and 50 in-lbs for 1 AWG). Do not guess by feel.
Frequently Asked Questions
Can I use aluminum wire for a 100A branch circuit to an EV charger?
Yes, but it is rare. Most EVSE (Electric Vehicle Supply Equipment) manufacturers specify copper in their installation manuals due to the tight bending radii and specific terminal designs of the chargers. Always check the equipment manufacturer's manual; if they mandate copper, using aluminum will void the warranty and violate NEC 110.3(B).
Do I need to upsize my 100A aluminum wire if it runs through a hot attic?
Potentially, yes. NEC Table 310.15(B)(1) requires ampacity derating for ambient temperatures above 86°F (30°C). If your attic routinely hits 110°F, you must apply a 0.76 correction factor to the 90°C column rating of your wire. In many cases, this forces you to upsize from 1 AWG to 1/0 AWG aluminum to maintain a safe 100A capacity after derating.
Is it safe to splice aluminum to copper in a junction box?
Only if you use specifically rated connectors, such as ILSCO MAC block connectors or Alumiconn lugs, and apply anti-oxidant paste. Never use standard wire nuts to twist aluminum and copper together; the galvanic corrosion and differing thermal expansion rates will cause the connection to fail and potentially ignite. For a 100A feeder, you should be using a Polaris insulated tap connector or a terminal block, not a wire nut.
