The correct aluminum wire size for a 100 amp service is 1 AWG when equipment terminations are rated for 75°C, or 1/0 AWG if the installation is limited to the 60°C ampacity column. Choosing aluminum over copper changes the physical installation by requiring larger conduit fill allowances, specific anti-oxidant compounds, and strict adherence to manufacturer torque specifications to prevent thermal creep. Builders most commonly confuse the 90°C ampacity column with the 75°C column, mistakenly believing 2 AWG aluminum is sufficient simply because the wire insulation itself is rated for 90°C. Understanding the intersection of wire ampacity, termination limits, and voltage drop is critical to passing inspection and preventing a fire hazard.
The Direct Answer: Sizing Aluminum for 100 Amps
When sizing conductors for a 100-amp residential or commercial feeder, the National Electrical Code (NEC) requires you to look at both the wire's insulation rating and the temperature rating of the equipment terminations. According to NFPA 70 (NEC) Article 110.14(C), the ampacity of a wire is determined by the lowest temperature rating of any connected device, conductor, or termination. Since almost all modern 100-amp breakers and panel lugs are rated for 75°C, you must use the 75°C column in NEC Table 310.16.
Aluminum (75°C column): 1 AWG
Copper (75°C column): 3 AWG
While 1 AWG aluminum is the strict code minimum for 100 amps at 75°C, many electricians routinely upsize to 1/0 AWG aluminum. This is not strictly required for ampacity, but it provides a buffer for voltage drop on longer runs and makes the wire more forgiving if lug torque is slightly off-spec.
| Wire Material | 60°C Column Ampacity | 75°C Column Ampacity | 90°C Column Ampacity |
|---|---|---|---|
| 2 AWG Aluminum | 75A | 90A | 100A |
| 1 AWG Aluminum | 85A | 100A | 115A |
| 1/0 AWG Aluminum | 100A | 120A | 135A |
| 3 AWG Copper | 85A | 100A | 115A |
Why Aluminum Changes the Installation
Aluminum is lighter and significantly cheaper than copper, but it introduces specific mechanical and chemical behaviors that change how you execute the installation. Aluminum has a higher coefficient of thermal expansion than copper. When current flows, the wire heats up and expands; when the load drops, it cools and contracts. Over hundreds of thermal cycles, this expansion and contraction can cause the wire to 'cold creep' or extrude from under the lug screw, reducing the clamping force.
This reduced clamping force increases the electrical resistance at the termination point. Higher resistance leads to localized I²R heating, which accelerates the creep in a destructive feedback loop. To combat this, modern installations require two specific practices:
- Anti-Oxidant Compound: Stranded aluminum wire must be treated with an anti-oxidant paste (commonly known by the brand name Noalox). Aluminum naturally forms a non-conductive oxide layer when exposed to air; the paste breaks this layer down and prevents moisture intrusion.
- Calibrated Torque: NEC 110.14(D) mandates the use of a calibrated torque tool (like a torque screwdriver or wrench) to tighten terminations to the exact inch-pound specification printed on the breaker or panel label. Hand-tightening by 'feel' is no longer code-compliant and is a primary cause of aluminum connection failures.
Where You Meet This in Practice
You will typically encounter the 100-amp aluminum sizing requirement in three specific residential and light-commercial scenarios:
- Detached Garage Subpanels: Running a 100-amp feeder from a main house panel to a detached garage workshop. Because these runs often exceed 50 feet, aluminum is heavily favored here to save hundreds of dollars on material costs compared to copper.
- Mobile Home Feeds: The NEC has specific articles governing mobile home service equipment. A 100-amp service drop or underground feeder to a mobile home disconnect is almost universally pulled with 1/0 AWG or 1 AWG aluminum USE-2 or XHHW-2 wire.
- EV Charger Upgrades: While most single Level 2 EV chargers require 60-amp circuits, homeowners installing dual-charger setups or future-proofing for heavy-duty electric trucks often pull 100-amp aluminum feeders to a secondary subpanel dedicated to vehicle charging.
