For a standard 200-amp residential service or feeder, the correct aluminum cable size is 4/0 AWG (based on the 75°C column of NEC Table 310.16). If you are running a 100-amp subpanel feeder, you need 2 AWG aluminum. Aluminum is lighter, significantly cheaper than copper, and perfectly safe for feeders and service entrance conductors—provided you size it using the correct temperature column and prep the terminations properly.
Many DIYers and even junior apprentices get tripped up by aluminum wire sizing because they look at the 90°C insulation rating instead of the 75°C termination limit, or they forget to apply conduit fill derating. This reference guide provides the exact lookup values, the physics behind the temperature columns, and a concrete decision path to finalize your wire purchase.
How to Read the Aluminum Ampacity Table (Columns & Temperature Ratings)
Before looking at the numbers, you must understand which column applies to your specific installation. The ampacity table is divided into three temperature columns: 60°C, 75°C, and 90°C. These numbers represent the thermal limits of the weakest link in your circuit, which is almost always the termination lug, not the wire insulation.
- The 60°C Column: Used for older equipment, NM-B (Romex) cable assemblies, and circuits rated 100A or less where the termination temperature is not explicitly marked. You rarely use this for modern aluminum feeders.
- The 75°C Column: This is your default sizing column. Nearly all modern residential breakers, panel lugs, and disconnect switches are rated for 75°C terminations. Even if your aluminum wire has 90°C insulation (like XHHW-2 or THHN), you must use the 75°C column to determine the base ampacity for overcurrent protection sizing.
- The 90°C Column: This column is only used as the starting point for derating calculations (ambient temperature or conduit fill adjustments). The final derated ampacity can never exceed the 75°C base ampacity for termination purposes.
Complete Aluminum Cable Ampacity Chart (NEC Table 310.16)
The following data is extracted from NEC Table 310.16 (formerly 310.15(B)(16) in older code cycles) for aluminum conductors with an ambient temperature of 30°C (86°F). The National Electrical Code restricts aluminum conductors to a minimum of 12 AWG, but practical termination limitations usually make 8 AWG the smallest aluminum used in branch circuits.
| AWG / kcmil | 60°C (140°F) | 75°C (167°F) (Default Sizing) |
90°C (194°F) (Derating Start) |
|---|---|---|---|
| 12 AWG | 15 | — | — |
| 10 AWG | 25 | — | — |
| 8 AWG | 30 | 40 | 45 |
| 6 AWG | 40 | 50 | 55 |
| 4 AWG | 55 | 65 | 75 |
| 3 AWG | 65 | 75 | 85 |
| 2 AWG | 75 | 90 | 100 |
| 1 AWG | 85 | 100 | 115 |
| 1/0 AWG | 100 | 120 | 135 |
| 2/0 AWG | 115 | 135 | 150 |
| 3/0 AWG | 130 | 155 | 170 |
| 4/0 AWG | 145 | 180 | 205 |
Bookmark Quick-Jump Notes for High-Frequency Queries:
- 100A Subpanel Feeder: Use 1/0 AWG (75°C column yields 120A, safely covering the 100A breaker). 2 AWG (90A) is too small unless specific 240.4(B) next-size-up rules apply, which they generally do not for standard feeders.
- 150A Subpanel Feeder: Use 2/0 AWG (75°C column yields 135A; wait, 135A is less than 150A. You must step up to 4/0 AWG for a strict 150A load, or use 2/0 AWG if the calculated continuous load is under 135A and you apply the next-size-up breaker rule. For a direct 150A breaker match without load calc exceptions, 4/0 AWG is the safe pick).
- 200A Main Service: Use 4/0 AWG (75°C column yields 180A, but NEC 230.42 and 310.12 specifically permit 4/0 AWG aluminum for a 200A residential service).
Derating Factors: When Base Ampacity Drops
The numbers in the table above assume two things: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. If your installation violates either assumption, the base ampacity drops.
How derating modifies the base value: You always start your derating math using the 90°C column, apply the adjustment factor, and then compare the result to the 75°C column. You must use the lower of the two numbers.
