The direct answer: 350 MCM (kcmil) aluminum wire has a base ampacity of 310 Amps in the 75°C column, 250 Amps in the 60°C column, and 350 Amps in the 90°C column, according to NEC Table 310.16. For nearly all residential and commercial feeder installations, the 75°C column is the legal limit due to standard equipment termination ratings.
Note on terminology: The NEC officially uses kcmil (thousand circular mils), while older trade slang and some distributor catalogs still use MCM (Roman numeral M for 1,000). They mean the exact same physical wire size. This reference guide uses the current NEC 2023/2020 standards to break down exactly how to apply these numbers on the jobsite, how derating shrinks your capacity, and the physical limitations the ampacity table leaves out.
350 MCM Aluminum Wire Ampacity Chart (NEC Table 310.16)
How to read this table: The columns represent the temperature rating of the wire's insulation. The rows represent the conductor size. To find your allowable ampacity, you must match the wire's insulation type (e.g., THHN is 90°C, XHHW is 75°C or 90°C depending on the wet/dry rating) to the correct column, while obeying the termination rules outlined in the next section. The values below assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway.
| Conductor Size (AWG/kcmil) | 60°C (140°F) Column | 75°C (167°F) Column | 90°C (194°F) Column |
|---|---|---|---|
| 250 kcmil | 205A | 255A | 290A |
| 300 kcmil | 230A | 285A | 320A |
| 350 kcmil (MCM) | 250A | 310A | 350A |
| 400 kcmil | 260A | 335A | 380A |
| 500 kcmil | 295A | 380A | 430A |
Which Temperature Column Applies to Your Installation?
A common mistake among DIYers and junior apprentices is looking at a spool of 90°C rated THHN or XHHW-2 wire, seeing the 350A rating in the 90°C column, and assuming they can run a 350A breaker. This is a code violation.
Under NEC 110.14(C), you are bound by the "weakest link" rule. The ampacity of your circuit is limited by the lowest temperature rating of any connected device, terminal, or splice. Nearly all modern breakers, panelboard busbars, and disconnect switches are tested and rated for 75°C terminations.
Therefore, even if your wire insulation can handle 90°C, the heat dissipation at the lug cannot. You must use the 75°C column (310 Amps) to determine your final allowable ampacity for overcurrent protection. The only time the 60°C column applies is if you are connecting to very old equipment explicitly marked for 60°C, or when working with specific small-gauge residential branch circuits (14, 12, and 10 AWG) under NEC 240.4(D).
Derating Factors That Reduce Your Base Ampacity
The 310A figure assumes perfect conditions: an ambient temperature of 30°C (86°F) and a maximum of three current-carrying conductors (CCC) in a single conduit. Real jobsites rarely offer perfect conditions. When conditions change, you must apply derating factors to the 90°C column (if using 90°C rated wire like XHHW-2 or THHN), and then compare that result to your 75°C termination limit. The lower of the two numbers becomes your final legal ampacity.
Scenario: Bundling and Ambient Heat
Imagine you are pulling four current-carrying conductors (e.g., two parallel sets of a multi-wire branch circuit, or a 3-phase 4-wire feeder with a non-linear neutral) through a conduit on a roof where the ambient temperature reaches 40°C (104°F).
- Start with the 90°C base: 350 kcmil Al = 350A.
- Ambient Temperature Correction (NEC Table 310.15(B)(1)): At 40°C, the correction factor for 90°C insulation is 0.91. (350A × 0.91 = 318.5A).
- Bundling Adjustment (NEC Table 310.15(C)(1)): 4 to 6 current-carrying conductors require an 80% adjustment factor. (318.5A × 0.80 = 254.8A).
- Compare to Termination Limit: Your derated wire ampacity is 254.8A. Your 75°C termination limit is 310A.
The Verdict: The final allowable ampacity drops to 254 Amps. You can no longer use this wire on a 300A feeder; you would need to upsize to 400 kcmil or split the conductors into separate conduits to eliminate the bundling derating.
What the Ampacity Table Cannot Tell You
NEC Table 310.16 is a thermal limit chart. It tells you the maximum current the wire can carry before the insulation melts or degrades. It does not account for power quality, physical installation constraints, or chemical reactivity. When working with 350 MCM aluminum, you must calculate the following manually.
1. Voltage Drop (The Hidden Capacity Killer)
The NEC does not strictly mandate a maximum voltage drop for most feeders (except for specific applications like elevators or sensitive medical equipment), but industry standard design practice and NEC Informational Notes recommend keeping feeder voltage drop under 3%. Aluminum has a higher resistance than copper, making voltage drop a critical factor for large runs.
Worked Example: You are running a 200-foot, 240V single-phase feeder using 350 kcmil Aluminum, loaded to 300 Amps. Using the standard single-phase voltage drop formula: VD = (2 × K × I × L) / CM
- K (Specific resistance for Al at 75°C) ≈ 21.2
- I (Current) = 300A
- L (Length) = 200 ft
- CM (Circular Mils for 350 kcmil) = 350,000
VD = (2 × 21.2 × 300 × 200) / 350,000 = 7.26 Volts.
Percentage = (7.26 / 240) × 100 = 3.02%.
At a 200-foot run, 350 MCM aluminum is right on the razor's edge of the 3% recommendation. If your run is 250 feet, you must upsize to 400 kcmil or 500 kcmil to prevent dimming lights and motor overheating at the subpanel, even though the wire is thermally rated for the current.
2. Termination Chemistry and Torque
Aluminum oxidizes within minutes of being exposed to air, forming aluminum oxide—a highly resistive layer that causes lugs to overheat and fail. The ampacity table assumes perfect connections. To achieve that on the bench:
- Anti-Oxidant Paste: You must coat the stripped aluminum conductor with a listed anti-oxidant compound (like Noalox or Penetrox) before inserting it into the lug. This displaces oxygen and prevents the resistive oxide layer from forming.
- Lug Compatibility: Ensure your lugs are rated for aluminum (marked "AL" or "AL/CU"). Never terminate aluminum wire into a copper-only lug.
- Calibrated Torque: Under NEC 110.14(D), you must use a calibrated torque tool. A 350 kcmil mechanical lug typically requires between 300 and 500 inch-pounds of torque (always verify the manufacturer's spec sheet printed on the breaker or lug). Hand-tightening large aluminum feeders is a primary cause of thermal failures and panel fires.
3. Physical Bending Radius
350 kcmil aluminum is incredibly stiff. NEC 300.34 dictates minimum bending radii for conductors to prevent damaging the insulation or stressing the metal. For a 350 kcmil shielded or unshielded feeder, expect to need a minimum bending radius of 7 to 12 times the cable diameter depending on the exact insulation and shielding. If your panel gutter is too shallow, you will not be able to land the wire without kinking it, which creates a localized hot spot not accounted for in Table 310.16.
