When sizing feeders or service entrance conductors for heavy loads, the 4/0 ampacity is one of the most frequently referenced benchmarks in residential and light commercial electrical work. For a direct answer: 4/0 AWG copper wire has an ampacity of 230 amps and 4/0 AWG aluminum wire has an ampacity of 205 amps when evaluated at the standard 75°C termination temperature rating.

However, simply memorizing those two numbers can lead to failed inspections or overheated terminations if you ignore insulation temperature ratings, ambient derating, and terminal limits. Below is the definitive reference data pulled directly from the National Electrical Code (NEC), followed by the practical rules for applying these numbers on the jobsite.

The 4/0 Ampacity Reference Table (NEC Table 310.16)

The following data is extracted from NEC Table 310.16 (formerly Table 310.15(B)(16)), which dictates the allowable ampacities for insulated conductors rated up to 2000 volts. This table assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable.

Table 310.16 Allowable Ampacities (Excerpt for Large Gauge Wire)
Size (AWG/kcmil) Copper 60°C Copper 75°C Copper 90°C Aluminum 60°C Aluminum 75°C Aluminum 90°C
2/0 145 175 195 115 135 150
3/0 165 200 225 130 155 175
4/0 195 230 260 180 205 230
250 kcmil 215 255 290 170 205 230
How to read this table: The columns represent the temperature rating of the wire's insulation (e.g., THHN is 90°C, XHHW is 90°C wet/75°C dry, THWN is 75°C). The rows represent the physical cross-sectional area of the metal conductor. To find your baseline ampacity, locate your wire size in the left column and trace it across to the column matching your insulation's temperature rating.

Note: For residential service entrance conductors, NEC Article 310.12 provides a specific sizing table that often permits 4/0 aluminum for 200-amp residential services, bypassing the standard 75°C ampacity limit of 205A. Always consult NFPA 70 (National Electrical Code) Article 310.12 for single-family dwelling services.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior electricians make is looking at a spool of 90°C-rated THHN wire, finding the 260A copper rating in the 90°C column, and assuming they can protect it with a 250A breaker. In almost all real-world scenarios, you cannot use the 90°C column for your final overcurrent protection sizing.

This restriction is governed by NEC 110.14(C) - Temperature Limitations of Equipment Terminations. The rule states that the ampacity of your wire is ultimately bottlenecked by the temperature rating of the lugs, breakers, and busbars it connects to.

  • Equipment rated 100A or less: Termination limits are generally assumed to be 60°C unless marked otherwise. You must use the 60°C column.
  • Equipment rated over 100A: Termination limits are generally assumed to be 75°C unless marked otherwise. You must use the 75°C column.

Because 4/0 AWG wire is almost exclusively used in equipment rated well over 100 amps (like 200A main panels or large subpanels), the 75°C column is your legal termination limit. Therefore, 4/0 copper is capped at 230A, and 4/0 aluminum is capped at 205A at the breaker lug.

So why does the 90°C column exist?

The 90°C column is not useless; it is strictly reserved as the starting baseline for derating calculations. If your wire runs through a hot attic or is bundled with other circuits, you apply your derating multipliers to the 90°C value, and then compare the result to the 75°C termination limit. The final allowable ampacity is whichever number is lower.

How Derating Factors Modify the Base 4/0 Value

Table 310.16 assumes ideal conditions: an ambient air temperature of 30°C (86°F) and a maximum of three current-carrying conductors in a conduit. When jobsite conditions deviate from this baseline, you must apply correction factors found in NEC Table 310.15(B)(1) for ambient temperature and Table 310.15(C)(1) for conduit fill.

A Real-World Derating Example

Imagine you are pulling four current-carrying conductors (e.g., two hots, a neutral carrying unbalanced non-linear load, and a grounded conductor) of 4/0 AWG copper THHN through a conduit located in an attic where the ambient temperature reaches 110°F (43°C).

  1. Start with the 90°C base value: 260A (from Table 310.16).
  2. Apply Ambient Temperature Correction: At 41-45°C, the correction factor for 90°C insulation is 0.82. (260A × 0.82 = 213.2A).
  3. Apply Bundling Adjustment: Four current-carrying conductors require an 80% adjustment factor. (213.2A × 0.80 = 170.5A).
  4. Compare to Termination Limit: The 75°C termination limit is 230A.

Your final derated ampacity is 170.5 amps. Even though you are using massive 4/0 wire, the heat and bundling have severely reduced its current-carrying capacity. You could not use this run for a 200A breaker; you would need to upsize the wire or change the routing.

What the Ampacity Table Cannot Tell You: Voltage Drop

NEC Table 310.16 only addresses thermal limits—meaning it tells you the point at which the wire's insulation will melt or degrade. It tells you absolutely nothing about voltage drop.

If you run 4/0 aluminum wire 300 feet from a utility transformer to a barn to supply a 150A load, the wire will not overheat (150A is well below the 205A thermal limit). However, the resistance of 300 feet of aluminum will cause the voltage to drop significantly below the nominal 240V, potentially damaging motors and tripping sensitive electronics. The NEC recommends (in Informational Notes) keeping voltage drop under 3% for branch circuits and feeders.

Pro-Tip for Long Runs: Always run your wire sizing through a dedicated voltage drop calculator, like the one provided by Southwire, after you have satisfied the NEC ampacity tables. For long 200A runs, it is incredibly common to see electricians upsize from 4/0 aluminum to 250 kcmil or 300 kcmil aluminum purely to mitigate voltage drop, even though the smaller wire would technically pass the thermal ampacity inspection.

Ultimately, mastering 4/0 ampacity means understanding that the printed table is just step one. True electrical design requires cross-referencing the insulation rating, the termination limits of your specific breaker model, the physical environment of the conduit, and the total length of the run to ensure a safe, code-compliant, and functional installation.