If you are sizing a feeder or service entrance, the direct answer for standard residential loads is this: a 100-amp subpanel feeder requires 1/0 AWG aluminum, while a 200-amp main residential service requires 4/0 AWG aluminum. However, pulling the right wire size goes deeper than just matching the breaker rating to a single number. You must account for termination temperature ratings, ambient heat, and conductor bundling.
This reference guide provides the complete aluminum wire gauge table based on the National Electrical Code (NEC) Table 310.16, along with the exact calculation methods needed to keep your installation safe and code-compliant.
How to Read the Aluminum Wire Gauge Table (NEC 310.16)
Before pulling wire, you must understand how to read the columns in the official NFPA 70 National Electrical Code ampacity tables. The table is divided into three temperature columns: 60°C, 75°C, and 90°C. These numbers represent the maximum temperature the wire insulation can safely withstand, not the ambient temperature of your attic or crawlspace.
For 95% of residential and light commercial jobs, the 75°C column is your baseline for sizing the breaker. Modern breakers, panelboard lugs, and terminal bars are almost universally rated for 75°C. The 90°C column is strictly used as your starting baseline for derating calculations (adjusting for heat and bundling). The 60°C column is generally reserved for older NM-B (Romex) cable or specific legacy equipment.
Below is the complete ampacity chart for common aluminum conductors (types THHN, XHHW-2, and USE-2). We have added bookmark-friendly anchor IDs to the most frequently queried sizes for quick jobsite reference.
| AWG / kcmil Size | 60°C (140°F) | 75°C (167°F) - Default Sizing | 90°C (194°F) - Derating Base |
|---|---|---|---|
| #8 AWG | 30A | 40A | 45A |
| #6 AWG | 40A | 50A | 60A |
| #4 AWG | 55A | 65A | 75A |
| #3 AWG | 65A | 75A | 85A |
| #2 AWG (100A Dwelling Service) | 75A | 90A | 100A |
| #1 AWG | 85A | 100A | 115A |
| 1/0 AWG (100A Standard Feeder) | 100A | 120A | 135A |
| 2/0 AWG | 115A | 135A | 150A |
| 3/0 AWG | 130A | 155A | 170A |
| 4/0 AWG (200A Service/Feeder) | 150A | 180A | 205A |
| 250 kcmil | 170A | 205A | 230A |
Applying Derating Factors to Your Base Ampacity
The numbers in the aluminum wire gauge table assume a perfect environment: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When real-world conditions deviate, you must apply derating factors to the 90°C column, regardless of the terminal temperature rating.
Here is how derating rows modify your base value in practice. Imagine you are running a 4-wire feeder (2 hots, 1 neutral, 1 ground) to a detached garage through an attic that reaches 110°F (43°C). You are using 4/0 AWG XHHW-2 aluminum.
- Identify the 90°C Base: 4/0 AWG at 90°C is 205A.
- Apply Ambient Temperature Correction: At 110°F, the NEC correction factor for 90°C insulation is 0.87. (205A × 0.87 = 178.35A).
- Apply Bundling Adjustment: You have 3 current-carrying conductors (the ground does not count, but the neutral carrying unbalanced load does). Wait, if it's a 120/240V split-phase feeder, the neutral only carries unbalanced current and is often exempt from bundling counts under NEC 310.15(B)(5). Let's assume you are running a 3-phase wye feeder with 4 current-carrying conductors instead. The adjustment factor for 4-6 conductors is 80% (0.80).
- Calculate Final Ampacity: 178.35A × 0.80 = 142.6A.
Even though you started with 4/0 AWG wire, your final allowable ampacity is 142.6A. You must size your breaker to the next standard size down, which is 125A. If you need a full 150A or 200A at the subpanel under these conditions, you must upsize to 250 kcmil or 350 kcmil aluminum.
What This Table Cannot Tell You
Relying solely on an ampacity chart leaves three critical engineering variables unaddressed. Always verify these before finalizing your wire procurement and installation plan:
- Voltage Drop: The NEC table only prevents the wire from melting; it does not guarantee usable voltage at the load. For runs over 100 feet, calculate voltage drop using the formula VD = (2 × K × I × D) / CM. Keep branch circuits under 3% drop and total feeders under 5%. Aluminum has a higher resistance (K=21.2) than copper (K=12.9), meaning voltage drop will force you to upsize aluminum wire on long runs much sooner than copper.
- Termination Limits (NEC 110.14): You cannot terminate a 90°C-rated wire into a 60°C-rated lug and claim the 90°C ampacity. The weakest link in the thermal chain dictates the final breaker size. Always check the stamped rating on your breaker lugs and panelboard bus bars.
- Conduit Fill Capacity: Upsizing wire to compensate for voltage drop or derating might push your bundle past the 40% conduit fill limit dictated in NEC Chapter 9. Always run a conduit fill calculation; 4/0 aluminum is notoriously stiff and difficult to pull through standard 1.5-inch PVC without specialized pulling lubricant.
Frequently Asked Questions
What size aluminum wire do I need for a 100-amp subpanel?
For a standard 100-amp subpanel feeder, you need 1/0 AWG aluminum (rated 120A at 75°C). While #1 AWG aluminum is rated exactly at 100A, NEC 240.4(B) requires the conductor ampacity to meet or exceed the load, and standard practice dictates using 1/0 AWG to accommodate the 100A breaker terminal sizing and provide a margin for voltage drop. Note: If this is a 100A main residential service entrance (not a subpanel), NEC 310.12 allows you to use #2 AWG aluminum, but 1/0 AWG remains the safer, universally accepted choice for feeders.
Can I mix copper and aluminum wire in the same circuit?
You cannot splice copper and aluminum directly together using standard wire nuts, as galvanic corrosion will cause a high-resistance fault and eventual fire. However, you can transition between them at a terminal block or breaker lug that is explicitly marked AL/CU (Aluminum/Copper rated). For inline splices, you must use specialized dual-rated compression connectors or split-bolt connectors separated by a physical divider, coated with an antioxidant compound like Noalox.
Why is my aluminum wire showing white powder at the lugs?
That white, crusty powder is aluminum oxide. When bare aluminum is exposed to oxygen, it rapidly forms an oxide layer that is highly resistive to electrical current. This resistance generates heat at the termination point. To prevent this, all aluminum terminations must be cleaned with a wire brush and immediately coated with an oxide-inhibiting compound (such as Ideal Noalox or Deox IT) before torquing the lug to the manufacturer's specified inch-pound rating.
Does the ground wire count toward the derating bundle?
No. According to NEC 310.15(B)(6), equipment grounding conductors (bare copper or green) are not counted when applying bundling adjustment factors. Furthermore, in a standard single-phase 120/240V residential feeder, the grounded (neutral) conductor is also exempt from the bundling count because it only carries the unbalanced current between the two hot legs. Therefore, a standard 4-wire SER cable or THHN feeder (2 hots, 1 neutral, 1 ground) only counts as two current-carrying conductors for derating purposes.
