The ampacity of 4/0 AWG copper wire ranges from 195 amps to 260 amps, strictly depending on the insulation temperature rating and termination limits defined in NEC Table 310.16. For standard residential 200A service entrances using THHN/THWN-2 in conduit, the effective allowable ampacity is 230A (based on the 75°C column).
Before pulling wire or sizing a breaker, you need to know how to read the NEC ampacity tables correctly. The table is divided into three temperature columns: 60°C (140°F), 75°C (167°F), and 90°C (194°F). The 60°C column applies to older NM-B (Romex) cable or equipment explicitly rated only for 60°C. The 75°C column is the default for most modern breakers, panel lugs, and THWN wire. The 90°C column applies to modern THHN/XHHW-2 wire but is almost never used for final overcurrent protection sizing; instead, it serves as the mathematical starting point for derating calculations.
The 4/0 Copper Ampacity Master Chart (NEC Table 310.16)
The following data is extracted directly from the Cerro Wire NEC Ampacity Charts, which mirror the allowances in NFPA 70 (NEC) Table 310.16. This table assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.
| Wire Size (AWG/kcmil) | 60°C (140°F) Copper | 75°C (167°F) Copper | 90°C (194°F) Copper | Common Insulation Types |
|---|---|---|---|---|
| 2/0 AWG | 145A | 175A | 195A | NM-B (60°C), THHN (90°C) |
| 3/0 AWG | 165A | 200A | 225A | XHHW-2, THWN-2 |
| 4/0 AWG | 195A | 230A | 260A | THHN, THWN-2, XHHW-2 |
| 250 kcmil | 215A | 255A | 290A | THHN, XHHW-2 |
Which Temperature Column Actually Applies to Your Install?
A common jobsite mistake is looking at the 90°C column for THHN wire and assuming you can protect 4/0 copper at 260A. This violates NEC 110.14(C), which governs termination temperature limits.
The rule is simple: your circuit is only as strong as its weakest termination. Almost all standard residential and commercial breakers, disconnects, and panel lugs are rated for 75°C. Even if you pull 90°C THHN-2 wire through your conduit, the moment that wire lands on a 75°C rated breaker lug, the 75°C ampacity column becomes your legal limit for overcurrent protection.
Therefore, for a 4/0 AWG copper wire terminating on standard 75°C equipment, your maximum allowable ampacity for breaker sizing is 230A. You would use the 90°C column (260A) exclusively when you need to apply derating factors for conduit fill or ambient heat, which we will cover next.
Derating 4/0 Wire: When 260A Drops Below 200A
Ampacity tables assume ideal conditions: 30°C ambient air and no more than three current-carrying conductors bundled together. When you bundle wires in a conduit, they trap heat. NEC Table 310.15(C)(1) requires you to reduce (derate) the wire's capacity based on the number of current-carrying conductors.
Here is where the 90°C column earns its keep. You apply the derating percentage to the 90°C value (260A for 4/0 Cu), not the 75°C value.
Real-World Derating Scenario
Imagine you are pulling a feeder to a subpanel that includes four current-carrying conductors (e.g., two ungrounded hots, a neutral carrying unbalanced nonlinear load, and a fourth wire for a multi-wire setup).
- Base 90°C Ampacity: 260A
- Adjustment Factor (4-6 conductors): 80%
- Derated Ampacity: 260A × 0.80 = 208A
Because 208A is still greater than your 200A load requirement, you can still legally protect this circuit with a 200A breaker. Furthermore, because 208A is greater than the 75°C termination limit requirement for the load, the installation is code-compliant.
When it fails: If you had 7 to 9 current-carrying conductors in that same conduit, the derating factor drops to 70%.
260A × 0.70 = 182A.
Your derated ampacity is now below 200A. You can no longer use a 200A breaker; you must drop to the next standard size down (175A) or upsize your wire to 250 kcmil.
What the Ampacity Table Cannot Tell You
Relying solely on NEC Table 310.16 leaves three critical physical and electrical realities unaddressed. Failing to account for these will result in failed inspections or poor system performance.
1. Voltage Drop Over Distance
Ampacity measures thermal limits (how much current before the insulation melts), not voltage delivery. Pushing 200A through 4/0 copper over a 250-foot run to a detached workshop will result in a voltage drop exceeding the NEC-recommended 3% threshold for branch circuits and feeders. At 240V, a 3% drop is 7.2V. For long runs, you must calculate voltage drop using the specific resistance of 4/0 copper (0.0608 ohms per 1,000 feet at 75°C) and may need to upsize to 250 kcmil or 350 kcmil purely to maintain voltage, even if the thermal ampacity is sufficient.
2. Conduit Fill Limits (Chapter 9)
4/0 AWG wire is physically massive. The ampacity table does not tell you if the wire will actually fit in your pipe. According to NEC Chapter 9, Table 1, conduit fill is limited to 40% for three or more wires. Three 4/0 THHN wires require a minimum of 1.5-inch EMT or 2-inch PVC Schedule 40. Attempting to force 4/0 into 1-inch or 1.25-inch conduit will result in jammed pulls, damaged insulation, and an immediate inspection fail.
3. Bending Radius and Pulling Tension
4/0 copper is incredibly stiff. NEC 300.34 and 334.24 dictate strict minimum bending radii for conductors. For 4/0 wire, you cannot use standard short-radius 90-degree sweeps. You must use long-sweep bends (typically a minimum of 12 inches of radius for 4/0) to prevent kinking the copper strands or tearing the THHN insulation. On runs longer than 100 feet or with more than two bends, hand-pulling 4/0 copper is nearly impossible; you will need wire pulling lubricant (like Polywater) and a mechanical tugger or a team of three pullers to keep tension within safe limits.
