The base ampacity for 400 kcmil (also known as 400 MCM) aluminum wire is 260 amps in the 75°C column and 290 amps in the 90°C column, according to NEC Table 310.16. However, due to terminal temperature limitations under NEC 110.14(C), your maximum usable ampacity for overcurrent breaker sizing is almost always capped at 260 amps for standard service equipment.
While the table provides the baseline thermal limits of the wire insulation, real-world jobsite conditions like conductor bundling, ambient heat, and voltage drop dictate your final installation parameters. Below is the complete reference data, followed by the exact calculations needed to keep your installation safe and code-compliant.
400 kcmil Aluminum Ampacity Reference Table (NEC 310.16)
How to read this table: The National Electrical Code (NEC) organizes ampacity by the maximum operating temperature of the wire's insulation. The 60°C column applies to older insulation types or specific wet-location constraints. The 75°C column is the standard baseline for most modern building wire and the default rating for nearly all panelboard and breaker terminals. The 90°C column is strictly used as a starting point for derating calculations; it is almost never used to size the final breaker.
| Insulation Type | 60°C (140°F) | 75°C (167°F) | 90°C (194°F) |
|---|---|---|---|
| TW, UF | 205A | — | — |
| THW, THWN, RHW, USE | — | 260A | — |
| THHN, THWN-2, XHHW-2 (Most Common) | — | 260A | 290A |
For deeper verification on specific manufacturer insulation ratings and metric conversions, cross-reference the Cerro Wire Ampacity Chart or consult the latest NFPA NEC guidelines.
Which Column Dictates Your Breaker Size? (Terminal Limits)
A common mistake on the bench and in the field is looking at a spool of 90°C-rated XHHW-2 aluminum wire, seeing the 290A rating, and attempting to pair it with a 300A breaker. This is a direct violation of NEC 110.14(C), which governs electrical continuity and mechanical strength of connections.
Here is the rule that dictates which column you must use:
- Equipment rated 100A or less: You must use the 60°C column, unless the equipment is specifically listed and marked for 75°C.
- Equipment rated over 100A: You must use the 75°C column, unless the equipment is specifically tested, listed, and marked for 90°C terminations (which is exceedingly rare for standard panelboards and disconnects).
Because 400 kcmil aluminum is typically used for 200A to 250A service entrances or heavy commercial feeders, your equipment will fall into the 'over 100A' category. Therefore, the 75°C column (260A) is your hard limit for sizing the overcurrent protective device, regardless of the fact that the wire insulation itself can physically withstand 90°C.
Derating Modifiers and What the Table Cannot Tell You
NEC Table 310.16 assumes an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. When your installation deviates from this baseline, you must apply derating factors. This is where the 90°C column finally earns its keep.
How Derating Rows Modify the Base Value
NEC rules require you to start your derating math using the 90°C column, and then compare the result to the 75°C terminal limit. The lower of the two numbers becomes your final allowable ampacity.
Worked Example: Conductor Bundling
Imagine you are pulling four current-carrying conductors (e.g., two hots, a neutral carrying unbalanced nonlinear load, and a grounded conductor) through a single conduit. Per NEC Table 310.15(C)(1), four current-carrying conductors require an 80% adjustment factor.
- Start at 90°C: 400 kcmil Al = 290A.
- Apply Adjustment Factor: 290A × 0.80 = 232A.
- Compare to Terminal Limit: The 75°C terminal limit is 260A.
- Final Result: 232A is lower than 260A. Your wire is now legally limited to 232A. You must size your breaker at or below this value (e.g., a 225A or 200A breaker).
Ambient Temperature Correction
If your conduit runs across a rooftop or through a boiler room where the ambient temperature reaches 113°F (45°C), you must apply a temperature correction factor. For 90°C wire at 45°C, the multiplier is 0.87.
Math: 290A × 0.87 = 252.3A. Since 252.3A is still below the 260A terminal limit, your final allowable ampacity drops to 252A.
What the Ampacity Table Cannot Tell You
Relying solely on Table 310.16 leaves three critical engineering factors unaddressed:
- Voltage Drop: The NEC does not strictly enforce voltage drop for most feeders (it is a recommendation, not a mandate, outside of specific AHJ jurisdictions), but a 3% maximum is the industry standard. 400 kcmil aluminum pushing 260A over a 200-foot run at 240V will experience approximately a 2.3% voltage drop. If your run exceeds 300 feet, 400 kcmil will fail the 3% threshold, and you will need to step up to 500 kcmil or 600 kcmil, regardless of the thermal ampacity table.
- Physical Bending Radius: 400 kcmil aluminum is incredibly stiff. NEC 300.34 dictates minimum bending radii to prevent damage to the insulation and the conductor shield. For shielded conductors, the bend radius is heavily restricted. Ensure your panel gutters and pull boxes are deep enough to accommodate the sweeping bends required for this gauge.
- Short-Circuit Thermal Withstand: Ampacity measures continuous heat dissipation. It does not tell you if the wire can survive the instantaneous magnetic and thermal shock of a 20,000-amp short circuit before the breaker clears the fault. For high-available-fault-current services, you must verify the conductor's thermal withstand rating against the specific let-through current of your upstream protective devices.
By anchoring your design to the 75°C terminal limits, applying 90°C derating math, and verifying voltage drop over distance, you ensure your 400 kcmil aluminum feeder will pass inspection and operate safely for decades.