Real-World Scenario: The Melted Lug Mistake
To understand why the 75°C rule and torque specs matter, consider this real-world failure mode observed in the field.
The Setup: A homeowner needed to feed a 100-amp subpanel in a new backyard shed. To save money, they purchased 2 AWG XHHW-2 aluminum wire. They looked at the wire's datasheet, saw that XHHW-2 insulation is rated for 90°C, and noted that 2 AWG aluminum carries exactly 100 amps in the 90°C column of NEC Table 310.16. They pulled the wire, stripped it, and tightened the lugs on the 100-amp breaker with a standard screwdriver until it felt 'snug'.
The Numbers: The breaker lugs were rated for 75°C. At 75°C, 2 AWG aluminum is only rated for 90 amps. Furthermore, without a torque screwdriver, the lug was under-torqued by roughly 30%.
The Outcome: For the first six months, the shed operated fine under light loads. When winter arrived, the homeowner plugged in a 1500W space heater and a heavy-duty table saw simultaneously, pulling a sustained 75 amps. The breaker did not trip (as 75A is below the 100A magnetic/thermal trip curve). However, the under-torqued, undersized connection at the breaker lug began to heat up to over 180°F. The aluminum crept, resistance spiked, and the plastic breaker housing around the lug melted, eventually causing an arc fault that destroyed the breaker and scorched the panel cover.
What Went Wrong: The installer violated NEC 110.14(C) by using the 90°C column for a 75°C termination, and violated 110.14(D) by failing to use a calibrated torque tool. The wire was physically capable of handling the heat, but the breaker lug was not.
Voltage Drop Calculations and FAQ
Ampacity is only half the battle; voltage drop dictates whether your equipment will actually run correctly at the end of the line. Let us run a worked numeric example for a 150-foot underground run to a 100-amp subpanel.
Formula: VD = (2 × K × I × L) / Circular Mils
K (Aluminum) = 21 | I = 100A | L = 150 ft
Using 1 AWG AL (83,690 CM): VD = (2 × 21 × 100 × 150) / 83,690 = 7.52V (3.13%)
Using 1/0 AWG AL (105,600 CM): VD = (2 × 21 × 100 × 150) / 105,600 = 5.96V (2.48%)
While the NEC recommends a maximum of 3% voltage drop for feeders, 1 AWG aluminum sits right on the edge of that limit at 150 feet. Upsizing to 1/0 AWG aluminum brings the drop comfortably under 3%, which is why 1/0 AWG is the industry-standard 'best practice' for 100-amp feeders of any significant length.
Frequently Asked Questions
Do I need to use Noalox on solid aluminum wire?
Anti-oxidant compound is strictly required for stranded aluminum conductors. While some inspectors allow bare solid aluminum (like SER cable) to be terminated without it if the lug is specifically rated for aluminum, applying a small amount of Noalox is a best practice that prevents future oxidation and is never penalized by an inspector.
Can I use 2 AWG aluminum if I derate the breaker to 90 amps?
Yes. If you have 2 AWG aluminum already pulled, you can swap the 100-amp breaker for a 90-amp breaker. The wire will then be properly protected at the 75°C column rating, and the installation will be code-compliant, provided the panel busbar and load calculations support a 90-amp feed.
What is the difference between SER and USE-2 aluminum cable?
SER (Service Entrance Cable) is rated for above-ground, indoor, or conduit-protected runs and typically features a bare aluminum grounding conductor wrapped around the insulated conductors. USE-2 (Underground Service Entrance) is rated for direct burial and wet locations but is not rated for flame retardancy, meaning it cannot be run inside a house without transitioning to a different cable type or running inside conduit. For more details on cable types and installation environments, refer to manufacturer specifications from suppliers like Schneider Electric or major wire distributors.
