The base ampacity for 350 kcmil aluminum wire is 310 Amps in the 75°C column and 350 Amps in the 90°C column, per NEC Table 310.16. For standard panel and breaker terminations, you must use the 75°C value (310A) as your legal maximum. The 90°C value is only used as a starting point for calculating derating adjustments.
Whether you are pulling feeders for a 300A residential service, a commercial HVAC disconnect, or a large subpanel, picking the right column and applying the correct adjustment factors is the difference between a safe installation and a failed inspection. Here is the exact data, the math, and the decision path you need.
350 kcmil Aluminum Wire Ampacity: The Quick Reference Table
The following data is sourced directly from NFPA 70 (NEC) Table 310.16, based on an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.
| Conductor Material | Common Insulation Types | Max Temp Rating | 60°C Column | 75°C Column (Terminations) | 90°C Column (Derating) |
|---|---|---|---|---|---|
| Aluminum | THHN / THWN-2 | 90°C | 250A | 310A | 350A |
| Aluminum | XHHW-2 | 90°C | 250A | 310A | 350A |
| Aluminum | USE-2 (Underground) | 75°C | 250A | 310A | N/A |
- Standard Breaker/Lug Limit: 310 Amps (75°C column)
- Derating Starting Point: 350 Amps (90°C column)
- Direct Burial (USE-2): 310 Amps (75°C column, no 90°C available)
How to Read the Ampacity Table and Pick Your Column
The most common mistake DIYers and junior electricians make is looking at the 90°C column (350A) and assuming they can protect the wire with a 350A breaker. In almost all real-world scenarios, you cannot.
NEC 110.14(C) Termination Rules: For circuits rated over 100 Amps, equipment terminations (breaker lugs, panel busbars, disconnect switches) are tested and rated for 75°C unless explicitly marked otherwise. Because the weakest link in the circuit dictates the maximum allowable current, your 90°C wire is legally choked down to the 75°C rating of the lug it lands on.
Therefore, the 75°C column (310A) is your default usable ampacity. You can only use the 60°C column (250A) if you are connecting to very old equipment explicitly marked for 60°C, which is practically nonexistent for 350 kcmil applications.
Derating 350 kcmil Aluminum: When the 90°C Column Saves You
While you cannot use the 90°C column for terminations, you must use it as the baseline for derating calculations. When you have more than three current-carrying conductors in a conduit, or when the ambient temperature exceeds 30°C (86°F), the wire's ability to shed heat drops.
Here is how derating modifies the base value in a real-world scenario:
You are pulling two 3-phase circuits (4 hot wires, 2 neutrals that don't count as current-carrying in this specific balanced setup, plus a ground) through a single conduit. NEC Chapter 9, Table 310.15(C)(1) requires an 80% adjustment factor for 4 current-carrying conductors.
- Start at 90°C: 350A (from Table 310.16)
- Apply Derating: 350A × 0.80 = 280 Amps
- Compare to Termination Limit: The 75°C termination limit is 310A.
- Final Rule: You must use the lower of the two values. Your final allowable ampacity is 280 Amps.
Because 280A is not a standard breaker size, NEC 240.4(B) allows you to round up to the next standard size, which is 300 Amps, provided your actual calculated load does not exceed 280A.
If you had mistakenly started your derating math at the 75°C column (310A × 0.80 = 248A), you would have unnecessarily oversized your wire or undersized your breaker. For deep technical details on conduit fill and adjustment factors, reference the Southwire Technical Resources ampacity correction tables.
Decision Path: Sizing Your Breaker and Lugs for 350 kcmil Al
Use this decision tree to finalize your breaker size and verify if 350 kcmil aluminum is actually the right wire for your load. This assumes standard 75°C terminations, 30°C ambient temperature, and no more than 3 current-carrying conductors.
| Calculated Continuous + Non-Continuous Load | Required Wire Size (Al) | Max Standard Breaker Size | Action / Concrete Pick |
|---|---|---|---|
| Up to 255A | 250 kcmil | 300A | Downsize to 250 kcmil to save money and pulling effort. |
| 256A to 300A | 350 kcmil | 300A | Use 350 kcmil Al with a 300A breaker. (Standard safe pick). |
| 301A to 310A | 350 kcmil | 350A | Use 350 kcmil Al with a 350A breaker. (Permitted via NEC 240.4(B) next-size-up rule, as 310A is not a standard breaker size). |
| 311A to 335A | 400 kcmil | 350A or 400A | Upsize to 400 kcmil. 350 kcmil is insufficient. |
| Over 335A | 500 kcmil or Parallel | 400A+ | Use 500 kcmil, or run two parallel sets of 350 kcmil (requires 400A+ panel). |
The Default Concrete Pick: If you are installing a standard 300A residential or light-commercial service, buy 350 kcmil XHHW-2 or THHN aluminum, land it on 75°C rated lugs, and protect it with a 300A molded case breaker.
What the Ampacity Table Cannot Tell You
Ampacity tables assume ideal conditions. When working with heavy gauge wire like 350 kcmil, physical and electrical realities outside the table will dictate your installation success. For comprehensive installation standards, always cross-reference ECM Web's NEC code guides.
- Voltage Drop: Table 310.16 does not account for distance. 350 kcmil aluminum has an approximate AC resistance of 0.032 ohms per 1,000 feet at 75°C. If your run exceeds 150 feet on a 240V system, calculate your voltage drop. If it exceeds 3%, you must upsize to 400 kcmil or 500 kcmil, even if the ampacity table says 350 kcmil is fine.
- Bending Radius and Pull Tension: 350 kcmil aluminum is incredibly stiff. NEC 300.34 dictates strict minimum bending radii (typically 4 to 5 times the conductor diameter depending on insulation and shielding). You will need specialized pulling compound (never use dish soap, which degrades insulation) and a fish tape or pulling grip rated for the physical weight of the wire.
- Lug Torque Requirements: NEC 110.14(D) mandates that all terminations be tightened to the manufacturer's specified torque using a calibrated instrument. For 350 kcmil aluminum, this typically ranges between 350 in-lbs and 500 in-lbs depending on the lug design. Aluminum creeps over time; under-torqued lugs will heat up, oxidize, and eventually cause a thermal failure or fire. Always use an oxide inhibitor compound (like Noalox) on stripped aluminum conductors before termination.
- Neutral Sizing: If your 350 kcmil feeders are for a single-phase 120/240V service with significant 120V line-to-neutral loads, the neutral conductor must also be 350 kcmil. If it is a strictly balanced 3-phase motor feed, the equipment grounding conductor (EGC) can be sized much smaller (e.g., 2 AWG Al per NEC 250.122 for a 300A breaker).