Conduit Fill Adjustment Factors (NEC Table 310.15(C)(1))
| Current-Carrying Conductors | Adjustment Factor |
|---|---|
| 1 to 3 | 100% (No derating) |
| 4 to 6 | 80% |
| 7 to 9 | 70% |
Worked Example: You are pulling four current-carrying conductors (two hots, one neutral, one equipment ground does not count, but let's say you have two separate 240V circuits sharing a conduit = 4 hots) through a conduit in a 30°C attic. You need to feed a 100A subpanel.
- Start at 90°C column for 1/0 AWG Aluminum: 135A.
- Apply 80% adjustment factor for 4 conductors: 135A × 0.80 = 108A.
- Check 75°C column for 1/0 AWG: 120A.
- The derated value (108A) is lower than the 75°C termination limit (120A). Therefore, the final allowable ampacity is 108A. Since 108A > 100A, 1/0 AWG is acceptable.
Decision Path: Sizing Your Aluminum Feeder or Branch Circuit
Follow this exact sequence to terminate your sizing debate and pick a wire size. Do not skip steps.
- Determine the Overcurrent Device (Breaker) Size: What is the rating of the breaker protecting this circuit? (e.g., 100A, 200A).
- Identify Termination Temperature: Check the breaker and panel lug markings. If unmarked or standard residential, assume 75°C.
- Lookup Base Size: Find the AWG in the 75°C column that equals or exceeds the breaker size. (e.g., for 100A, 1/0 AWG yields 120A).
- Count Conductors in Conduit: Are there more than 3 current-carrying conductors?
- If NO: Stop here. Your base size is your final size.
- If YES: Proceed to Step 5.
- Apply Derating: Take the 90°C ampacity of your chosen wire, multiply by the adjustment factor (e.g., 0.80).
- If the result is ≥ Breaker Size: Wire is approved.
- If the result is < Breaker Size: Go up one AWG size and repeat Step 5.
- Final Concrete Pick: Purchase the wire, a tube of Noalox (anti-oxidant paste), and verify your torque screwdriver is calibrated.
What This Table Cannot Tell You (Limitations & Edge Cases)
Ampacity tables are strictly thermal limits for steady-state current. They do not account for the physical realities of jobsite installation. Here is what the table leaves out:
1. Voltage Drop Over Distance
NEC Table 310.16 does not calculate voltage drop. If you are running a 200A aluminum feeder to a detached garage 150 feet away, 4/0 AWG will keep the wire from melting, but you will experience roughly a 4.5% voltage drop at full load. While the NEC recommends keeping branch circuits and feeders under 3% each (5% total), long runs require upsizing the wire purely for voltage drop, not thermal ampacity. Use the Southwire Voltage Drop Calculator to verify long runs.
2. Physical Lug Fitment and Bending Radius
Aluminum wire has a larger diameter than copper for the same ampacity. A 100A breaker designed primarily for copper might physically struggle to accept the thicker 1/0 AWG aluminum conductor. Furthermore, aluminum is stiffer. You must maintain the manufacturer-specified bending radius inside the panel to avoid stressing the termination lug, which can lead to micro-fractures and high-resistance hot spots over time.
3. Oxidation and Torque Requirements
Unlike copper, aluminum forms a non-conductive oxide layer almost immediately when exposed to air. You must brush the conductor strands with a wire brush and apply an anti-oxidant compound (like Noalox or Penetrox) before inserting it into the lug. Finally, aluminum creeps under pressure more than copper. You must use a calibrated torque screwdriver or torque wrench set to the exact inch-pound value printed on the breaker or lug label (typically 35 to 45 in-lbs for smaller lugs, up to 250 in-lbs for large service lugs). Hand-tightening aluminum feeders is a leading cause of residential panel fires.
For authoritative code interpretations and updates on termination requirements, refer to the NFPA 70 National Electrical Code documentation and consult your local Authority Having Jurisdiction (AHJ) for any regional amendments.
