When looking up 1 0 aluminum wire ampacity (properly written as 1/0 AWG or "one-aught"), the direct answer depends on your termination temperature rating. Per NEC Table 310.16, 1/0 aluminum wire has a baseline ampacity of 120 amps in the 75°C column and 100 amps in the 60°C column, assuming an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway.
For a standard modern subpanel feeder using XHHW-2 or THHN conductors in conduit, you will use the 75°C column, allowing you to protect the circuit with a 120A breaker. If you are using Service Entrance (SE) cable or terminating into older equipment rated only for 60°C, your maximum ampacity drops to 100A, requiring a 100A breaker.
The Master Ampacity Table for 1/0 Aluminum (NEC Table 310.16)
The following data is extracted directly from NFPA 70 (National Electrical Code) Table 310.16. This table dictates the allowable ampacities for insulated conductors rated up to 2000 volts.
| Insulation Type | Max Operating Temp | 60°C (140°F) Column | 75°C (167°F) Column | 90°C (194°F) Column |
|---|---|---|---|---|
| THHN / THWN-2 | 90°C | — | 120A (Termination Limit) | 135A (Derating Base) |
| XHHW-2 | 90°C | — | 120A (Termination Limit) | 135A (Derating Base) |
| USE-2 / RHW-2 | 90°C | 100A | 120A | 135A (Derating Base) |
| Quick-Jump: Standard Subpanel Feeder | Use 75°C column (120A) for XHHW-2/THHN in conduit to a modern panel. | |||
| Quick-Jump: SE Cable / Older Panels | Use 60°C column (100A) for NM-B, SE cable, or 60°C rated lugs. | |||
How to read this table: The columns represent the temperature rating of the weakest link in your circuit—usually the breaker or panel lug, not the wire insulation itself. Even if you buy THHN wire with 90°C insulation, you cannot use the 90°C column (135A) for your final breaker sizing unless both the breaker and the panel lugs are explicitly rated for 90°C, which is exceptionally rare in residential and light commercial gear. The 90°C column is strictly reserved as a starting point for calculating derating factors.
Which Temperature Column Applies to Your Installation?
Choosing the wrong column is the most common mistake DIYers make when sizing feeders. Here is how to determine which column governs your 1/0 aluminum wire ampacity:
- The 60°C Column (100A): Applies if you are using SE (Service Entrance) cable, NM-B (Romex) equivalents, or terminating into equipment explicitly marked for 60°C. It also applies if you are working on a legacy panel where the manufacturer's documentation limits terminations to 60°C.
- The 75°C Column (120A): This is the default for almost all modern residential and commercial installations. If you are pulling individual XHHW-2 or THHN conductors through PVC or EMT conduit to a standard Square D, Eaton, or Siemens load center, the lugs are rated for 75°C. Your ampacity is 120A.
- The 90°C Column (135A): You never use this column to size your breaker. You only use this column as the base number when you are forced to apply ambient temperature or conductor bundling derating factors.
Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that causes heat buildup at the lug. Always apply a UL-listed antioxidant compound (like Noalox or Penetrox) to stripped 1/0 aluminum conductors before termination. Torque the lug to the manufacturer's exact specification (typically 250–350 in-lbs for standard 120A panel lugs) using a calibrated torque wrench.
Derating 1/0 Aluminum: Ambient Temperature and Conductor Bundling
The base ampacities in Table 310.16 assume two things: an ambient temperature of 30°C (86°F) or less, and no more than three current-carrying conductors (CCCs) in a single raceway. If either condition changes, you must derate the wire using the 90°C column (135A) as your starting math value, per standard industry ampacity charts and NEC Article 310.15.
Worked Example: Conductor Bundling
Imagine you are pulling two 240V circuits through the same 1-inch PVC conduit. You now have four current-carrying conductors (the two hots for circuit A, and the two hots for circuit B; neutrals carrying only unbalanced load do not count, but in a pure 240V setup, both wires count).
- Look up the 90°C ampacity for 1/0 AL: 135A.
- Check NEC Table 310.15(C)(1) for 4 to 6 CCCs: The adjustment factor is 80%.
- Calculate the derated ampacity: 135A × 0.80 = 108A.
- Compare to the termination limit: Your panel lugs are 75°C (120A limit). Since 108A is less than 120A, the derated value governs.
- Final Result: Your 1/0 aluminum wire is now legally limited to 108A. You must drop down to a 100A breaker (the next standard size down per NEC 240.4(B)).
Decision Tree: Sizing Your 1/0 Aluminum Feeder Breaker
Use this decision matrix to terminate your planning phase and select the exact breaker size for your 1/0 AWG aluminum feeder.
| Installation Scenario | Conditions | Concrete Pick: Breaker Size |
|---|---|---|
| Standard Subpanel Feeder | 3 CCCs in conduit, XHHW-2 wire, modern 75°C panel lugs, run under 100 feet. | 120A Breaker (Use 75°C column) |
| SE Cable to Main Disconnect | Using 1/0-1/0-2 AL SE-S style cable, terminating in a main breaker enclosure. | 100A Breaker (Use 60°C column per NEC 338.10(B)) |
| Bundled Conductors | 4 to 6 CCCs in a single conduit, XHHW-2 wire, standard 75°C lugs. | 100A Breaker (Derate 90°C column: 135A × 0.8 = 108A) |
| High Ambient Heat | Conduit running across a roof or in an attic reaching 110°F (43°C), 3 CCCs. | 110A or 100A Breaker (Derate 90°C column: 135A × 0.87 = 117A; size to next standard breaker below termination limit) |
What the Ampacity Table Cannot Tell You
NEC Table 310.16 only tells you the thermal limit of the insulation and termination. It completely ignores voltage drop and physical lug sizing. Ignoring these two factors will result in a system that is legally compliant but functionally defective.
1. Voltage Drop Over Distance
If your 1/0 aluminum feeder run exceeds 100 feet, you must calculate voltage drop. The NEC recommends a maximum 3% drop for feeders. Let's run the math for a 200-foot run of 1/0 AL carrying a full 120A load at 240V:
Formula: VD = (2 × K × I × D) / CM
K (AC resistance for AL) ≈ 21.2
I (Current) = 120A
D (Distance) = 200 ft
CM (Circular Mils for 1/0) = 105,600
VD = (2 × 21.2 × 120 × 200) / 105,600 = 9.63 Volts
A 9.63V drop on a 240V system is a 4.01% drop. This exceeds the 3% recommendation and will cause noticeable dimming and motor strain at the subpanel. The fix: If your run is 200 feet at 120A, you must upsize to 2/0 AWG aluminum (which yields a 2.5% drop) or 3/0 AWG aluminum (1.9% drop), regardless of what the ampacity table says.
2. Physical Lug Capacity
Not every 100A or 120A breaker is manufactured to accept 1/0 AWG wire. Some compact 100A breakers max out at #1 AWG or #2 AWG. Before purchasing your 1/0 aluminum wire and 120A breaker, pull the datasheet for the specific breaker model (e.g., Square D QOM2120VH or Eaton BR2120) and verify the "Wire Size Range" explicitly lists "1/0". If it does not, you will be forced to use a lug reducer or upsize the breaker frame.
