If you are sizing aluminum conductors for a residential feeder or service entrance, here are the direct answers for the most common main breaker sizes based on the 75°C termination column: 100A requires 2 AWG aluminum, 150A requires 1/0 AWG aluminum, and 200A requires 4/0 AWG aluminum. These values assume standard THHN/THWN-2 or XHHW-2 insulation in a raceway or cable assembly with no more than three current-carrying conductors.
Aluminum wire is significantly lighter and cheaper than copper, making it the standard choice for heavy feeder circuits like main service panels, subpanels, and large HVAC disconnects. However, reading the National Electrical Code (NEC) ampacity tables incorrectly is a frequent cause of failed inspections and overheated lugs. Below is the complete reference data you need to size your conductors correctly.
How to Read the Aluminum Wire Size Chart (NEC Table 310.16)
Before looking at the numbers, you must understand how the NEC structures ampacity tables. The chart below is derived directly from NEC Table 310.16 (2023/2026 editions), which dictates the allowable ampacities for insulated conductors rated up to 2000 volts.
The table is divided into three temperature columns: 60°C, 75°C, and 90°C. These columns correspond to the thermal rating of the wire's insulation. Modern aluminum building wire (like THHN/THWN-2 or XHHW-2) is almost universally rated for 90°C in dry locations. However, you rarely get to use the 90°C column for your final breaker sizing.
The Master Aluminum Wire Ampacity Chart (75°C & 90°C Columns)
The following data applies to aluminum conductors with standard insulations (THWN, THHN, XHHW, XHHW-2) installed in a raceway or cable, with an ambient temperature of 30°C (86°F).
• 100-Amp Service/Subpanel: Look at 2 AWG (90A in the 75°C column allows a 100A breaker under specific residential service rules, but 2 AWG is the minimum physical size; 1/0 AWG is often used for continuous 100A loads).
• 150-Amp Service: Look at 1/0 AWG (120A in 75°C column, but 150A breaker permitted for 1/0 under NEC 230.42/310.12 residential service calculations).
• 200-Amp Service: Look at 4/0 AWG (180A in 75°C column, permitted for 200A residential service via NEC 310.12).
| Wire Size (AWG/kcmil) | 60°C Column (TW, UF) | 75°C Column (THWN, XHHW) | 90°C Column (THHN, XHHW-2) |
|---|---|---|---|
| 8 AWG | 30A | 40A | 45A |
| 6 AWG | 40A | 50A | 55A |
| 4 AWG | 55A | 65A | 75A |
| 3 AWG | 65A | 75A | 85A |
| 2 AWG (100A Feeder) | 75A | 90A | 100A |
| 1 AWG | 85A | 100A | 115A |
| 1/0 AWG (150A Service) | 100A | 120A | 135A |
| 2/0 AWG | 115A | 135A | 150A |
| 3/0 AWG | 130A | 155A | 170A |
| 4/0 AWG (200A Service) | 150A | 180A | 205A |
| 250 kcmil | 170A | 205A | 230A |
Source: Adapted from NFPA 70 (NEC) Table 310.16. Always verify against your local adopted code cycle, as municipal amendments can alter these baseline values.
Derating, Limitations, and What the Chart Cannot Tell You
An ampacity chart is a baseline, not a final answer. Real-world installations require adjustments for heat buildup and physical distance. Here is how derating modifies the base values, followed by the critical limitations of the chart.
How Derating Modifies the Base Value
When you pull more than three current-carrying conductors through a single raceway (conduit), the wires heat each other up. The NEC requires you to apply an adjustment factor (derating) to the wire's ampacity. Derating math is always performed using the 90°C column, even if your terminations are rated for 75°C.
Worked Example: You are pulling four current-carrying aluminum conductors (two hots, one neutral, one grounded equipment conductor if it carries unbalanced current, though typically we count 4 for a multi-wire branch circuit or two circuits) through a single PVC conduit for a 60A subpanel feeder.
- According to NEC Table 310.15(C)(1), 4 to 6 conductors require an 80% adjustment factor.
- Look at the 90°C column for 4 AWG aluminum: 75A.
- Multiply by the derating factor: 75A × 0.80 = 60A.
- Now, check the termination limit in the 75°C column for 4 AWG: 65A.
- The final allowable ampacity is the lower of the two numbers: 60A. You can protect this circuit with a 60A breaker.
What the Table Cannot Tell You
Relying solely on NEC Table 310.16 will leave you blind to three major installation hazards:
The chart assumes a standard run length. It does not account for voltage drop. If you are running a 200A feeder to a detached garage 250 feet away, 4/0 AWG aluminum will experience roughly a 3.5% to 4% voltage drop under full load. While the NEC recommends keeping branch circuits and feeders under 3% (and the combined total under 5%), best practice for a 250-foot 200A run is to upsize to 250 kcmil aluminum to maintain tight voltage regulation and prevent motor burnout on heavy tools.
2. Conduit Fill Limits (Chapter 9)
The ampacity chart tells you how much current the wire can handle, but it does not tell you if the wire will physically fit in your conduit. Aluminum wire is stiff and has a larger outer diameter than copper for the same ampacity. Pulling 4/0 AWG aluminum through a 1.5-inch Schedule 40 PVC conduit with tight bends will result in damaged insulation or a stuck pull. Always cross-reference manufacturer conduit fill calculators and NEC Chapter 9, Table 1 (maximum 40% fill for 3 or more conductors).
3. Aluminum Termination Physics (Oxidation and Creep)
The chart assumes perfect connections. In reality, aluminum oxidizes rapidly when exposed to air, creating a highly resistive layer that generates intense heat. Furthermore, aluminum is subject to "cold creep" (thermal expansion and contraction that loosens mechanical lugs over time). To prevent a melted lug or a panel fire:
- Always apply a UL-listed anti-oxidant compound (like Noalox) to the stripped aluminum conductor before insertion.
- Use a calibrated torque screwdriver or torque wrench to tighten the breaker and panel lugs to the exact inch-pound specification printed on the equipment label. Hand-tightening aluminum wire is a guaranteed failure point.
- Never terminate aluminum wire directly to a device (like a receptacle) unless the device is explicitly marked "CO/ALR". Standard "CU/AL" devices are not rated for aluminum branch circuit wiring.
