The Direct Answer: 4/0 AWG Ampacity at a Glance
The maximum allowable ampacity for 4/0 AWG copper wire is 230 amps when terminated on standard 75°C equipment, and 260 amps for derating calculations using the 90°C column. For 4/0 AWG aluminum, the ratings are 180 amps (75°C) and 205 amps (90°C).
Bookmark Quick-Jump: If you are wiring a standard 200A residential service entrance, NEC 310.12 permits 4/0 AWG Aluminum (XHHW-2). If you are wiring a 200A subpanel feeder or a 225A panel, use 4/0 AWG Copper (THHN/THWN-2).
These values assume an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway. Sizing 4/0 wire incorrectly is a common point of failure in heavy feeder runs, usually stemming from confusion between the wire's insulation rating and the termination rating of the breaker lugs.
How to Read NEC Table 310.16 for 4/0 Wire
Before pulling wire, you must understand how to read the National Electrical Code (NEC) ampacity tables. The table below is extracted from NEC 2023 Table 310.16, which governs the allowable ampacities of insulated conductors rated up to 2000 volts.
How to read this table: The columns represent the temperature rating of the wire's insulation (60°C, 75°C, 90°C). The rows represent the American Wire Gauge (AWG) or circular mil (kcmil) size. To find your baseline ampacity, locate the 4/0 row and read across to the column that matches your installation conditions. Most modern building wire (THHN/THWN-2 or XHHW-2) is rated for 90°C, but as explained in the next section, you rarely get to use the full 90°C value for the final breaker sizing.
| Wire Size (AWG/kcmil) | 60°C Column (140°F) | 75°C Column (167°F) | 90°C Column (194°F) |
|---|---|---|---|
| 2/0 AWG | 145A (Cu) / 115A (Al) | 175A (Cu) / 135A (Al) | 195A (Cu) / 150A (Al) |
| 3/0 AWG | 165A (Cu) / 130A (Al) | 200A (Cu) / 155A (Al) | 225A (Cu) / 170A (Al) |
| 4/0 AWG | 195A (Cu) / 150A (Al) | 230A (Cu) / 180A (Al) | 260A (Cu) / 205A (Al) |
| 250 kcmil | 215A (Cu) / 170A (Al) | 255A (Cu) / 205A (Al) | 290A (Cu) / 230A (Al) |
Source: NFPA 70: National Electrical Code (NEC) 2023 Edition, Table 310.16. Cu = Copper, Al = Aluminum.
Which Temperature Column Actually Applies to Your Install?
The most frequent mistake DIYers and junior apprentices make is looking at a spool of 4/0 THHN (which is stamped 90°C on the jacket), reading the 260A value from the 90°C column, and slapping it on a 250A breaker. This is a code violation and a fire hazard.
Under NEC 110.14(C), the ampacity of your circuit is limited by the lowest temperature rating of any connected component. While your wire insulation might handle 90°C, the mechanical lugs inside standard residential and light-commercial breakers, panels, and disconnects are almost universally rated for 75°C. Older equipment may even be limited to 60°C.
The Rule of Thumb: You must use the 75°C column to determine the maximum size of your overcurrent protective device (breaker). Therefore, a 4/0 AWG copper feeder is capped at 230 amps for termination purposes. You would protect this wire with a 225A breaker (the next standard size down, or exactly 225A if available). If your specific equipment manufacturer explicitly lists their lugs as rated for 90°C and the wire is 90°C, only then can you use the 260A figure for termination sizing.
Derating 4/0 AWG: When the Base Value Drops
This is where the 90°C column finally becomes useful. When you have more than three current-carrying conductors (CCCs) in a single conduit, or when ambient temperatures exceed 30°C (86°F), the wires heat each other up. The NEC requires you to apply a derating factor.
How derating modifies the base value: You always start your derating math using the 90°C column (because the insulation itself can handle the heat, even if the breaker lug cannot). After applying the derating multiplier, you compare the result to the 75°C column termination limit. The final allowable ampacity is the lower of the two numbers.
