The base ampacity for #1 AWG aluminum wire is 100 amps in the 75°C column and 115 amps in the 90°C column, according to NEC Table 310.16. For the vast majority of residential and light commercial feeder applications, the 75°C rating of 100A is the governing limit due to terminal temperature restrictions at the breaker and busbar lugs.
While aluminum is lighter and significantly cheaper than copper, it requires strict adherence to termination protocols and derating math. Below is the exact reference data, the rules for selecting the correct temperature column, and the physical limitations the ampacity chart will not show you.
The #1 Aluminum Wire Ampacity Quick-Reference Table
How to read this table: The values below are extracted for common aluminum feeder sizes to provide context around #1 AWG. The columns represent the maximum allowable ampacity based on the insulation's temperature rating. Most modern building wire (THWN-2, XHHW-2) is rated for 90°C, but the usable ampacity is often capped by the 75°C or 60°C column depending on the equipment terminals. Bookmark the #1 AWG row for quick jobsite reference.
| AWG Size (Aluminum) | 60°C Column (140°F) | 75°C Column (167°F) | 90°C Column (194°F) |
|---|---|---|---|
| #2 AWG | 75 Amps | 90 Amps | 100 Amps |
| #1 AWG (Target) | 75 Amps | 100 Amps | 115 Amps |
| #1/0 AWG | 100 Amps | 120 Amps | 135 Amps |
| #2/0 AWG | 115 Amps | 135 Amps | 150 Amps |
Which Temperature Column Actually Applies to Your Install?
The most common mistake DIYers and junior electricians make is sizing the breaker using the 90°C column because the wire jacket says "90°C". To determine the actual allowable ampacity, you must follow NEC 110.14(C) - Temperature Limitations.
- The 60°C Rule: Equipment rated 100 amps or less defaults to the 60°C column unless the manufacturer explicitly marks the lugs as 75°C rated. For #1 aluminum, the 60°C column is only 75A.
- The 75°C Rule: Most modern 100A subpanel breakers, meter sockets, and disconnects feature lugs rated for 75°C. When using 90°C-rated #1 aluminum wire with 75°C-rated lugs, you are permitted to use the 75°C column (100 Amps) for your overcurrent protection sizing.
- The 90°C Exception: You are only allowed to use the 90°C column (115A) for ampacity derating calculations (such as adjusting for high ambient temperatures or conduit bundling), provided the final derated value does not exceed the terminal temperature limits.
For a standard 100-amp subpanel feeder, #1 AWG aluminum is the correct minimum size, provided your panel lugs are rated 75°C. If your lugs are only rated 60°C, you must upsize to #1/0 AWG aluminum (which yields 100A in the 60°C column).
How Derating Factors Modify the Base Ampacity
The 100A and 115A figures in the table assume ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a single raceway. When conditions change, you must apply derating factors.
Ambient Temperature Correction
If your conduit runs through an attic in the summer or across a hot roof, the ambient temperature exceeds 30°C. According to the temperature correction factors at the bottom of NEC Table 310.16, if the ambient temperature is 40°C (104°F), you must multiply the base ampacity by 0.91.
- Using the 90°C column for derating: 115A × 0.91 = 104.6 Amps.
- Since 104.6A still exceeds the 75°C terminal limit (100A), your maximum breaker size remains 100A.
Conduit Fill (Bundling) Adjustment
If you pull two separate 240V circuits in the same conduit, you will have four current-carrying conductors. NEC Table 310.15(C)(1) requires a derating factor of 80%.
- Using the 90°C column: 115A × 0.80 = 92 Amps.
- Because 92A is less than the 75°C terminal limit, your allowable ampacity drops to 92A. You can no longer use a 100A breaker; you must drop to a 90A breaker or upsize the wire to #1/0 AWG.
What the Ampacity Table Cannot Tell You
Ampacity charts only address thermal limits under ideal, short-distance conditions. They fail to account for three critical jobsite realities:
- Voltage Drop Over Distance: NEC Table 310.16 does not factor in resistance over length. #1 aluminum has a resistance of roughly 0.253 ohms per 1,000 feet. If you run a 100A load at 240V for 150 feet, you will experience a voltage drop of approximately 7.6V (3.1%). This exceeds the NEC Informational Note recommendation of 3% for feeders. For runs over 100 feet at full 100A load, upsize to #1/0 or #2/0 aluminum to maintain voltage stability, even if #1 meets the thermal ampacity requirement.
- Termination Hardware Compatibility: You cannot land aluminum wire on lugs designed exclusively for copper. Many older breakers and busbars feature tin-plated copper lugs that may not be rated for aluminum due to galvanic corrosion risks. Always verify the lug markings (e.g., "AL" or "AL/CU").
- Termination Preparation: Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that generates heat. The ampacity table assumes a perfect connection. In reality, you must wire-brush the aluminum conductor, apply an anti-oxidant compound (like Noalox or Penetro-oxide), and torque the lug to the manufacturer's exact inch-pound specification using a calibrated torque screwdriver or wrench.
Frequently Asked Questions About #1 Aluminum Wire
Can I use #1 aluminum wire for a 100-amp subpanel?
Yes, #1 AWG aluminum wire is the standard, code-compliant choice for a 100-amp subpanel feeder, provided the wire insulation is rated 75°C or higher (like THWN-2 or XHHW-2) and the terminating lugs on both the main breaker and the subpanel are rated for 75°C. If your equipment is strictly rated for 60°C, you must upsize to #1/0 AWG aluminum.
What size breaker can I use with #1 aluminum wire?
Under standard installation conditions (30°C ambient, max 3 current-carrying conductors), you can use a 100-amp breaker with #1 aluminum wire, relying on the 75°C ampacity column. You cannot use a 110A or 125A breaker, as standard overcurrent protection rules (NEC 240.4) require the breaker to protect the wire at its lowest applicable ampacity limit, which is 100A for standard terminations.
Does #1 aluminum wire need anti-oxidant paste?
Yes. While the NEC does not explicitly mandate anti-oxidant paste for all aluminum terminations in every single scenario, manufacturer instructions (which are enforceable under NEC 110.3(B)) almost universally require it. Aluminum oxide is an electrical insulator; without a compound like Noalox to break down and seal out oxygen, the connection will develop high resistance, overheat, and potentially cause a fire at the lug.
How does voltage drop affect #1 aluminum wire ampacity?
Voltage drop does not change the thermal ampacity (the wire will still safely carry 100A without melting the insulation), but it dictates the practical usability of the circuit. If your 100A load is located more than 100-120 feet from the main panel, the voltage at the destination will drop below 230V, causing motors to run hot and electronics to malfunction. For long runs, you must upsize the wire to #1/0 or #2/0 aluminum to compensate for resistance, regardless of the ampacity chart.
