400 MCM Aluminum Wire Ampacity: NEC Chart, Derating & Sizing Guide

The direct answer for your lookup: 400 MCM aluminum wire has a base ampacity of 280 amps in the 75°C column and 315 amps in the 90°C column, per NEC Table 310.16. This assumes an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway or cable.

Before pulling wire, it is critical to note that the National Electrical Code (NEC) officially transitioned from the term 'MCM' (thousands of circular mils) to 'kcmil' decades ago, though tradespeople and suppliers still use both interchangeably. 400 MCM and 400 kcmil are the exact same conductor. Because this size is almost exclusively used for heavy commercial feeders, large subpanels, or 300A/400A residential service entrances, misunderstanding the temperature columns or derating factors can result in catastrophic lug failure or a failed inspection.

The Master Ampacity Table (NEC Table 310.16)

To use this chart correctly, you must match the wire's insulation type to the correct temperature column. Common aluminum building wire like XHHW-2 and THWN-2 features 90°C rated insulation, but as we will cover below, you rarely get to use the full 90°C value for your final breaker sizing. The rows represent the cross-sectional area of the conductor in kcmil.

For a complete breakdown of insulation types and their corresponding temperature ratings, refer to the Southwire Ampacity Tools or NEC Table 310.104(A).

Which Column Applies to Your Installation?

The most common mistake made by DIYers and junior electricians when sizing 400 MCM aluminum is looking at the 90°C column (315A) and assuming they can protect the wire with a 300A breaker. In almost all real-world scenarios, you must use the 75°C column (280A) for your final ampacity and overcurrent protection sizing.

This restriction comes from NEC 110.14(C), which governs termination temperature provisions. The rule states that equipment terminals for circuits rated over 100 amps are generally tested and rated for 75°C. Even if your XHHW-2 aluminum wire has 90°C insulation, the lug on the breaker, panelboard, or disconnect switch is likely only rated to dissipate heat up to 75°C. If you push 315 amps through a 75°C rated lug, the lug will overheat, oxidize rapidly, and potentially cause a fire.

Derating Rows: Modifying the Base Value

The 280A and 315A figures in the table above are 'ideal laboratory' numbers. On the jobsite, you must apply derating factors that reduce the wire's allowable ampacity based on environmental heat and conductor bundling. This is where the 90°C column finally earns its keep: NEC allows you to start your derating math from the 90°C column (315A), provided your final derated number is still greater than or equal to your calculated load, and you do not exceed the 75°C termination limit.

Let's run a concrete jobsite scenario. You are pulling four current-carrying conductors (e.g., two hots, a neutral carrying unbalanced non-linear load, and a grounded phase) through a conduit in an attic where the ambient temperature reaches 110°F (43°C).

1. Start with the 90°C base: 315A.
2. Apply Ambient Temperature Correction (NEC Table 310.15(B)(1)): For 90°C insulation at 41-45°C ambient, the multiplier is 0.87. (315A × 0.87 = 274A).
3. Apply Bundling Adjustment (NEC Table 310.15(C)(1)): For 4 current-carrying conductors, the multiplier is 0.80. (274A × 0.80 = 219.2A).

Your final derated ampacity is 219 amps. If your calculated continuous load is 250 amps, this installation fails. You would need to upsize to 500 kcmil aluminum or move the conduit out of the hot attic. Notice how the table alone cannot tell you this; you must synthesize the base value with the correction tables.

What the Table Cannot Tell You (Edge Cases & Sizing)

Ampacity charts only address thermal limits under steady-state conditions. When working with 400 MCM aluminum, three critical physical and electrical realities fall outside the scope of Table 310.16.

1. Voltage Drop Over Distance

NEC Table 310.16 does not account for voltage drop. Aluminum has a higher resistivity than copper (roughly 1.6 times higher for the same cross-sectional area). If you are running a 400 kcmil aluminum feeder 300 feet to a remote barn or detached workshop carrying 250 amps, you will experience significant voltage drop. To maintain the recommended 3% maximum drop for feeders (per NEC informational notes), you will likely need to upsize to 500 kcmil or even 600 kcmil aluminum, regardless of the thermal ampacity chart.

2. Aluminum Termination Preparation

Unlike copper, aluminum oxidizes almost instantly when exposed to air, forming a highly resistive layer that generates severe heat under load. The ampacity table assumes a perfect connection. To achieve that, you must use a wire brush to clean the strands immediately before termination and apply an anti-oxidant compound (like Noalox or Deox). Furthermore, you must torque the mechanical lugs to the manufacturer's exact specification—typically between 375 and 500 inch-pounds for 400 kcmil lugs. Under-torquing causes arcing; over-torquing cold-flows the aluminum and severs strands.

3. Physical Bending Radius and Pull Tension

400 MCM aluminum is exceptionally stiff. The NEC mandates specific bending radii for conductors to prevent damage to the insulation and the metal shield. If you are pulling this wire through multiple 90-degree sweeps, you must calculate the maximum pull tension. Exceeding the tensile limit of the aluminum will stretch the conductor, physically reducing its cross-sectional area and secretly lowering its ampacity below what the chart promises. Use appropriate wire pulling lubricant and a calibrated tension meter for long runs.

For comprehensive code compliance and safety standards regarding large-gauge conductor installations, always cross-reference your local AHJ requirements with the NFPA National Electrical Code guidelines.