Worked Derating Example: You are pulling two 4/0 AWG copper hot wires, one neutral, and one ground through a conduit to a subpanel. You have 3 current-carrying conductors (the ground does not count). No derating is required. Your limit is 230A (75°C column).
Now, imagine you are pulling two separate 120/240V multi-wire branch circuits in the same conduit (4 hot wires, 2 neutrals = 6 CCCs). NEC Table 310.15(C)(1) requires an 80% derating factor.
Math: 4/0 Cu at 90°C = 260A.
260A × 0.80 = 208A.
Compare 208A (derated 90°C value) to 230A (75°C termination limit). The lower number is 208A. Your wire is now legally capped at 208 amps, meaning you must drop your breaker size to 200A.
Decision Tree: Sizing Your 200A to 225A Feeder
Use this decision matrix to select the exact wire type and size for your specific heavy-load application. Do not guess; follow the path that matches your installation.
| Installation Scenario | NEC Rule Applied | Concrete Wire Pick |
|---|---|---|
| 200A Residential Service Entrance (Utility meter to main panel, powering the whole house) | NEC 310.12(A) allows an 83% derating exception for whole-dwelling services. | 4/0 AWG Aluminum (XHHW-2). Rated 180A at 75°C, but legally permitted for a 200A main breaker under this specific residential exception. |
| 200A Subpanel Feeder (Main panel to detached garage or subpanel, not a whole-house service) | Standard NEC 310.16 rules apply. The 83% residential exception does NOT apply to feeders. | 4/0 AWG Copper (THHN/THWN-2) OR 250 kcmil Aluminum. (4/0 Al is only rated 180A for feeders and cannot be put on a 200A breaker here). |
| 225A Subpanel or Service | Standard NEC 310.16 rules. Must meet or exceed 225A in the 75°C column. | 4/0 AWG Copper (THHN/THWN-2). Rated 230A at 75°C. (Aluminum would require stepping up to 350 kcmil). |
| High Ambient Heat (Conduit running across a hot roof or boiler room >110°F) | NEC Table 310.15(B)(1) ambient temperature correction factors apply. | Step up to 250 kcmil Copper or run the conduit in a shaded raceway to avoid severe ampacity penalties. |
What the Ampacity Table Cannot Tell You
Ampacity tables only tell you how much current the wire can carry before the insulation melts. They completely ignore the physics of the physical installation. Before buying 4/0 wire, account for these three critical field realities:
1. Voltage Drop Over Distance
NEC 310.15(B) recommends a maximum voltage drop of 3% for feeders. 4/0 copper carrying 200A at 240V will experience roughly a 1.5% voltage drop at 100 feet. However, if your detached garage is 250 feet away, that drop exceeds 3.5%, which can cause hard-starting issues for well pumps, air compressors, and EV chargers. For runs over 150 feet at 200A, step up to 250 kcmil or 300 kcmil copper to maintain voltage stability, even though 4/0 is technically legal for the ampacity.
2. Bending Radius and Box Sizing
4/0 AWG wire is incredibly stiff. According to Southwire technical specifications, the minimum bending radius for 4/0 THHN is roughly 5 times the cable diameter (about 3.5 inches). You cannot stuff this wire into standard 4x4 junction boxes or shallow panel gutters. You must use deep junction boxes, large pull boxes, or sweep elbows to avoid kinking the conductor or damaging the insulation against the metal box edges.
3. Lug Torque Requirements
Under NEC 110.14(D), you cannot just tighten 4/0 lugs with a standard wrench until they 'feel tight'. Loose connections on 200A feeders create high-resistance hotspots that will melt the breaker lug and start a fire. You must use a calibrated torque wrench or torque screwdriver set to the exact inch-pound value printed on the breaker or panel label (typically between 250 and 350 in-lbs for 4/0 wire). If the manufacturer's torque spec is missing, you must defer to the default values in NEC Table 110.14(D), but checking the equipment label is the mandatory first step.
